p.p. See 'pro parte'.

pachy- (Gr. pakhys: thick) prefix. Meaning thick.

pachycarpous (Gr. pakhys: thick; karpos: fruit) adj. With large, thick fruit.

pachycaul (Gr. pakhys: thick; kaulos: stalk) adj. Thick-stemmed, not 'twiggy; unbranched, or the branches few and thick; with thick or massive primary construction.

pachycladous (Gr. pakhys: thick; klados: branch) adj. With thick branches.

pachydermous adj. Thick-skinned; applied to exceptionally thick-walled cells of moss capsules and stems.

pachyphyllous (Gr. pakhys, thick; phyllon, leaf) adj. With thick leaves.

pachytene n. The double thread or 4 strand stage of meiosis.

pagina (L. pagina: page, leaf) n. A flat surface, such as the blade of a leaf.

paired adj. Associated in groups of two; e.g. the peristome teeth of Fabronia, the margingal teeth on the leaves of Rhizogonium spiniforme.

pairing n. The close association between homologous chromosomes that develops during the first prophase of meiosis. The two chromosomes move together and an exact pairing of corresponding points along their lengths occurs as they lie side by side. The resulting structure is called a bivalent.

palaeo- (Gr. palaios: old) prefix. Meaning ancient, very old.

Palaeocene See Paleocene.

palaeopolyploid n. Early or primitive polyploid; e.g., one of ancient origin. Also paleopolyploid.

palate (L. palatum: palate) n. A raised appendage on the lower lip of a corolla which partially or completely closes the throat.

pale n. A membrane dividing distinct parts of a plant structure.

palea (L. palea: chaff) n. (pl. paleae) The upper, and usually shorter and thinner, of two membranous, chaffy bracts which subtend a grass floret, often partially surrounded by the lemma. In Asteraceae, a scale-like bract that subtends an individual flower on the receptacle, absent in some genera, restricted to a ring separating ray and disk flowers in most tarweed species.

paleaceous (L. palea: chaff) adj. Chaffy; with chaffy scales.

paleae See palea.

palearctic adj. Belonging or pertaining to a geographical division comprising Europe, Africa north of the tropic of Cancer, the northern part of the Arabian Peninsula, and Asia north of the Himalayas.

paleo- (Gr. palaios: old) prefix. Meaning ancient, very old.

paleobotanic adj. See paleobotany. Also paleobotanical.

paleobotanical See paleobotanic. Also paleophytological.

paleobotany n. The branch of paleontology that deals with plant fossils and ancient vegetation.

Paleocene (Gr. palaios: old; eôs: dawn; kainos: new) n. The lowest epoch of the Tertiary Period, about 65-56.5 Millions years ago. The name is derived from the Greek palaios 'ancient', eos 'dawn', and kainos 'new', and means 'the old part of the Eocene' (the subsequent epoch).

paleoherb (Gr. palaios: old; L. herba: plant, herb) n. Any member of a group of basal flowering herbs which may be the closest relatives of the monocots. They include the water lilies, Piperales, and Aristolochiales.

paleola (diminutive L. palea: chaff) n. (pl. paleolae) A tiny palea; a lodicule.

paleolae See paleola.

paleolate adj. With a paleola or paleolae.

paleomorphic (Gr. palaios: old; morphę: shape) adj. Flowers without symmetry; usually with an indefinite number of stamens and carpels, and usually subtended by bracts or discolored upper leaves; e.g., Salix discolor. Syn. amorphic, amorphous.

paleontology n. The science of reconstructing extinct species, including their behavior and their living conditions, primarily through the study of fossils.

paleophytological See paleobotanical.

paleopolyploid See palaeopolyploid.

paleotropical adj. Belonging or pertaining to a geographical division comprising the Ethiopian and Oriental regions.

Paleozoic (Gr. palaios: old; zôon: animal) n. The first (570 to 248 million years ago) of the three eras of the Phanerozoic. The Cambrian, Ordovician, and Silurian Periods together form the Lower Palaeozoic sub-era; the Devonian, Carboniferous, and Permian the Upper Palaeozoic sub-era. During the Palaeozoic, two major orogenies occurred: the Caledonian during the Lower Palaeozoic, and the Variscan in late Palaeozoic times. The faunas of the Palaeozoic are noted for the presence of many invertebrate organisms, including trilobites (Trilobita), graptolites (Graptolithina), brachiopods (Brachiopoda), cephalopods (Cephalopoda), and corals. By the end of the era, amphibians and reptiles were major components of various communities and giant tree-ferns, horsetails, and cycads gave rise to extensive forests.

palinactinodromous] adj. Actinodromous, the primary veins with one or more subsidiary radiations above the primary one.

palisade adj. Said especially of leaf mesophyll, but of tissues in general, where the cells are elongated, closely packed, and upright.

palisade cell One of the columnar cells of palisade parenchyma.

palisade parenchyma A leaf tissue composed of columnar cells containing numerous chloroplasts in which the long axis of each cell is perpendicular to the leaf surface.

pallid adj. Pale.

palmate (L. palmatus: stamped with the shape of a hand) adj. Of leaves divided into lobes arising from a common center, like the fingers of a hand. Digitate, radiating, fan-like from a common point, as leaflets of a palmately compound leaf or veins or palmately-veined leaf. Having three or more veins, leaflets, or lobes radiating from one point; digitate. Cf. pinnate.

palmately adv. See palmate.

palmately compound Having veins or leaflets arranged like the fingers on a hand.

palmate-pinnate adj. With the primary leaflets palmately arranged and the secondary leaflets pinnately arranged. See palmate, pinnate.

palmate-veined adj. With the principal veins arising from the end of the leafstalk and radiating toward the edge of the leaf. See palmate.

palmatifid (L. palmatus: stamped with the shape of a hand; findere: to split) adj. Palmately cleft or lobed. Of a leaf, deeply divided into several lobes which arise (almost) at the same level. Cf. pinnatifid.

palmatinerved adj. Said of palmate venation in general.

palmatisect (L. palmatus: stamped with the shape of a hand; sectus: cut) adj. Palmately divided. Sectioned or divided palmately into distinct segments. A condition intermediate between palmate and palmatifid, with the green tissue of the lamina completely divided into several segments, but the segments not fully separated at the base. See palmate, palmatifid.

palmitic acid A 16-carbon saturated fatty acid found in most fats and oils, particularly associated with stearic acid.

pals n. A peat hummock with a core of frozen peat and/or mineral soil rising to a height of 0.5-10 m above a mire surface within the discontinuous permafrost zone. Also palsa

palsa See pals.

paludicolous adj. Growing in marshy places. Cf. paludose.

paludose (L. palus, paludis: swamp, marsh) adj. Growing in wet meadows or marshes. (same as palustrine)

palustrine (L. palustris: marshy, swampy) See paludose

palustrine wetlands Inland marshes and swamps as well as bogs, fens, tundra and floodplains. Palustrine systems include any inland wetland which lacks flowing water and contains ocean derived salts in concentrations of less than 0.05 %. Nontidal wetlands covering areas less than 20 acres with water up to 6.6 feet deep.

palynology (Gr. palyno: to scatter flour; logos: word) n. The study of living and particularly of fossil pollen grains, spores, and certain other microfossils (e.g. dinoflagellates and coccolithophorids). Palynology was developed from pollen analysis and deals principally with structure, classification, and distribution. It has many applications, e.g. in medicine, archaeology, petroleum exploration, and palaeoclimatology.

palynomorph n. The fossil spores and pollen; also dinoflagellates.

pampiniform (L. pampinus: young shoot, shoot of vine with foliage; forma: shape) adj. Tendril-like.

pampinus (L. pampinus: young shoot) n. A tendril

pandurate (L. pandura: pandore) adj. Shaped somewhat like a violin, as some leaves; obovate with sinus or indentation on each side near base and with two small basal lobes. Syn. panduriform.

panduriform (L. pandura: pandore; forma: shape) adj. See pandurate

panicle (L. panicula: panicle) n. A branched, racemose inflorescence with flowers maturing from the bottom upwards. An indeterminate inflorescence whose primary axis bears branches of pedicelled flowers (at least basally so); a branching raceme. A loose irregularly compound inflorescence with pedicellate flowers. A branched racemose inflorescence often applied more widely to any branched inflorescence. A compound raceme; an indeterminate inflorescence in which the flowers are borne on branches of the main axis or on further branches of these. A type of flowering shoot common in the grass family. The primary axis bears groups of racemes and is itself racemose, as the youngest groups of flowers are at the top (e.g. oat). The term may be used loosely for any form of branched racemose inflorescence; for example, the horse chestnut is a raceme of cymes. Both these arrangements are seen in the family Polygonaceae (docks and sorrels).

panicled (L. panicula: panicle) adj. Borne in a panicle; resembling a panicle.

paniculate (L. panicula: panicle) adj. Panicled; arranged or growing in panicles; indeterminate and much branched.

paniculiform (L. panicula: panicle; forma: shape) adj. An inflorescence with the general appearance, but not necessarily the structure, of a true panicle.

panmixia n. The random mating without restriction (usually extended to include random mating under the restrictions of sex or incompatibility). Also panmixis, panmixy.

panmixis See panmixia.

panmixy See panmixia.

panniform (L. pannus: tag; forma: shape) See pannose.

pannose (L. pannosus: in rags, in tatters) adj. Covered with a short, dense, felt-like tomentum; having the texture of felt or wooeln cloth. Syn. pannose.

pantoporate (Gr. pas, pan, pantos: all; L. porus: pore) adj. Of a pollen grain, with rounded apertures all over the surface.

papaveraceous (L. papaver: poppy) adj. Resembling a poppy; belonging to the Papaveraceae.

papilionaceous (L. papilio, papilionis: butterfly) adj. Butterfly-like, as the irregular corolla of a pea, with a banner petal, two wing petals, and two fused keel petals, as in the Fabaceae.

papilla (L. papilla: nipple) n. (pl. papillae) Any cone-shaped protuberance projecting from the surface of an organ or organism. Papillae occur, for example, on the tongue, in the kidneys, and, in plants, on the surface of many petals. A glandular hair with one secreting cell above the epidermis level.

papillae See papilla.

papillary (L. papilla: nipple) adj. Of, pertainng to, or of the nature of papilla or papillae.

papillate (L. papillatus: with buds) adj. Having papillae.

papillose (L. papilla: nipple) adj. Descriptive of a surface beset with short, blunt, rounded, or cylindric projections; having minute papillae; covered with minute tubercles.

pappiferous (L. pappus: down of thistle; ferre: to carry) adj. Pappus-bearing.

pappose (L. pappus: down of thistle) adj. Having or forming a pappus, downy. See pappiferous.

pappus (L. pappus: down of thistle) n. A group of modified sepals, often in the form of a ring of silky hairs, bristles, awns or scales, borne above the ovary and outside the corolla in Asteraceae and possibly representing the calyx. For example, when the fruit of a dandelion or histle matures a pappus of hairs persists at the top of a thin stalk forming a parachute-like structure, which serves to disperse the fruit.

papyraceous (L. papyrus: paper) adj. Papery in texture and usually color.

parabiosis (Gr. para: near; bios: life) n. The natural or artificial joining or grafting of two organisms.

paracarp (Gr. para: near; karpos: fruit) n. Two or more carpels merged completely, forming a cylindical ovule, where the seeds are located either at the central placenta (free central placentation) or externally at the inner walls of the carpel (parietal placentation). An aborted ovary.

paracarpous adj. Said of a syncarpous gynoecium where the carpellary units are congenitally united by their margins, the placentation thus being parietal.

paraclade n. Lateral branches of a synflorescence.

paracytic adj. Said of stomata, with two cells surrounding and parallel to the guard cells (see also brachyparacytic).

paradermal adj. Applied to section cut parallel to the surface; commonly employed to study the air pore morphology of thalloid liverworts.

parageneon n. A species in which apomixis is not present and with little morphological or genetic variation but contains some aberrant genotypes; all individuals are infertile.

parallelodromous (Gr. parallęlos: parallel; dromos: race) With two or more primary veins originating beside one another at the blade base and running more or less parallel to the apex where they converge. See parallel-veined.

parallel-veined adj. With the main veins parallel to the leaf axis or to each other. Cf. net-veined.

parallell evolution Same as convergence.

paramecium (Gr. paramekęs: long, oblong) n. Any member of the genus Paramecium.

Paramecium (Gr. paramekęs: long, oblong) n. A genus of microscopic single-celled organisms (protozoa), sometimes called slipper animalcules, found in fresh water. They are slipper-shaped, 0.1 to 0.3 mm long, and covered with cilia, which are used for swimming and to waft bacteria and small protozoans into the gullet. They reproduce asexually by binary fission and sexually by conjugation. Phylum: Ciliophora.

paranema n. A common colorless form of flagellate algae with a characteristic stiff flagellum. See also paraphysis.

parapatric adj. Said of distributions of two taxa or populations, having non-overlapping but contiguous ranges.

paraphyletic (Gr. para: near; phyletikos: bout a tribe) adj. Term applied to a group of organisms which includes the most recent common ancestor of all of its members, but not all of the descendants of that most recent common ancestor. For example, the taxonomic group 'reptiles' used in evolutionary systematics is paraphyletic because it excludes birds and mammals, which share the same common ancestor as the reptiles. In cladistics such groups are regarded as invalid when constructing classification schemes, since cladists allow only monophyletic groups, or clades, as a basis for taxonomic groupings. However, in evolutionary systematics paraphyletic groups, or evolutionary grades, are sometimes permitted in order to reflect biological similarities. Thus 'reptiles' is used as a group because its members are closely related to each other and they share certain essential characteristics, such as ectothermy ('cold-bloodedness'), that differ markedly from their fellow clade members, the birds and mammals. Cf. polyphyletic.

paraphyllia See paraphyllium.

paraphyllium (Gr. para: near; phyllon: leaf) n. (pl. paraphyllia) One of the branched, filamentous or stipuliform outgrowths of the stem, between the leaves of certain mosses.

paraphyllum (Gr. para: near; phyllon: leaf) See stipule.

paraphyly n. Term applied to a group of organisms that include an ancestor and some, but not all of its descendants; cf. monophyly, monophyletic groups, polyphyly, polyphyletic groups.

paraphyletic group Same as paraphyly.

paraphyses See paraphysis,

paraphysis (Gr. para: near; physis: forming, production) n. (pl. paraphyses). A sterile filament occurring among the sporangia of some ferns. Any of the sterile filaments of cells that are found around the sex organs of mosses (Bryophyta), brown algae (Phaeophyta), and certain fungi (the Ascomycota and Basidiomycota). A sterile, upright, basally attached fungal filament in a hymenium, growing between asci. Syn. paranema.

parasite (L. parasitus: parasite) n. An organism living on or in a different organism, called the host, and deriving nourishment and protection from it; the relationship may or may not be harmful to the host. A facultative parasite is one that becomes parasitic only under certain conditions, while an obligate parasite must always live parasitically. Many parasites have complex life cycles, with one or more intermediate hosts (of different species) supporting the parasite in the larval stages of its development. The study of parasites-parasitology-is of great importance in medicine since many parasites either cause or transmit disease. Disease-causing parasites include bacteria, fungi, and other microorganisms and tapeworms; mites, ticks, and fleas are examples of external parasites that transmit disease. Some plants are partly parasitic, usually on other plants; mistletoe is an example. Cf. saprophyte, epiphyte.

parasitic adj. Relating to or caused by parasites. Also parasitical.

parasitical See parasitic.

parasitism (L. parasitus: parasite) n. An association in which one organism (the parasite) lives on (ectoparasitism) or in (endoparasitism) the body of another (the host), from which it obtains its nutrients. Some parasites inflict comparatively little damage on their host, but many cause characteristic diseases (these are, however, never immediately fatal, as killing the host would destroy the parasite's source of food; Cf. parasitoid). Parasites are usually highly specialized for their way of life, which may involve one host or several (if the life cycle requires it). They typically produce vast numbers of eggs, very few of which survive to find their way to another suitable host. Obligate parasites can only survive and reproduce as parasites; facultative parasites can also live as saprotrophs. The parasites of man include fleas and lice (which are ectoparasites), various bacteria, protozoans, and fungi (endoparasites causing characteristic diseases), and tapeworms (e.g. Taenia solium, which lives in the gut). See also hemiparasite.

parasitize v. To live on or in (a host) as a parasite.

parastemon (Gr. para: near; stęmôn: thread) See staminode.

parastichy n. Of the arrangement of parts along an axis, a regular oblique line formed by joining a primordium to the next youngest primordium initiated in contact with it.

parasymbiont n. An organism symbiotic with a pre-existing symbiosis (e.g., a lichenicolous fungus) not damaging its host.

parateniola n. (pl. parateniolae) The extension of teniola into the sheathing base, either replacing the teniola proximally or lying just outside it and distinguished by being unistratose. Also mesohyalocysts.

parathecia See parathecium.

parathecium n. (pl. parathecia) Of apothecia, the outside hyphal layer.

paratracheal adj. Said of axial parenchyma, associated with the vessels.

paratype (Gr. para: near; typos: mark, image) n. A specimen, other than the holotype, that was cited with the original publication of a name. Cf. lectotype, neotype.

parenchyma (Gr. para: near; en; in; khymos: sap, juice) n. A tissue of higher plants that consists of thin-walled living photosynthetic or storage cells, relatively undifferentiated, frequently with air spaces between them, which may vary in structure and function, capable of division even when mature and that makes up much of the substance of leaves and roots, the pulp of fruits, and parts of stems and supporting structures; these cells may manufacture or store food, and can often divide or differentiate into other kinds of cells. The cortex and pith are composed of parenchyma cells.

parenchyma sheath A parenchymatous, single-layered bundle sheath surrounding a vascular bundle, homologous to the endodermis.

parenchymatous See parenchyma.

parenthesome n. A curved double membrane (which may be perforate, continuous or vesiculate) on each side of a dolipore septum in the Basidiomycota, septal pore cap.

parichnos n. An interconnected system of parenchymatous strands with many air spaces that extend throughout the vegetative organs of some arborescent lycopsids/

parichnos scars Small scars that marks the position of the parichnos strands on stem fossils of the some arborescent lycopsids.

parietal (L. parietalis: of, belonging to walls) adj. Attached to the margins of a structure; of placentation, having the ovules attached to placentas on the wall of the ovary.

parietal placentation Ovules attached to the walls of the ovary.

paripinnate (L. par, paris: even; pinnatus: feathered, winged) adj. Having an even number of pinnae by virtue of having a pair in the terminal position. Cf. imparipinnate.

Parkeriaceae n. A family of the phylum Filicinophyta.

paroicous (Gr. para: near; oikos: house) adj. Having its male and female organs in the same cluster, but not mixed, the antheridia being in the axils of the perichaetial bracts below the archegonia.

parsimony (L. parsimonia: savings, thrift) n. Refers to a rule used to choose among possible cladograms, which states that the cladogram implying the least number of changes in character states, with the simplest explanation is the best, although the most parsimonious explanation is not always the correct one..

parted (L. pars, partis: part) adj. Deeply cleft, usually more than half the distance to the base or midvein. Syn. partite.

parthenocarpic (Gr. parthenos: virgin; karpos: fruit) adj. Exhibiting parthenocarpy. Syn. parthenocarpical, parthenocarpicous.

parthenocarpical See parthenocarpic.

parthenocarpically See parthenocarpical.

parthenocarpous See parthenocarpic.

parthenocarpy (Gr. parthenos: virgin; karpos: fruit) n. The formation of fruit without prior fertilization of the flower by pollen. The resulting fruits are seedless and therefore do not contribute to the reproduction of the plant; examples are bananas and pineapples. Plant growth substances may have a role in this phenomenon, which can be induced by auxins in the commercial production of tomatoes and other fruits.

parthenogenesis (Gr. parthenos: virgin; genesis: creation, production, generation) n. The development of an organism from an unfertilized egg. This occurs sporadically in many plants (e.g. dandelions and hawkweeds) and in a few animals, but in some species it is the main and sometimes only method of reproduction. For example, in some species of aphid, males are absent or very rare. The eggs formed by the females contain the full (diploid) number of chromosomes and are genetically identical. Variation is consequently very limited in species that reproduce parthenogenetically. See arrhenotoky; thelytoky.

parthenogenetic See parthenogenesis.

parthenogenetically See parthenogenetoc.

parthogenesis n. The development of an organism from a sex cell but without fertilization.

partite (L. partitus: divided) adj. See parted.

parturition n. The act or process of giving birth; childbirth.

passage cells Cells of the endodermis opposite the protoxylem that remain thin-walled and retain their Casparian band, or a similar cell in the exodermis.

patella (L. patella: cup, plate) n. (pl. patellae) A panlike or cuplike formation.

patellae See patella.

patellate (L. patella: cup, plate) adj. Having a patella; patelliform.

patelliform (L. patella: cup, plate; forma: shape) adj. Kneecap shaped; broadly round, convex on one side and concave on the other. Syn. patellate.

patent (L. patere: to spread) adj. Spreading or expanded, as in widely spreading branches or broadly expanded petals; of leaves spreading from stem at an angle of 45š or more.

pathognomonic adj. Characteristic or symptomatic of a particular disease or condition.

patulous (L. patulus: [wide] open) adj. Of an inflorescence, open or spreading loosely; spreading slightly, as a calyx; spreading, as a tree or its boughs; spreading at an angle between 45 and 90š. Syn. effuse.

paxillate adj. Said of fine venation in which the areoles occur in oriented fields, often appearing as if laid down by a series of brush strokes.

pearl gland A small sessile spherical multicellular gland.

peat n. A piece of turf cut for use as a fuel; a mass of partially carbonized plant tissue formed by partial decomposition in water of various plants and especially of mosses of the genus Sphagnum, widely found in many parts of the world, varying in consistency from a turf to a slime used as a fertilizer, as stable litter, as a fuel, and for making charcoal. Peat represents an early stage in the formation of lignite and more bituminous forms of coal, being formed less than 1 million years ago. If dried out, peat will burn, although it produces ammonia fumes. Peat is also cut for use as a soil enricher in horticulture. It is common in Canada, and northern and central Europe, and forms in swampy areas. The decaying organic matter may, in marshlands, emit an inflammable gas (phosphine), whose spontaneous combustion gives rise to the phosphorescent light (ignis fatuus) seen hovering over peaty ground.

peaty adv. Composed of peat; abounding in peat; resembling peat.

pectin n. A highly hydrophilic polysaccharide built up of monomers of alpha-galacturonic acid, an important component of cell walls.

pectinate (L. pectinatus: comb-shaped) adj. Comb-like; with close, regularly spaced divisions, appendages, or hairs, often in a single row, like the teeth of a comb. Syn. ctenoid.

pectoral adj. Useful in relieving disorders of the chest or respiratory tract.

pedate (L. pedatus: that has feet) adj. Of a leaf, having divisions like toes or a bird's claws. Of a palmate or palmately-lobed leaf, having the lateral segments divided again. Palmately divided, with the lateral lobes 2-cleft. Palmate, but with the first and fifth lobes tridentate, to give a 7-fid leaf overall.

pedately adv. See pedate.

pedatifid (L. pedatus: that has feet; findere: to split) adj. Pedately cleft.

pedatilobate (L. pedatus: that has feet; Gr. lobos: lobe) adj. Of a leaf pedately lobed.

pedicel (L. pediculus: little foot, peduncle) n. A tiny stalk; the stalk of a flower in an inflorescence; the ultimate division of a common peduncle; the stalk of the sporangia. See also seta.

pedicellate (L. pediculus: little foot, peduncle) adj. Having a pedicel or pedicels. Syn. pedicelled.

pedicelled See pedicellate.

pedigree n. A record of the ancestry of an individual, family, or strain.

pedigree breeding A system of breeding in which individual plants are selected in the segregating generations from a cross on the basis of their desirability judged individually and on the basis of a pedigree record.

pediophilous adj. Thriving in upland or level country.

peduncle (L. pedunculus: little foot, peduncle) n. In ferns, the stalk of a sporocarp. The stalk of a flower, inflorescence, (flower cluster) or fruit, especially a main stalk bearing a solitary flower or subordinate stalks.

peduncled See pedunculate.

pedunculate (L. pedunculus: little foot, peduncle) adj. Having a peduncle. Syn. pedunculated, peduncled

pedunculated See pedunculate.

pellicle (L. pellicula: small skin) n. A thin, membranous or skin-like covering.

pellis n. The surface layer of cells, same as cuticle.

pellucid (L. pellucidus: transparent, diaphanous) adj. Transparent or translucent, clear.

peloria (Gr. pelôron: monster) n. Regularity of structure occuring abnormally in flowers normally irregular, e.g. radial symmetry in flowers normally bilaterally symmetrical.

pelorian adj. See peloria.

peloriate adj. See peloria.

peloric adj. See peloria.

pelta (L. pelta: small shield) n. (pl. peltae) A scale or bract attached at the middle. A shieldlike structure, especially of a lichen.

peltae See pelta.

peltafid (L. pelta: small shield; findere: to split) adj. Peltate and divided into segments.

peltate (L. pelta: small shield) adj. Shield-shaped; leaves that are shaped like a shield and attached to the stem at the center or by some point distinctly within the margin, and having the petiole inserted into the undersurface of the lamina not far from the center.

peltiform (L. pelta: small shield; forma: shape) adj. Rounded with petiole attached to center of blade or apparently to laminar tissue. See peltate.

pencilled adj. See penicillate.

pendent (L. pendere: to be suspended, hanging) adj. Drooping, hanging down. See pendulous.

pendulous (L. pendulus: hanging down) adj. More or less hanging or declined; drooping; of ovules, attached at the top of the ovary and hanging downwards from an apical placenta. See pendent.

penetrance n. The frequency with which a gene produces a recognizable effect in individuals which carry it.

penicil (L. penicillus: [paint] brush) n. A brush-like tuft of short hairs.

penicillatate (L. penicillus: [paint] brush) adj. Brush-shaped. Syn. muscariform.

penicillate (L. penicillus: [paint] brush) adj. Having the form of a pencil, tufted like an artist's brush; terminating in tuft of hairs as certain grasses; penicilliform.

penicillately adv. See penicillate.

pennate (L. pennatus: feathered, winged) adj. Winged, feathered. See pinnate.

pennatifid See pinnatifid.

penninerved (L. penna: feather; nervus: tendon, ligament, nerve) adj. Having conspicuous lateral veins divergent from the midrib and lying approximately parallel to each other, lile a feather. Syn. penniveined.

penniparallel (L. penna: feather; parallelus: parallel) adj. With veins extending from midrib to margins, essentially parallel.

penniveined (L. penna: feather; cena: vein): parallel) See penninerved.

Pennsylvanian n. 1. The Late Carboniferous subperiod, preceded by the Mississippian, comprising the Bashkirian, Moscovian, Kasimovian, and Gzelian Epochs, and dated at 322.8 to 290 million years ago. 2. The name of the corresponding N. American sub-system, comprising the Morrowan, Atokan (Derryan), Desmoinesian, Missourian, and Virgilian series, and roughly contemporaneous with most of the Silesian sub-System (i.e. above Namurian A).

penta- (Gr. pente: five) prefix. Meaning five.

pentacamerous (Gr. pente: five; L. camera: vault) adj. With five locules.

pentacarpellary (Gr. pente: five; karpos: fruit) adj. With five carpels.

pentacyclic (Gr. pente: five; kyklos: circle) adj. With five whorls, five cycles.

pentadactylous (Gr. pente: five; daktylos: finger) adj. Divided into five finger-like segments.

pentadelphous (Gr. pente: five; adelphos, brother) adj. With the stamens arranged into five groups or clusters.

pentagonal (Gr. pente: five; gônia: angle) adj. Five-angled.

pentagynous (Gr. pente: five; gynę: woman) adj. With five pistils or styles.

pentamerous (Gr. pente: five; meros: part) adj. With five parts arranged in sets or multiples of five. Of a flower, having five parts in each floral whorl (not necessarily including the gynoecium).

pentandrous (Gr. pente: five; anęr, andros: male) adj. With five stamens.

pentangular (Gr. pente: five; L. angulus: angle) adj. Having five angles.

pentapetalous (Gr. pente: five; petalon: leaf) adj. With five petals.

pentaploid adj. n. Having five haploid sets of chromosomes. A pentaploid individual.

pentapterous (Gr. pente: five; pteron: wing) adj. With five wings.

pentasepalous (Gr. pente: five; sepalous: see sepal) adj. With five sepals.

pentastichous (Gr. pente: five; stikhos: line, row) adj. In five vertical rows or ranks

pentose n. A five-carbon sugar, e.g. xylose.

pepo (L. pepo: melon) n. Literally, a pumpkin; a fleshy, indehiscent, many-seeded fruit with a tough rind (a firm skin), a pulpy interior, and a single locule, as a melon or a cucumber.

peptidoglycan n. Carbohydrate polymer cross-linked by proteins. It is found in the cell wall of Gram positive bacteria, where it stains with the dye crystal-violet. It is a component of the cell wall of eubacteria; it is not found in eukaryotes. Consisting of chains of amino sugars (N-acetylglucosamine and N-acetylmuramic acid) linked to a tripeptide (of alanine, glutamic acid, and lysine or diaminopimelic acid), it confers strength and shape to the cell wall. Archaebacteria possess a similar polysaccharide, pseudopeptidoglycan, with N-acetyltalosaminuronic acid instead of N-acetylmuramic acid.

percurrent (L. percurrere: to run through) adj. Extending through the entire length, as a midrib through a leaf.

perennate (L. perennis: lasting the whole year) adj. Maintaining a dormant, vegetative state throughout non-growing seasons.

perennating adj. Maintaining a dormant, vegetative state throughout the non-growing seasons.

perennation (L. perennis: lasting the whole year) n. The survival of biennial or perennial plants from one year to the next by vegetative means. In biennials and herbaceous perennials the aerial parts of the plant die down and the plants survive by means of underground storage roots (e.g. carrot), rhizomes (e.g. couch grass, Solomon's seal), tubers (e.g. dahlia), bulbs (e.g. daffodil, snowdrop), or corms (e.g. crocus, gladiolus). These perennating organs are also frequently responsible for vegetative propagation. Woody perennials survive the winter by reducing their metabolic activity (e.g. by leaf loss in deciduous trees and shrubs).

perennial (L. perennis: lasting the whole year) adj., n. (Said of) a plant that grows for 3 or more years and usually flowers each year.

perfect adj. Containing both stamens and pistils.

perfect flower A flower with both essential and accessory organs; a monoclinous flower.

perfoliate (L. per: through; folium: leaf) adj. Said of opposite or whorled leaves or bracts that are united into a collarlike structure around the stem that bears them, so that the stem appears to pass through the leaf. Also perfoliated.

perfoliated See perfoliate.

perforate (L. perforatus: pierced) adj. With holes or perforations.

peri- prefix. Meaning surrounding.

perianth (Gr. peri: around; anthos: flower) n. The part of a flower situated outside the stamens and carpels. In dicotyledons it consists of two distinct whorls, the outer of sepals (see calyx) and the inner of petals (see corolla). In monocotyledons the two whorls are similar and often brightly coloured. In wind-pollinated flowers both whorls may be reduced or absent. In many horticultural varieties the number of perianth parts is multiplied, but the resulting 'double' flowers are often sterile.

periblem (Gr. periblęma: rampart, clothes) n. The histogen in plants which gives rise to the cortex.

pericarp (Gr. peri: around; karpos: fruit) n. Technically, the wall of the fruit, the ovary wall of a flowering plant after fertilization which develops to form the fruit. A term used by some to designate a fruit; technically, the ovary wall. The type of fruit that develops in a plant depends on whether the pericarp becomes dry and hard or soft and fleshy. The pericarp can be made up of three layers. The outer skin (epicarp or exocarp) may be tough and hard; the middle layer (mesocarp) may be succulent as in peach, hard as in almond, or fibrous as in coconut; and the inner layer (endocarp) may be hard and stony as in many drupes, membranous as in citrus fruits, or indistinguishable from the mesocarp, as in many berries.

pericarpial adj. See pericarp.

pericarpic adj. See pericarp.

pericarpoidal adj. See pericarp.

pericarpous (Gr. peri: around; karpos: fruit) adj. Arond the fruit.

perichaetia See perichaetium.

perichaetial adj. See perichaetium.

perichaetium (Gr. peri: around; khaitę: flowing hair, mane of a helmet, foliage) n. (pl. perichaetia) A whorl of bracts at the base of reproductive organs. In mosses, those surrounding the archegonia and base of seta.

pericladial (Gr. peri: around; klados: branchlet) asd. With a sheathing base, as in the Apiaceae.

periclinal adj., n. (Of) a plane of division parallel to the surface; applied to a cell division that is parallel to a structure. Curved in the direction of, or parallel to, the surface or the circumference. Cf. anticlinal.

periclinia See periclinium.

periclinium (Gr. periklinęs: slanting all around) n. (pl. periclinia) An involucre.

pericycle (Gr. perikyklos: to surround as with a circle) n. A plant tissue characteristic of the roots, located between the endodermis and phloem.

periderm (Gr. peri: around; derma: skin) n. The outer layer of the stem of a woody plant, consisting of the cork (bark), the cork cambium, and underlying cortical cells derived from the cork cambium.

peridermal adj. See periderm.

peridermic adj. See periderm.

peridia See peridium.

peridial adj. See peridium.

peridiiform (Gr. pęridion: small bag, small pouch; L. forma: shape) adj. In the shpe of a peridum.

peridinin n. Carotenoid pigment found in dinoflagellates.

peridiole (Gr. pęridion: small bag, small pouch) b, Any of several lenticular structures containing the basidiospores in the peridium of the bird's-nest fungi. Syn. peridiolum.

peridiolum See peridiole.

peridium (Gr. pęridion: small bag, small pouch) n. (pl. peridia) The outer enveloping coat of the fruit body in many fungi, sometimes itself differentiated into outer and inner layers, exoperidium and endoperidium respectively.

peridroma (Gr. peri: around; droma: race) n. The rachis of a fern frond.

perigenous adj. Said of stomatal ontogeny in which the subsidiary cells are not produced from the same cell (meristemoid, initial) that gives rise to the guard cell initials.

perigonal adj. See perigone.

perigone (Gr. peri: around; gonos: generation) n. Any organ inclosing the essential organs of a flower; a perianth. In mosses, the involucral bracts of a male flower, the envelope of modified leaves surrounding the antheridia. Syn. perichaetium, perigonium.

perigonia See perigonium.

perigonial adj. See perigone.

perigonium n. (pl. perigonia) The androecium; strictly the cluster of modified leaves or underleaves (bracts; bracteoles) enclosing the antheridia. Cf. perigone.

perigynia See perigynium.

perigynium (Gr. peri: around; gynę: female) n. (pl. perigynia). Some unusual appendage about the pistil, as the bottle-shaped body in the sedges, and the bristles or scales in some other genera of the Sedge family, or Cyperaceæ; in bryophytes, a somewhat fleshy, tubular structure around the archegonial cluster and subsequent sporophyte, derived from axial cells peripheral to the archegonial cluster, a perigynium may or may not be associated with a shoot calyptra, e.g. the hollow perigynium of Isotachis.

perigynous (Gr. peri: around; gynę: female) adj. With stamens, petals, and sepals borne on a calyx tube (hypanthium) surrounding, but not actually attached to the superior ovary, i.e. with stamens, petals, and sepals adnate to the perianth, and therefore around the ovary and not at its base. Cf. epigynous, hypogynous. See perigyny.

perigyny (Gr. peri: around; gynę: female) n. A floral arrangement in which the ovary is situated in a cup-shaped or flattened receptacle, from the margin of which the perianth and stamens arise. The perianth and stamens are said to be perigynous with respect to the ovary, as seen in cherry and plum flowers. Cf. epigyny, hypogyny.

perine n. An extraneous deposit of remnant endothecial debris deposited on the exine of a spore.

period (Gr. periodos: circuit, way around) n. Second-order geologic time unit which is the equivalent of the chronostratigraphic unit 'system'. Periods are subdivided into epochs; together, several periods constitute an era. When used formally the initial letter of the term is capitalized, e.g. the Devonian Period.

periphysis n. A hair-like projection inside the ostiole of a perithecium or pycnidial conidioma.

periphysoid n. Short, interascal filaments growing down from the top of a perithecium.

periphyton n. Sessile organisms, such as algae and small crustaceans, that live attached to surfaces projecting from the bottom of a freshwater aquatic environment.

peripterous (Gr. peri: around; pteron: wing) adj. With a surrounding border or wing.

perisperm (Gr. peri: around; sperma: seed) n. Nutritive tissue in an angiospermous seed, that surrounds the embryo-sac in some seeds and formed from the nucellus. Cf. endosperm.

perispermous (Gr. peri: around; sperma: seed) adj. With food reserve in perisperm, derived from diploid nucellus or integuments.

perispore (Gr. peri: around; spora: seed) n. A membrane surrounding a spore, as in Equisetum spores.

peristome (Gr. peri: around; stoma: mouth) n. A ring of toothlike structures around the opening of a moss capsule (sporangium). In many mosses, the teeth are sensitive to humidity and tend to bend and twist in dry weather, so opening the mouth of the capsule and allowing the spores to escape, aiding in spore dispersal. In wet weather they close over the opening of the capsule.

perithecia See perithecium.

perithecium (Gr. peri: around; thękę: case, box) n. (pl. perithecia) The fructification of certain fungi, typically a minute, more or less completely closed, globose or flask-shaped body enclosing the asci. A type of ascoma, the more or less globular or flask-like fruiting body of Pyrenomycetes and angiocarpic lichens, opening by a pore at top.

permafrost n. Permanently frozen subsoil, occurring throughout the Polar Regions and locally in perennially frigid areas.

Permian n. Final period (290 to 248 million years ago) of the Palaeozoic era, which is named after the central Russian province of Perm. The period is often noted for the widespread continental conditions that prevailed in the northern hemisphere and for the extensive nature of the southern hemisphere glaciation. Many groups of animals and plants, including the rugose corals (Rugosa), trilobites (Trilobita), and blastoid echinoderms (Blastozoa), vanished at the end of the Permian, in a mass extinction that was one of the great crises in the history of life. The period is divided into seven ages: the Asselian, Sakmarian, Artinskian, and Kungurian Ages in the Early Permian Epoch (290 to 256.1 million years ago); and the Ufimian, Kazanian, and Tatarian Ages in the Late Permian Epoch (256.1 to 248 million years ago).

permineralization n. A preservation type in which mineral matter has infilled intercellular and intracellular spaces, but has not replaced the cell walls; compare to petrifaction and silification.

persicicolor (L. persicum: peach; color: color) adj. Peach-colored.

persistent (L. persistere: to persist) adj. Remaining attached after the normal function has been completed; not falling or non-deciduous, long lasting.

personate (L. personatus: masked) adj. Having the lower lip pushed upwards so as to close the gap between the lips, as in te snapdragon; i.e. two-lipped, with the throat closed by a prominent projection (palate); Of a bilabiate corolla, masklike.

perspicuate See perspicuous.

perspicuous (L. perspicuus: transparent, diaphanous) adj. Transparent. Syn. perspicuate.

pertussis See whooping cough

perula n. (pl. perulae) One of the scales of a leaf bud. A pouchlike portion of the perianth in certain orchides. Also perule.

perulae See perula.

perulate adj. Describes leaf buds which are covered with scales. See perula.

perule See perula.

petal (Gr. petalon: leaf) n. One of the parts of the flower that make up the corolla. Petals of insect-pollinated plants are usually brightly coloured and often scented. Those of wind-pollinated plants are usually reduced or absent. Petals are onsidered to be modified leaves but their structure is simpler. Epidermal hairs may be present and the cuticle is often covered by lines or dots known as honey guides, which direct insects to the nectar.

petalantherous (Gr. petalon: leaf; anthęros: flowered) adj. Of a stamen with a petaloid filament, as in Saxifraga.

petaliferous (Gr. petalon: leaf; L. ferre: to carry) adj. Bearing petals.

petaline (Gr. petalon: leaf) adj. Of or resembling petals

petalode (Gr. petalon: leaf; eidô: to look like) n. An organ (usually a stamen) which resembles a petal.

petalody (Gr. petalon: leaf; eidô: to look like) n. A condition in which various organs in a flower, such as stamens, become petals or become petaloid, as in some double flowers

petaloid (Gr. petalon: leaf; eidô: to look like) adj. Like a petal; soft in texture and coloured conspicuously.

petalostemonous (Gr. petalon: leaf; stęmôn: thread) adj. With the staminal filaments fused to the corolla and the anthers free.

petalous (Gr. petalon: leaf) adj. With petals.

petiolar (L. petiolus: petiole, peduncle) adj. Pertaining to the petiole; growing from the petiole.

petiolate (L. petiolus: petiole, peduncle) adj. Growing on, or provided with, a petiole.

petiole (L. petiolus: petiole, peduncle) n. The slender stalk or stem of a leaf, also called a leaf stalk.

petiolular See petiolulate.

petiolulate (diminutive of L. petiolus: petiole, peduncle) adj. With a petiolule. Syn. petiolular.

petiolule (diminutive of L. petiolus: petiole, peduncle) n. The stalk portion of a leaflet of a compound leaf.

petrifaction n. A type of preservation in which mineral matter infills intercellular spaces and replaces the cell walls; cf permineralization and silification.

petrocolous adj. Inhabiting rocks.

petrophilous adj. Preferring rocks.

petrorhizous (Gr. petra: rock; riza: root) adj. With roots on rock.

pH Abbreviation for potential Hydrogen, a log scale measurement of the acidity and alkalinity of a solution with 1 being extremely acidic, 10 being extremely alkaline, and 7 being neutral. Most plants prefer a soil within a certain range of pH.

phaenantherous (Gr. phainô: to shine; anthęros: flowered) adj. With stamens exserted from the corolla.

phaenocarpous (Gr. phainô: to shine; karpos: fruit) adj. With the ovary (fruit) free from the surrounding floral parts.

phaenogamous adj. Having flowers with stamens and pistils and producing seeds.

Phaeophyta n. A phylum of algae in which the green chlorophyll pigments are usually masked by the brown pigment fucoxanthin. Brown algae are usually marine (being abundant in cold water) and many species, such as the wracks (Fucus), inhabit intertidal zones. They vary in size from small branched filaments to ribbon-like bodies (known as kelps) many metres long.

phagedaena n. A cutaneous ulcer that rapidly spreads peripherally, destroying tissues as it increases in size. Also called phagedenic ulcer. Also phagedena

phagedena n. See phagedaena.

phagemid n. A bacterial plasmid that can be propagated both as a plasmid and a bacteriophage.

phaierocotylar (Gr. phaneros: visible, manifest; kotylos: cup) adj. With the cotyledons emergent from seed, usually appearing above ground. Syn. epigeous, epigeal.

phalaenophily (Gr. phalaina; night butterfly) n. Pollination by moths.

phalange (Gr. phalagx: army) n. Two or more stamens joined by their filaments. Syn. phalanx.

phalanx See phalange.

phanerocotylar adj. Said of germination, when the cotyledons are exposed and photosynthetic, whether or not the hypocotyl is elongated.

phanerogam (Gr. phaneros: visible, manifest; gamos: marriage) n. In old classification systems, any plant that reproduces by means of flowers, i.e. with conspicuous reproductive parts, and seeds. Phanerogams comprised the angiosperms (flowering plants) and the gymnosperms (conifers, etc.). Their common feature is that they reproduce, and are dispersed, by means of seeds as opposed to the spores of cryptogams. The seed contains the developing plant embryo and a store of food for the initial growth of the germinating plant. The whole seed is enclosed within a hard protective coat, or testa. Cf. cryptogam.

phanerogamian adj. See phanerogam.

phanerogamic adj. See phanerogam.

phanerogamous adj. See phanerogam.

phanerogamy n. See phanerogam.

phanerophyte (Gr. phaneros: visible, manifest; phyton: plant) n. Woody or herbaceous evergreen perennial, taller than 20 inches, whose shoots do not die back. In other words, these plants are trees and large shrubs.

phanerophytic adj. See phanerophyte.

phaneropore adj. With superficial stomata; with guard cells at the same level as other exothecial cells and not sunken in chambers. Cf. cryptopore.

Phanerozoic (Gr. phaneros: visible, manifest; zôon: animal) n. Period of geologic time comprising the Palaeozoic Mesozoic, and Cenozoic Eras, It began approximately 570 million years ago at the start of the Cambrian period and is marked by the accumulation of sediments containing the remains of animals with mineralized skeletons. Although the name is derived from the Greek phaneros meaning 'visiblé and zôon meaning 'any living being, animal', the term is no longer used in the sense of 'visible life', but merely defines the base of the Cambrian.

phascoid adj. With a sessile and cleistocarpous capsule; with a bud-like habit as in Phascum.

phellem (Gr. phellos: cork) n. Cork.

phelloderm (Gr. phellos: cork; derma: skin) n. A layer of tissue, in certain plants, formed from the inner cells of phellogen and consisting usually of chlorenchyma.

phellodermal adj. See phelloderm.

phellogen (Gr. phellos: cork; genea: what is generated) n. Cork cambium, a layer of tissue or secondary meristem external to the true cambium, giving rise to cork tissue one the outside and phelloderm on the inside.

phellogenetic adj. See phellogen.

phellogenic adj. See phellogen.

phenetic adj. Describing a system of classifying organisms that relies on observable characters rather than evolutionary relationships. Phenetic classification is of practical use in, for example, field guides which enable plants and animals to be identified under field conditions. Cf. phyletic.

phenetic classification The grouping of taxa by apparent similarities rather than evolutionary genetics.

phenetic systematics See phenetics.

phenetics n. A method of generating phylogenies that is based on large numbers of quantifiable (measureable) characters which groups organisms with respect to overall similarity. Cf. phenetic.

phenogen n. A morphologically indistinguishable segment of a species which is intrafertile but intersterile.

phenology n. The scientific study of periodic biological phenomena, such as flowering, breeding, the color change of leaves and migration, in relation to climatic conditions. The relationship between a periodic biological phenomenon and climatic conditions.

phenon n. A phenotypically homogeneous species whose individuals are sexually reproduced (i.e., apomixis is not present) but has intersterile segments.

phenotype (Gr. phainô: to point out; typos: mark, image) n. The observable characteristics of an organism. These are determined by its genes (see genotype), the dominance relationships between the alleles, and by the interaction of the genes with the environment. Those observable properties of an organism that are produced by the genotype in conjunction with the environment. Organisms with the same overall genotype may have different phenotypes because of the effects of the environment and of gene interaction. Conversely, organisms may have the same phenotype but different genotypes.

phenotypic (Gr. phainô: to point out; typos: mark, image) adj. A set of characters arising from reaction to environmental stimulus. See phenotype.

phenotypical See phenotypic

phenotypically See phenotypic.

phenotypic plasticity The degree to which phenotypic expression is exhibited in a plant community under various environmental conditions.

phenovariant n. The altered external appearance of an individual as a result of environment and genotype interaction; a taxon exhibiting phenotypic plasticity is comprised of a range of phenovariants.

phenylpropanoid n. Any compound bearing a 3-carbon chain attached to a 6-carbon aromatic ring (C6-C3 compounds), most being formed from cinnamic or p-coumaric acids, volatile compounds in essential oils.

pheromone n. Biologically active substance that is secreted outside of an organism, and that triggers a specific reaction in other individuals of the same species.

phialides n. Conidiogenous cells producing conidia in basipetal succession through one or several openings.

phialoconidia See phialoconidium.

phialoconidium n. (pl. phialoconidia) A conidium produced on a special conidiogenous cell (phialide).

philopatry n. The tendency of an organism to stay in, or return to, its home area.

phloem (Gr. phloios: [inner] bark) n. A complex plant tissue specialized to transport synthesized foods, mainly sugars (carbohydrates), in a vascular plant. It is composed of sieve elements, parenchyma cells and sometimes also of fibers and sclereids. It consists then principally of tubelike cells that lack nuclei, the end of one cell being linked to the next by means of a porous wall (sieve plate). The cells are controlled by small neighbouring cells, known as companion cells.

phloroglucinol n. Simple phenolic compound composed of an aromatic ring and three hydroxyl groups, an aglycone of many glycosides.

phoranthia See phoranthium.

phoranthium (Gr. phora: carrying; anthos: flower) n. (pl. phoranthia) The receptacle of the flower head of the Compositae (Asteraceae).

phosphate n . An ion consisting of a phosphorus atom and four oxygen atoms. Among other things, it is used in the constuction of nucleic acids.

photoallergy n. A hypersensitivity that develops against the photoproduct of an exogenous chemical. A sensitivity to light that causes allergic reactions.

photoautotroph (Gr. phos, photos: light; autos: self; trophę: food) n. An autotrophic organism, such as a green plant or a photosynthetic bacterium, that synthesizes its organic materials from inorganic components using energy derived from the sun (solar energy) in the process of photosynthesis. See autotrophic nutrition, phototroph.

photobiont n. The photosynthetic partner in a lichen symbiosis, either a green alga or a member of the cyanobacteria.

photobiotic (Gr. phos, photos: light; bios: life) adj. Living or thriving only in the presence of light.

photodermatitis n. An abnormal skin reaction to sunlight, or more specifically to ultraviolet (UV) rays. It can be acute (sudden) or chronic (ongoing). It may develop rash, blisters, or scaly patches.

photoheterotroph (Gr. phos, photos: light; eteros: other; trophę: food) n. An organism using light as a source of energy and organic materials as carbon source. Any organism that uses energy derived from the sun to manufacture organic compounds from organic precursors in photosynthesis. For example, under certain conditions some purple sulphur bacteria use organic acids (rather than hydrogen sulphide) as a source of hydrogen. See phototroph.

photolysis (Gr. phos, photos: light; lysis: dissolution) n. A chemical reaction produced by exposure to light or ultraviolet radiation. The photolysis of water, using energy from sunlight absorbed by chlorophyll, produces gaseous oxygen, electrons, and hydrogen ions and is a key reaction in photosynthesis. See photophosphorylation, photosystems I and II.

photomorphogenesis n. The light-mediated changes in plant growth and development.

photoperiod (Gr. phos, photos: light; periodos: circuit, way around) n. The interval in a 24-hour period during which a plant or animal is exposed to light.

photoperiodic adj. See photoperiodism.

photoperiodism (Gr. phos, photos: light; periodos: circuit, way around) n. The response, as affecting growth, reproduction, etc., of a plant or animal to the length of exposure to light in a 24-hour period.

photophilous adj. Sun-loving; light-loving.

photophosphorylation n. The formation of ATP from ADP and inorganic phosphate using light energy in photosynthesis (Cf. oxidative phosphorylation). There are two pathways, noncyclic and cyclic photophosphorylation, which occur in the thylakoid membranes of the chloroplasts. In noncyclic photophosphorylation electrons derived from the photolysis of water are raised to higher energy levels in photosystems I and II and pass along an electron transport chain of carrier molecules (see ferredoxin; plastocyanin; plastoquinone) to NADP reductase. This enzyme transfers electrons to NADP+ to make NADPH, which provides reducing power for the dark reaction of photosynthesis. In cyclic photophosphorylation the electrons from photosystem I that are raised to a higher energy level are recycled through the electron carrier system back to photosystem I. Both pathways of electron flow cause H+ ions to be pumped by a group of cytochromes, the cytochrome b6-f complex, across the thylakoid membrane. This creates a proton gradient that drives the phosphorylation of ADP to ATP by the enzyme ATP synthetase (see chemiosmotic theory).

photophytic adj. Of plants of sunny habitats.

photosensitivity n. Abnormal reactivity of the skin to sunlight. An abnormal cutaneous response involving the interaction between photosensitizing substances and sunlight or filtered or artificial light at wavelengths of 280-400 mn. There are two main types: photoallergy and photoxicity.

photosymbiodeme n. Either of one or two morphologically distinct structures formed by the interaction of a single mycobiont with two different photobionts.

photosynthate (Gr. phos, photos: light; synthesis: combining, putting together) n. A chemical product of photosynthesis.

photosynthesis (Gr. phos, photos: light; synthesis: combining, putting together) n. Biochemical process in which light energy is absorbed by chlorophyll, and is used to fuel the building of sugar molecules. The chemical process by which green plants and other phototrophs synthesize organic compounds from carbon dioxide and water in the presence of sunlight. In plants and most algae it occurs in the chloroplasts and there are two principal reactions. In the light-dependent (or light) reaction, which requires the presence of light, energy from sunlight is absorbed by photosynthetic pigments (chiefly the green pigment chlorophyll) and used to bring about the photolysis of water: H2O 2H+ + 2e- + ―O2 The electrons released by this reaction pass along a series of electron carrier molecules; as they do so they lose their energy, which is used to convert ADP to ATP in the process of photophosphorylation. The electrons and protons produced by the photolysis of water are used to reduce NADP: 2H+ + 2e- + NADP+ NADPH + H+ The ATP and NADPH produced during the light reaction provide energy and reducing power, respectively, for the ensuing light-independent (or dark) reaction, which can take place in light or darkness. During this reaction carbon dioxide is reduced to carbohydrate in a metabolic pathway known as the Calvin cycle. Photosynthesis can be summarized by the equation: CO2 + 2H2O [CH2O] + H2O + O2 Since virtually all other forms of life are directly or indirectly dependent on plants for food, photosynthesis is the basis for all life on earth. Furthermore virtually all the atmospheric oxygen has originated from oxygen released during photosynthesis.

photosynthesize (Gr. phos, photos: light; synthesis: combining, putting together) v. To synthesize by the process of photosynthesis.

photosynthetic adj. See photosynthesis.

photosynthetically adv. See photosynthesis.

photosynthetic pigments The pigments responsible for the capture of light energy during the light reaction of photosynthesis. In plants, algae, and cyanobacteria the green pigment chlorophyll a is the primary pigment, absorbing blue and red light (see photosystems I and II). The carotenoids and various other pigments are accessory pigments, absorbing light energy and passing this on to the chlorophyll a molecules.

photosystems I and II The two systems of photosynthetic pigments in the thylakoid membranes of chloroplasts that are involved in the light reaction of photosynthesis. Each photosystem contains about 300 chlorophyll molecules that trap light energy, which is then passed to a reaction centre, comprising a chlorophyll a molecule, in each photosystem. In photosystem II a chlorophyll a molecule, known as P680, utilizes light of wavelength 680 nm; in photosystem I the chlorophyll a molecule, known as P700, absorbs light at a wavelength of 700 nm. Light energy is used in each reaction centre to raise electrons to higher energy levels to enable them to be taken up by electron acceptors. This causes P680 and P700 to become positively charged, or oxidized. The chlorophyll in photosystem II replaces its lost electrons with ones supplied by an associated complex of proteins called the oxygen-evolving complex, which is responsible for the photolysis of water: 2H2O 4H+ + 4e- + O2 The oxygen produced is given off as a gas, and the H+ ions, together with the electrons from photosystem I, reduce NADP+ (see photophosphorylation).

phototactic adj. See phototaxis.

phototactically adv. See phototaxis.

phototaxis (Gr. phos, photos: light; taxis: arrangement) n. Movement of an organism toward or awy from a source of light. Syn. phototaxy.

phototaxy n. See phototaxis.

phototonus (Gr. phos, photos: light; tonos: intensity, strength) n. The normal condition of sensitiveness to light in plants or plant organs; the irritability exhibited by protoplasm when exposed to light of a certain intensity.

phototoxicity n. An acute toxic response that is elicited after the first exposure of the skin to certain chemicals and the subsequent exposure to light, or a response induced similarly by skin irradiation after systemic administration of a chemical substance.

phototroph (Gr. phos, photos: light; trophę: food) n. Any organism that uses energy derived from the sun to manufacture organic compounds by photosynthesis. Most phototrophs are photoautotrophs; a few bacteria are photoheterotrophs.

phototropic (Gr. phos, photos: light; tropos: direction) Growing towards light. Since light is the major source of energy for plants, they often grow toward light to increase their light-gathering ability. Some plants, such as the tulip tree (Liriodendron tulipifera), however, grow away from gravity rather than towards light. Because the tulip tree does not bend towards light, its stem is very straight.

phototropism (Gr. phos, photos: light; tropos: direction) n. Growth or movement of a sessile organism toward or away from a source of light. See phototropic.

phragma (Gr. phragma: fence) n. (pl. phragmata) A septum or partition.

phragmata See phragma.

phragmobasidia See phragmobasidium.

phragmobasidium n. (pl. phragmobasidia) A basidium which is divided by primary septa, usually transverse or cruciate.

phragmoplast n. Cell division structure occurring only in higher plants.

phreatic (Gr. phreatia: water tank) adj. Referring to groundwater situated below the water-table. The phreatic zone is permanently saturated. A phreatic eruption of a volcano is one in which meteoric water is mixed with the lava. This water may be given off as a geyser or as steam.

phreatophyte (Gr. phreatia: water tank, phyton: plant) n. A plant with its root system typically in soil saturated with water. A class of desert plant with very long tap roots which develop to reach the phreatic zone.

phthisic n. Any illness of the lungs or throat, such as asthma or a cough.

phycobiliprotein n. Any one of a class of pigments in the blue-green bacteria, red algae, and photosynthetic cryptophytes that act as accessory pigments in photosynthesis, being attached to the thylakoid membranes. Each comprises a protein bound to a phycobilin, a coloured tetrapyrrole prosthetic group (see porphyrin). The main phycobiliproteins are phycocyanin and phycoerythrin.

phycobiont n. The green algal photosynthetic partner in a lichen symbiosis.

phycocyanin (Gr. phykos: alga; kyanos: dark blue) n. An accessory photosynthetic pigment occurring mainly in the blue-green bacteria and red algae. It is a phycobiliprotein, in which the pigmented prosthetic group is phycocyanobilin, which gives phycocyanin its blue colour.

phycoerythrin n. An accessory photosynthetic pigment occurring mainly in the red algae and blue-green bacteria (cyanobacteria). It is a phycobiliprotein, in which the pigmented prosthetic group is phycoerythrobilin, which gives phycoerythrin its red colour.

phylad n. A phylogenetic group.

phyletic (Gr. phyletikos: about a tribe) n. Describing a system of classifying organisms that reflects their evolutionary relationships. Cf. phenetic.

phyletic gradualism Theory holding that macroevolution is merely the operation of microevolution over relatively long periods of time. Thus gradual changes will eventually accumulate to the point at which descendants of an ancestral population diverge into separate species, genera, or higher-level taxa.

phyllary (Gr. phyllarion: little leaf) n. One of the bracts under the flower head of a plant, especially in Asteraceae.

phyllichnia See phyllichnium.

phyllichnium n. (pl. phyllichnia) In Casuarinaceae, the ridge of a branchlet article.

phyllidia See phyllidium.

phyllidium n. (pl. phyllidia) The leaf of mosses or leafy liverworts. A small, corticate, scale-like, dorsiventral structure developed at the margins or on the upper surface of a thallus, usually attached by a narrow stalk. Cf. caulidium.

phylloclade (Gr. phyllon: leaf; klados: branchlet) n. A very leaf-like, photosynthetic stem of a plant whose true leaves are much reduced, then a stem with the form and function of a leaf. Syn. cladophylls, phyllocladium.

phyllocladia See phyllocladium.

phyllocladium n. (pl. phyllocladia) A small, corticate thalline structure, granular, verrucose, coralloid, squamiform, digitate, or peltate, containing a green photobiont (in Argopsis and Stereocaulon). See phylloclade.

phyllocladous (Gr. phyllon: leaf; klados: branchlet) adj. Having phylloclades.

phyllode (Gr. phyllon: leaf; eidô: to seem) n. A flat expanded, winged, petiole that replaces the blade of a foliage leaf, that fulfills the same functions, and is analogous to a cladophyll; i.e. a leaf whose blade is much reduced or absent, and whose petiole and rachis have assumed the functions of the whole leaf. Cf. cladode, cladophyll.

phyllodia See phyllodium.

phyllodial adj. See phyllode.

phyllodioicous adj. Nanandrous; with dwarf male plants resting on leaves or tomentum of larger female plants.

phyllodium n. (pl. phyllodia) See phyllode.

phyllody (Gr. phyllon: leaf; eidô: to seem) n. The abnormal transformation of a floral structure into a foliage leaf.

phyllogenetic (Gr. phyllon: leaf; genesis: creation, production, generation) adj. Of or pertaining to the development of leaves.

phylloid (Gr. phyllon: leaf; eidô: to seem) adj. Leaflike.

phyllomania (Gr. phyllon: leaf; mania: madness) n. The production of leaves in abnormal number or places.

phyllome (Gr. phyllon: leaf) n. A leaf of a plant; a structure corresponding to it.

phyllophore (Gr. phyllon: leaf; phora: carrying) n. The terminal bud of a stem, especially of the stem of a palm.

phyllopode (Gr. phyllon: leaf; pous, podos: foot) n. The dilated leaf base of an Isoetes leaf.

phyllopodia See phyllopodium.

phyllopodic (Gr. phyllon: leaf; pous, podos: foot) adj. With the lowest leaves well developed, not reduced to scales. Cf. aphyllopodic.

phyllopodium (Gr. phyllon: leaf; pous, podos: foot) n. (pl. phyllopodia) An outgrowth of the rhizome to which the frond is joined in some ferns.

phyllotactic adj. See phyllotaxis.

phyllotactical adj. See phyllotaxis.

phyllotaxic adj. See phyllotaxis.

phyllotaxis (Gr. phyllon: leaf; taxis: arrangement) n. The arrangement of leaves on a plant stem. The leaves may be inserted in whorls or pairs at each node or singly up the stem. When arranged in pairs the two leaves arise on opposite sides of the stem and are usually at right angles to the leaf pairs above and below them. Single leaves may be inserted alternately or in a spiral pattern up the stem. Phyllotaxis generally results in the minimum of shading of leaves by those above them. The three common positions are alternate, opposite, and verticillate. When the arrangement of leaves on a stem is spiral, it is often expressed quantitatively as the fraction of the circumference of the stem that separates two successive leaves; the numerator indicates the number of turns around the stem, and the denominator indicates the number of internodes between two leaves in direct vertical alignment on the stem. Syn. phyllotaxy.

phyllotaxy See phyllotaxis.

phylogenetic (Gr. phylę: tribe; genea: what is generated) adj. Of or relating to phylogeny; based on natural evolutionary relationships; acquired in the course of phylogenetic development. Syn. phylogenetical, phylogenic.

phylogenetic systematics Same as phylogenetics, cladistics; a method of grouping organisms that is based on synapomorphies or shared derived traits or characters; cf. numerical taxonomy.

phylogenetical See phylogenetic.

phylogenetically adv. See phylogenetical.

phylogenetics (Gr. phylę: tribe; genea: what is generated) n. Field of biology that deals with the relationships between organisms on the basis of their degree of evolutionary relatedness. It includes the discovery of these relationships, and the study of the causes behind this pattern.

phylogenic See phylogenetic.

phylogenist n. See phylogeny.

phylogeny (Gr. phylę: tribe; genea: what is generated) n. The history of the evolution of a species or other group of organisms. By studying the fossil record, comparative anatomy, embryology, biochemistry, and geographical distribution of the groups involved, one can establish the probable lines of descent and degrees of relationship between broad groups of plants or animals to produce a phylogenetic tree. For example, it is believed that whales and carnivorous land mammals developed from insectivorous mammals living more than 65 million years ago. Phylogeny forms the basis for the classification of organisms (see taxonomy). It should be distinguished from ontogeny, which is the succession of developmental stages through which an individual organism passes during its lifetime. See also biogenetic law.

phylum (Gr. phylę: tribe) n. A taxon of high rank, the major unit of classification; the five kingdoms are divided into phyla, which in turn each contain a number of classes; in the plant kingdom, it is usually replaced by the division. Examples of phyla are the Rhodophyta, Ascomycota, Bryophyta, Annelida, and Chordata. Organisms belonging to the same phylum share basic features but are divided into classes, orders, genera, and species according to their supposed degrees of relationship. For example, the phylum Arthropoda is a very diverse group including the insects, spiders, scorpions, crustaceans, etc., which all share the features of a tough segmented external skeleton and jointed limbs.

physic n. A medicine that purges; cathartic; laxative. Any medicine; a drug or medicament.

physiognomy n. The analysis of leaf features that are sensitive to the environment.

phytobiology (Gr. phyton: plant; bios: life; logos: word) n. The branch of biology dealing with plants.

phytochemistry n. The chemistry of plants, plant processes, and plant products.

phytoclimatologic adj. See phytoclimatology.

phytoclimatological adj. See phytoclimatology.

phytoclimatology (Gr. phyton: plant; klima: tendency; logos: word) n. The study of local climatic condition, the analysis of climatic profiles of plants and plants communities.

phytocoenosis (Gr. phyton: plant; koinos: common) n. The plants of a given area considered as a whole.

phytophotodermatitis n. (also known as 'Berloque dermatitis') is a chemical reaction which makes skin hypersensitive to ultraviolet light. It is frequently mistaken for hereditary conditions such as atopic dermatitis or chemical burns, but it is caused by contact with the photosensitizing compounds found naturally in some plants.

phytogenesis (Gr. phyton: plant; genesis: creation, production, generation) n. The origin and development of plants.

phytogenetic adj. Of or relating to phylogeny. See also phytogenesis.

phytogenetical adj. See phytogenesis.

phytogenetically adv. See phytogenetical.

phytogenic adj. Having a plant origin, as coal. Also phytogenous.

phytogenous See phytogenic.

phytogeographer n. See phytogeography.

phytogeographic adj. See phytogeography.

phytogeographical adj. See phytogeography.

phytogeographically adv. See phytogeography.

phytogeography (Gr. phyton: plant; gę: earth; graphô: to write) n. The science dealing with the geographical relationships of plants.

phytographer n. See phytography.

phytographic adj. See phytography.

phytographical adj. See phytography.

phytographist n. See phytography.

phytography (Gr. phyton: plant; graphô: to write) n. The branch of botany dealing with the description of plants.

phytohormone (Gr. phyton: plant; ormos: what helps in binding) n. A plant hormone. See growth substance.

Phytolaccaceae n. The pokeweed family.

phytology n. The study of plants; botany.

phytomelanin (Gr. phyton: plant; melas, anos: black) n. A papery 'sooty' black layer over the seed of plants in the Asparagales, which includes agaves, aloes, onions and hyacinths. It is an important character for defining the group.

phytomere (Gr. phyton: plant; meros: part) n. A section of a grass shoot including an internode, the leaf and a portion of the node at the top of the internode, and a portion of the node at the bottom of the intemode.

phytopathogen n. An organism that is pathogenic to a plant.

phytopathogenic adj. See phytopathogen.

phytophage adj. That feeds primarily on living plant tissue.

phytophagous (Gr. phyton: plant; phagos: big eater, glutton) adj. Herbivorous.

Phytophthora n. A genus of the phylum Oomycota.

phytoplankton n. Minute, free-floating aquatic plants.

phytoplasm (Gr. phyton: plant; plasma: something molded) adj. The protoplasm of a plant or plants.

phytosociology (Gr. phyton: plant; L. socius: companion, associate; Gr. logos: word) adj. The branch of ecology dealing with the origin, composition, structure, and the classification of plants communities.

phytotherapy n. The study or use of medicinal herbs to prevent and treat diseases and ailments or to promote health and healing.

phytotoxic (Gr. phyton: plant; toxikos: poison used on arrows) adj. Poisonous to plants.

phytotoxicity See phytotoxic.

phytotoxin (Gr. phyton: plant; toxikos: poison used on arrows) n. Any toxin produced by a plant and having an effect similar to any of certain toxins produced by micoorganisms.

picoplankton n. Single-cell organisms (primarily cyanobacteria) about two microns in diameter, which flourish in the uppermost layers of the oceans.

pigment (L. pigmentum: color (for painting), make-up) n. Any colorful compound, used by living things to absorb or block sunlight, and in sexual displays, etc. For example, haemoglobin in vertebrate erythrocytes gives blood its characteristic red colour and enables the transport of oxygen throughout the body. Other biological pigments include chlorophyll, a photosynthetic pigment in plants that is responsible for their green coloration; and melanin, a brown pigment in animals that provides protection from ultraviolet light and can be used in camouflaging colorations.

pileate (L. pileus: cap) adj. With a cap.

pilei See pileus.

pileitrama n. hyphae within the cap of a basidiomycete.

pileus (L. pileus: cap) n. (pl.pilei) Umbrella-shaped structure of mushrooms or toadstools. Spores are produced from gills or pores on the lower surface.

piliferous (L. pilus, pili: hair; ferre: to carry) adj. Tipped with a fine hair-like structure.

piliform (L. pilus, pili: hair; forma: shape) adj. With the form of a hair.

piloglandulose (L. pilus, pili: hair; glandula: gland) adj. With glandular hairs

pilose (L. pilosus: hairy, shaggy) adj. Hairy; pubescence comprised of scattered long, slender, soft hairs; e.g. the calyptra of some Orthotrichum species.

pilosulose (diminutive of L. pilosus: hairy, shaggy) adj. Bearing minute, long, soft, straight hairs. Syn. pilosulous.

pilosulous See pilosulose.

pilum n. A sexine element, usually standing directly on the nexine, consisting of a rod-like part, the columella, and a swollen apical caput.

pilus (L. pilus, pili: hair) n. (pl. pili) A hair or hairlike structure.

pinna (L. pinna: feather, wing, fin) n. (pl. pinnae or pinnas) A leaflet or a primary division of a compound leaf or frond; a primary division of a pinnate leaf, attached to the main rachis.

pinnae See pinna.

pinnate (L. pinnatus: feathered, winged) adj. Said of a compound leaf (somewhat vaguely resembling a feather) having leaflets arranged on either side of the stem, usually in pairs opposite each other. The feather vein pattern of simple leaves. Cf. bipinnate, tripinnate.

pinnated adj. See pinnate.

pinnately adv. See pinnate.

pinnately compound With leaflets arranged in two rows along an axis. Leaves which are divided up like a feather are said to be pinnately compound.

pinnatifid (L. pinnatus: feathered; findere: to split) adj. Pinnately cleft or lobed half the distance or more to the midrib, but not reaching the midrib, i.e. with leaflets not entirely separate. Syn. pinnatipartite. cf palmatifid.

pinnatilobate (L. pinnatus: feathered; lobos: lobe) adj. With pinnately arranged lobes. Syn. pinnatilobed.

pinnatilobed See pinnatilobate.

pinnation (L. pinnatus: feathered, winged) n. Pinnate condition or development.

pinnatipartite (L. pinnatus: feathered, winged; partitus: divided) adj. Pinnately parted.

pinnatinerved adj. Pinnate when said of venation.

pinnatisect (L. pinnatus: feathered, winged; sectus: cut) adj. Dissected down to the midrib but having the segments confluent with it.

pinninerved (L. pinna: feather, wing, fin; nervus: tendon, ligament, nerve) adj. Pinnately veined.

pinnipalmate (L. pinna: feather, wing, fin; palmatus: stamped with the shape of a hand) adj. Intermediate between pinnate and palmate, as in a leaf with the first pair of veins larger and more distinctive than the others.

pinnula n. (pl. pinnulae) See pinnule.

pinnulae See pinnula

pinnulate (L. pinnula: small feather, small wing) adj. Having pinnules. Syn. pinnulated.

pinnulated See pinnulate.

pinnule (L. pinnula: small feather, small wing) n. The pinnate division of a pinna in a bipinnately compound leaf, or the ultimate divisions of a leaf which is more than twice pinnately compound. Syn. pinnula.

Pinopsida n. A phylum of trees and shrubs in which the sex organs are borne in cones; seeds are produced on the surface of the cone scales rather than being enclosed in an ovary, as in the Magnoliopsida. Typical members of the phylum have needle-like leaves and most are evergreens; examples are pines, junipers, spruces and firs. Syn. Gymnospermae.

pinworm n. See pinworms.

pinworms n. Small, white worms that can live in the intestines, are common in young children, and are easily treated.

pip n. A small seed, especially of a fleshy fruit, as an apple or orange.

pippin n. A seed.

piriform (L. pirum: pear; forma: shape) adj. Pear-shaped. Also pyriform.

pisaceous (L. pisum: pea) adj. Pea-green.

pisiferous (L. pisum: pea; ferre: to carry) adj. Bearing peas.

pisiform (L. pisum: pea; forma: shape) adj. Pea-shaped.

pistil (L. pistillum: pestle) n. The part of a flower consisting of the female reproductive organs. It consists of one or more carpels (ovary, stigma, and style when present), which may be united into a single structure. Some plants, such as the cucumber, have separate male and female flowers: the latter are described as pistillate. Cf. gynoecium.

pistillate (L. pistillum: pestle) adj. Said of a flower bearing a pistil or pistils but not stamens, may refer also to a plant having only pistillate flowers.

pistillode (L. pistillum: pestle) n. A sterile, rudimentary pistil in an otherwise staminate flower.

pistillody (L. pistillum: pestle; eidô: to look like) n. The metamorphosis of an organ, as a sepal, into a pistil.

pit n. The stony endocarp of a drupe, as in a peach or cherry. The small openings in the walls of tracheids and vessel elements which allow water to move from cell to cell. The equivalent structures in primary cell walls are called primary pit fields, areas where plasmodesmata are concentrated and pit development usually occurs. A pit comprises a pit membrane, consisting of the middle lamella plus the primary wall; and a pit cavity, the depression in the secondary wall. Pits usually occur in pairs (called pit pairs) on either side of the middle lamella between two adjacent cells.

pith n. The soft, spongy parenchyma tissue, in the center of certain plant stems, inside the vascular cylinder. It is light in weight and has been put to various commercial uses, notably the manufacture of pith helmets.

pith helmet A lightweight sun-helmet made from the dried pith of the sola, etc.

pits n. Thin regions of the cell wall in xylem conducting cells. Their structure is an important characteristic for recognizing different kinds of wood. See pit.

pitted adj. Marked with small depressions or pits.

placenta (L. placenta: [flat] cake) n. (pl. placentae) The part of the ovary wall (a ridge of tissue) in flowering plants, to which the ovules arise and are attached. The arrangement of ovules on the placenta (placentation) is variable, depending on the number of carpels and whether they are free (see apocarpy) or fused (see syncarpy). In ferns and related plants, the tissue giving rise to sporangia.

placentate (L. placenta: [flat] cake) adj. Having a placenta.

placentation (L. placenta: [flat] cake) n. The manner in which the placenta, and hence of ovules, are arranged in the ovary. See axile, basal, free central, and parietal placentation.

placentoid adj. Said of an inpushing of parenchymatous tissue into the anther loculus.

placodioid adj. Crustose at the centre but lobed and plicate at the circumference.

placodiomorph n. A 2-celled spore with a thickened septum which may or may not have a pore. Cf. polarilocular.

plagiotropic (Gr. plagios: oblique; tropos: direction) adj. Of growth characterized by horizontal growth; The lateral branches of a tree are often plagiotropic; pin oaks (Quercus palustris) have plagiotropic mid branches; in fact, the lower branches decline at a slight angle, a characteristic that can be used to identify the species.

plagiotropism (Gr. plagios: oblique; tropos: direction) n. Plagiotropic tendency of growth.

plait n. A fold or pleat, as in some corollas.

plane (L. planus: flat) adj. With a flat surface; not curved or wavy, when referring to leaf margins or blade.

plankton n. The collection of small or microscopic organisms, including algae and protozoans, that float or drift in great numbers in fresh or salt water, especially at or near the surface, and serve as food for fish and other larger organisms.

planoconvex See plano-convex.

plano-convex (L. planus: flat; convexus: rounded, curved) adj. Flat on one side and convex on the other. Compressed and flattened. Also planoconvex.

planogamete (Gr. planos: wandering; gametęs: spouse) n. A motile gamete.

plant (L. planta: plant) n. Any member of a kingdom (Plantae) of eukaryotic multicellular organisms containing chlorophyll, (usually) obtaining energy by photosynthesis, typically lacking locomotive movement or obvious sensory organs, generally making its own food, possessing cell walls, and of unlimited growth.

plant hormone See growth substance.

plantlet n. A little plant.

plasmalemma See cell membrane.

plasma membrane See cell membrane.

plasmid n. Autonomously replicating extrachromosomal DNA molecule that contains genetic information that is not essential to the host cell.

plasmodesma (Gr. plasma: something molded; desmę: link) n. (pl. plasmodesmata) Fine cytoplasmic strands that connect the protoplasts of adjacent plant cells by passing through their cell walls. Plasmodesmata are cylindrical in shape (about 20 to 40 nm in diameter) and are lined by the plasma membrane of the two adjacent cells. The endoplasmic reticula of the two adjacent cells are connected by a narrower structure, the desmotubule, which runs through the centre of a plasmodesma. Plasmodesmata tend to occur in groups, forming distinct areas called primary pit fields (see pit).

plasmodesmata See plasmodesma.

plasmodia See plamodium.

plasmodial adj. See plamodium.

plasmodium (Gr. plasma: something molded; eidô: to look like) n. (pl. plamodia) An amoeboid, multinucleate mass or sheet of protoplasm characteristic of some stages of organisms, as a myxomycete or slime mold.

plasmogamy (Gr. plasma: something molded; gamos: marriage) n. The fusion of the protoplast of cells.

plasmolysis (Gr. plasma: something molded; lysis: dissolution) n. The contraction of the protoplasm in a living cell when water is removed by exosmosis.

plasmon n. The genetic elements contained in the cytoplasm.

plaster n. A pastelike mixture applied to a part of the body for healing or cosmetic purposes.

plastic (Gr. plastikos: that may be molded) n. A polymeric material that can be moulded into shape easily. There are two important classes, thermosoftening plastics, e.g. the polyalkenes such as PVC and polythene, which can be melted and reshaped several times, and thermosetting plastics, such as phenol-formaldehyde resins, which can be shaped only once. Plastics were first made in the 1920s.

plastid (Gr. plastęs: modeler, maker) n. An organelle within a plant cell, often occurring in large numbers. Apart from the nucleus, plastids are the largest solid inclusions in a plant cell. For convenience they are classified into those containing pigments (chromoplasts) and those that are colourless (leucoplasts), although changes from one to the other frequently occur. Plastids develop from proplastids, colourless bodies found in meristematic and immature cells; they also arise by division of existing plastids. See also chloroplast.

plastome n. Inheritance of plastids by maternal cytoplasm of the egg.

platanoid adj. Said of a leaf tooth in which the medial secondary vein narrows to the foraminate glandular apex, higher order laterals forming brochidodromous loops that converge on the apex, but do not join it.

platyspermic (Gr. platys: large and flat; sperma: seed) adj. Having seeds which are flattened and disc-like. Contrast with radiospermic.

pleated adj. Of leaves creased along their length.

plectenchyma n. A thick tissue formed by hyphae becoming twisted and fused together, looking like a parenchyma of plants.

plectostele (Gr. plęktron: plectrum; stęlę: stele) n. In a plectostele, the phloem surrounds the xylem; the xylem appears to be dissected into plate-like sectors; an endodermis surrounds the entire stele. See also stele.

pleio- (Gr. pleiôn: more) prefix. Meaning multiple, excessive.

pleiochasia See pleiochasium.

pleiochasium (Gr. pleiôn: more; kazo: separate) n. (pl. pleiochasia) A cymose inflorescence with more than two branches from the main axis.

pleiomery (Gr. pleiôn: more; meros: part) n. The condition of having more than the usual number of floral whorls.

pleiopetalous (Gr. pleiôn: more; petalon: leaf) adj. With many petals.

pleiophylly (Gr. pleiôn: more; phyllon: leaf) adj. An abnormal increase in the number of leaves.

pleiosepalous (Gr. pleiôn: more; skepas: covering, shelter; [pe]talon: leaf) adj. With many sepals.

pleiospermous (Gr. pleiôn: more; sperma: seed) adj. With many seeds.

pleiotaxis See pleiotaxy.

pleiotaxy (Gr. pleiôn: more; taxis: arrangement) adj. An increase in the normal number of parts. Syn. pleiotaxis.

pleiotrophy n. The property of a gene by which it affects two or more characters.

Pleistocene n. A geological era marked by the alternative extension and withdrawal of glaciers and the appearance of human beings. It began some 2.5 million years ago and ended about 10,000 years ago with the end of the last ice age.

pleomorphic adj. See pleomorphism.

pleomorphism (Gr. pleiôn: more; morphę: shape) n. The existence of a plant in two or more distinct forms during the life cycle; polymorphism. Syn. pleomorphy.

pleomorphous adj. See pleomorphism.

pleomorphy See pleomorphism.

pleonanthic adj. Daid of flowering with respect to growth, when an axis that is not determinate by flowering, the inflorescences being axillary.

plesiomorph adj. Primitive (of a character state); the opposite of apomorph.

plesiomorphic adj. Applied to features that are shared by different groups of biological organisms and are inherited from a common ancestor. The term means 'old-featured' and the features to which it is applied were formerly called 'primitive'.

plesiomorphy (Gr. plesios: near; morphę: shape) n. A primitive character state for the taxa under consideration; a primitive trait is inferred to be the original character state of that character within the clade under consideration.

pleurocarpic See pleurocarpous.

pleurocarpous (Gr. pleuron: side; karpos: fruit) adj. Of certain mosses bearing the fructifications along the main stem or lateral branches. Syn. pleurocarpic.

pleurotropous adj. Said of the curvature of an ovule with respect to the ovary axis, lateral.

plica (L. plicare: to fold) n (pl. plicae) A plait or fold.

plicae See plica.

plicate (L. plicatus: folded) adj. Folded into plaits, usually lengthwise; arranged in pleats, as a fan, as in the leaves of palms, cyclanthoids, and some orchids; wrinkled. Syn. plicated.

plicated See plicate.

pliestesial adj. Monocarpic but living several to many years before flowering and fruiting, and then dying, as an Agave. Syn. multiperennial.

Pliocene (Gr. pleiôn: more; kainos: new) n. The last (5.2 to 1.64 million years ago) of the Tertiary epochs, comprising the Zanclian (Tabianian) and Piacenzian Ages.

plococarpia See plococarpium.

plococarpium (Gr. plokos: what is folded or plaited; karpos: fruit) n. (pl. plococarpia) A fruit consisting of follicles around an axis.

plumbeous (L. plumbum: lead) adj. Lead-colored

plumose (L. plumosus: feathered) adj. With hairlike branches, feathery.

plumule (L. plumula: small feather) n. The part of a plant embryo that develops into the shoot system. It consists of the stem apex and first leaves. In seedlings showing epigeal germination the plumule grows above the soil surface together with the cotyledons; in seeds showing hypogeal germination, the plumule alone emerges. Same as epicotyl. Cf. radicle.

pluri- (L. plures: many, more) prefix. Meaning many or several.

pluriaxial (L. plures: many, more; axis: axle) adj. Having flowers that grow from a secondary branch or stem.

pluricellular (L. plures: many, more; cellula: cell [of a monk]) adj. Of many cells.

pluricipital (L. plures: many, more; caput, capitis: head) adj. With many heads, as in a highly branched caudex.

plurilocular (L. plures: many, more; loculus: a cell, box) See pluricellular.

pluriovulate (L. plures: many, more; ovum: egg) adj. With many ovules.

pluriseriate (L. plures: many, more; series: line) adj. In many series or rows.

pneumatophore (Gr. pneuma, pneumatos; phora: carrying) n. An air-vessel; an organ containing aerenchyma; in particular, an aerial negatively geotropic root that acts as an organ of gaseous exchange. Pneumatophores are produced by mangroves and other plants that grow above the substratum, in waterlogged oxygen-deficient soils.

pneumatophorous adj. See pneumatophore.

poculiform (L. poculum: cup; forma: shape) adj. Cup-shaped.

pod n. Any dry, dehiscent (burst open) fruit characteristic of the Leguminosae, or pea family, though other fruit types may be included. Pods usually split down one side to expose a row of seeds, though some, like the peanut, are indehiscent (do not burst open).

podetiform (Gr. pous, podos: foot; L. forma: shape) adj. Sahped like a podetium

podetia See podetium.

podetium (Gr. pous, podos: foot) n. (pl. podetia) In lichens (or algae) a hollow upright structure bearing an apothecium; any stalklike elevation. A lichenised, stem-like portion (stipe, or discopodium) bearing the hymenial discs and sometimes conidiomata in a fruticose apothecium (especially in Cladonia).

podocarp (Gr. pous, podos: foot; karpos: fruit) n. A fruit borne on a gynophore.

podogyne (Gr. pous, podos: foot; gynę: female) n. The elongation of the base of a gynoecium. See carpopodium.

point mutation See mutation.

polarilocular adj. Said of lichen ascospores, two-celled, the two lumina separated by a thick septum through which a narrow canal passes. Cf. placodiomorph.

polarity of characters The states of characters used in a cladistic analysis, either original or derived. Original characters are those acquired by an ancestor deeper in the phylogeny than the most recent common ancestor of the taxa under consideration. Derived characters are those acquired by the most recent common ancestor of the taxa under consideration.

polar nuclei Two haploid nuclei in the centre of the embryo sac of flowering plants. These nuclei fuse with a male gamete nucleus to form a triploid endosperm nucleus, which subsequently divides to form the endosperm. See also double fertilization.

pollen (L. pollen, pollinis: fine flour, mill dust) n. The microspores of seed plants, containing the microgametophyte (the structure that generates the male nucleus, the gamete). Pollen is produced in the microsporangia (anthers of flowering plants), and released when ripe to be carried by wind or animals to the stigma (or female cone in conifers) during pollination. Under appropriate conditions, the pollen grain germinates to produce a pollen tube which grows down the style, carrying the male gamete to the female gamete in the ovule.

pollened adj. See pollen.

pollen grain A microspore of a seed plant, containing the partially developed male gametophyte when it is dispersed, and surrounded by a complex wall made up of sporopollenin that usually has elaborate surface ornamentation.

pollenless adj. See pollen.

pollenlike adj. See pollen.

pollen sac The microsporangium of a seed plant where pollen is produced; the upper portion of the stamen, containing pollen grains; the anther.

pollen tube The slender tube that is emitted by a pollen grain which penetrates and fertilizes the ovule.

pollenate v. See pollinate.

pollenation See pollination.

pollinaria See pollinarium.

pollinarium n. (pl. pollinaria) The complex structure found in flowers of Apocynaceae-Asclepiadoideae and Orchidaceae where pollinia are attached by translators to a central corpusculum. The combination of pollinia, caudicles, stipes and viscidium.

pollinate (L. pollen, pollinis: fine flour, mill dust) v. To transfer pollen from an anther to the stigma of a flower. Also pollenate, polinize.

pollination (L. pollen, pollinis: fine flour, mill dust) n. The transfer of pollen from an anther (the male reproductive organ), where it is formed, to a stigma (the receptive region of the female reproductive) organ, either of the same flower (self-pollination) or of a different flower of the same species (cross-pollination). Cross-pollination involves the action of a pollinating agent to effect transfer of the pollen. Pollination may be accomplished by the use of wind, water, insects, birds, bats, or other means. Pollination is usually followed by fertilization, in which sperm are released from the pollen grain to unite with the egg cell. See anemophily, entomophily, hydrophily, fertilization. incompatibility. Sometimes writtent pollenation.

pollinator n. An insect (or an animal) that carries pollen from one flower to another. See pollination.

pollinia See pollinium.

pollinic adj. See pollen.

pollinical adj. See pollen.

polliniferous (L. pollen, pollinis: fine flour, mill dust; ferre: to carry) adj. Bearing pollen.

pollinium (L. pollen, pollinis: fine flour, mill dust) n. (pl. pollinia) A coherent mass of pollen grains. They are the product of only one anther, but are transferred, during pollination, as a single unit. This is regularly seen in various plants, such as orchids and many species of milkweeds (Asclepiadoideae).

pollinize See pollinate.

polsterform (German polster: carpet; L. forma: shape) adj. Shaped like a low, compact mound.

poly- (Gr. polys: many) prefix. Meaning many or much.

polyad n. Pollen aggregated into units of many grains each.

polyadelphous (Gr. polys: many; adelphę: sister) adj. Borne in several distinct groups, as the stamens of some flowers. Cf. monadelphous and diadelphous.

polyalkene n. A polymer made from monomers that are alkenes. The table shows how similar all of the monomers are, the double bond being characteristically broken in the polymerization process to produce a long-chain alkane-like molecule which is unreactive. This unreactivity of polyalkenes is responsible for concern over their indiscriminate use, as they are nonbiodegradable.

|               Some examples               |
|   Polyalkene      |   Structure of monomer|
|   Polyethylene    |       H2C=CH2         |
|   Polypropylene   |       H2C=CHCH3       |
|   Polystyrene     |       H2C=CHC6H5      |
|   PVC             |       H2C=CHCl        |
|   PTFE            |       F2C=CF2         |

polyamide n. A polymer that contains a substituted amide link; nylon is one of a variety of polyamide fibres made by condensation polymerization.

polyandrous (Gr. polys: many; anęr, andros: male) adj. With many stamens (usually more than ten).

polyandry (Gr. polys: many; anęr, andros: male) adj. The condition of being polyandrous.

polyanthous (Gr. polys: many; anthos: flower) adj. With many flowers, especially when clustered together in an involucre.

polyaploid adj., n. A haploid from a polyploid; i.e., a di-haploid, tri-haploid, or tetra-haploid.

polycarpellary (Gr. polys: many; karpos: fruit) adj. Having or consisting of many carpels. Also polycarpous.

polycarpic See polycarpous.

polycarpous (Gr. polys: many; karpos: fruit) adj. With many carpels; in bryophytes: with two or more sporophytes per perichaetium or perianth, e.g. Ptychomitrium lepidomitrium. Syn. polycarpic. Also polycarpellary.

polycarpy n. See polycarpous.

polycephalous (Gr. polys: many; kephalę: head) adj. With many flower heads.

polychasi See polychasium.

polychasial adj. See polychasium.

polychasium (Gr. polys: many; kazo: move apart, separate) n. (pl. polychasia) A cymose inflorescence in which each axis produces more than two lateral axes.

polychrome (Gr. polys: many; khrôma: color) adj. Many colored. Syn. polychromic.

polychromic See polychrome.

polycotyledon (Gr. polys: many; kotylędon: cavity, hollow) n. A plant having more than two cotyledons, as certain gymnosperms.

polycross n. The open pollination of a group of genotypes (generally selected) in isolation from other compatible genotypes in such a way as to promote random mating inter se.

polycyclic (Gr. polys: many; kyklos: circle) adj. With many whorls.

polyembryonic adj. See polyembryony.

polyembryony n. The dvelopment of more than one embryo from a single egg or ovule.

polyester n. A polymer with repeating units containing ester groups, formed from a dicarboxylic acid reacting with a dihydroxy compound. A good example is Terylene.

polyethylene n. The polymer of ethene, whose systematic name is poly(ethene), which was the first polyalkene to be discovered and remains among the most useful polymers known. It was discovered by chance in 1933 by two ICI chemists (Fawcett and Gibson) when a leak in their apparatus allowed a tiny quantity of oxygen to catalyse the radical polymerization of ethene. The conditions required for the reaction were extreme; producing 1500 atm pressure in particular is very expensive. In 1953 a much cheaper route was found using the Ziegler-Natta catalysts, which also produced a denser material. Later versions of these catalysts were tailored to produce stereospecific polymers (See stereospecific reaction). Syn. polythene.

polygamodioecious See polygamo-dioecious.

polygamo-dioecious adj. Having bisexual and male flowers on some plants, and bisexual and female flowers on others, i.e. mostly dioecious, but with some perfect flowers. The genus Ilex is either dioecious or polygamodioecious.

polygamomonoecious See polygamo-monoecious.

polygamo-monoecious adj. Polygamous but chiefly monoecious, i.e. mostly monoecious, but with some perfect flowers.

polygamous (Gr. polys: many; gamos: marriage) adj. Having some bisexual flowers, some pistillate flowers (with pistils only), and sime staminate flowers (with stamens only) on the same individual plant; in bryophytes, with several forms of gametoecia on the same plant. Cf. heteroicous.

polygene See polygenes.

polygenes n. Genes whose effects are too slight to be identified individually but which, through similar and supplementary effects, can have important effects on total variability.

polygenic adj. Genetically controlled by many genes.

polygynous (Gr. polys: many; gynę: woman) adj. With many pistils or styles.

polygyny (Gr. polys: many; gynę: woman) n. The state or condition of having many pistils or styles.

polymer (Gr. polys: many; meros: part) n. A large molecule constructed from many smaller identical units. These include proeins, nucleic acids, and starches.

polymerization (Gr. polys: many; meros: part) n. An important process in which many (hence 'poly') small molecules called monomers join together to make one gigantic molecule, the polymer. Many different classifications are significant. Chain polymerization produces a polymer of high molar mass almost immediately; the reaction mixture contains only monomer, high molar mass polymer and a few growing chains. Step polymerization produces polymers with a wide range of molar masses, the average molar mass rising steadily with longer reaction times. Thermosoftening plastics can be made by either mechanism whereas thermosetting plastics are necessarily step polymers. Common chain polymers include addition polymers, such as polyalkenes (see table at polyalkene). The most important step polymers are the condensation polymers, such as polyesters and polyamides. Other distinctions include artificial polymers such as nylon and natural polymers such as rubber (See polynucleotide, polysaccharide, protein). The polymers have much larger molar masses than the monomers, typically by factors ranging from thousands to hundreds of thousands.

polymerous (Gr. polys: many; meros: part) adj. With many parts, as in a floral whorl with many members.

polymorphic (Gr. polys: many; morphe: shape) adj. Having, assuming, or occurring in various forms, characters, or styles; variable. Syn. polymorphous.

polymorphism (Gr. polys: many; morphe: shape) n. Existance of a plant in several form or color varieties. The occurrence together in the same population of two or more distinct forms at frequencies too great to be explained by recurrent mutation.

polymorphous See polymorphic.

polynucleotide n. A polymer (long chain) of nucleotides, of which molecules of DNA and RNA are made. See DNA, RNA.

polyoicous See heteroicous.

polyp n. A usually nonmalignant growth or tumor protruding from the mucous lining of an organ such as the nose, bladder, or intestine, often causing obstruction.

polypetalous (Gr. polys: many; petalon: petal) adj. With many separate petals. A corolla of completely separate petals. Syn. apopetalous; Cf. gamopetalous, sympetalous.

polypetaly n. See polypetalous.

polyphyletic (Gr. polys: many; phylę: tribe) adj. The occurrence in taxa of members that have descended via different ancestral lineages. True polyphyletism has traditionally been distinguished from errors of classification, especially at the higher taxonomic levels, where organisms, as a result of convergent or parallel evolution, have been placed wrongly in the same natural group; but modern phyletic taxonomists would hold that any taxon found to be polyphyletic is unnatural, and so an 'error', and must be disbanded. Cf. monophyletic.

polyphyletic group See polyphyly.

polyphyllous adj. Said of a foliose thallus, divided into many lobes.

polyphyly n. A group of organisms with different most recent ancestors. Cf. polyphyletic.

polyploid (Gr. polys: many; eidos: shape) adj. Describing a nucleus that contains more than two sets of chromosomes (see diploid) or a cell or organism containing such nuclei. For example, triploid plants have three sets of chromosomes and tetraploid plants have four. Polyploidy is far more common in plants than in animals; many crops, in particular, are polyploid (bread wheat, for example, is hexaploid, i.e. 6n). It can be induced chemically with colchicine. See also allopolyploid, autopolyploid.

polyploidy n. See polyploid.

polyplotype n. Race or form of plant that differs from another in chromosome number, usually in ploidy. An increasing number of wild species have been ascertained to consist of two to several races that differ in ploidy level and are intersterile. Such species are better termed `species-complexes'.

Polypodiaceae n. A family of the phylum Filicinophyta.

polysaccharide n. A polymer of monosaccharide molecules. Polysaccharides include starch, cellulose, inulin, glycogen and agarose. Their exact structure depends on the nature of the monomer and how the monomers are bonded together.

polysepalous (Gr. polys: many; sepalous: see sepal) adj. Having a calyx made up of separate sepals. Cf. synsepalous, gamosepalous.

polysety n. Producing more than one sporophyte from a single gametoecium, each from a separate archegonium and thus each with its own calyptra. Cf. polyembryony.

polysomaty n. Chromosome condition of a plant such as olive in which 2n = 55 instead of the usual 2n = 2x = 46 for the genus.

polystachyous (Gr. polys: many; stakhys, stakhyos: ear of wheat) adj. With many spikes, as in a grass with many ears or spikes.

polystelic adj. Having more than one stele.

polystemonous (Gr. polys: many; stęmôn: thread) adj. With many stamens (more than twice the number of petals or sepals).

polystichous (Gr. polys: many; stikhos: row) adj. Arranged into several rows.

polystigmous (Gr. polys: many; stigma, stigmatos: prick, spot) adj. With many stigmas.

polystyllous (Gr. polys: many; stylos: column) adj. With many styles.

polysymmetric adj. Symmetrical about more than two planes passing through the axis of the flower.

polythene See polyethylene.

polyterpene See polyterpenes.

polyterpenes n. pl. A terpene with a large number of isoprene units. See terpenes.

polytomic adj. Dividing into many branches usually at one node.

polytrichoid adj. Having the habit or characters of Polytrichum, e.g. with large rigid leaves or a somewhat bristly or hairy calyptra.

polytrichous (Gr. polys: many; thrix, trikhos: hait, bristle) adj. Hairy.

polytypic (Gr. polys: many; typos: image) adj. Describing a species that exists in a variety of forms or subspecies that inhabit different geographical regions. Cf. monotypic.

pome (L. pomum: fruit[with pit or pip]) n. A type of fleshy, indehiscent (false) fruit represented by the apple, pear and related genera, resulting from a compound inferior ovary, in which the receptacle or hypanthium has enlarged to enclose the true fruit. The flesh of the fruit develops from the receptacle of the flower, which completely encloses the fused carpels. After fertilization the carpels form the 'core' of the fruit, which contains the seeds. The characteristic fruit of the apple family, as the apple, the pear, the quince, etc. See also pseudocarp.

pomiferous (L. pomum: fruit [with pit or pip], ferre: to carry) adj. Bearing pomes or pomelike fruits.

pontoperculum n. A distincly delimited and thickened ectexinous or sexinous structure that covers part of an ectoaperture of a pollen grain, but is linked to the general pollen surface at the ends of the aperture.

porandrous (Gr. poros: passage, anęr, andros: male) adj. With anthers opening by pores

porate (Gr. poros: passage) adj. Of a pollen grain, with rounded apertures only. Cf. colporate.

pore (Gr. poros: passage) n. An opening, very small and almost circular; the opening in the wall of some cells.

poricidal (Gr. poros: passage; L. cædes: cutting) adj. A type of dehiscence in which the pollen is released through pores at the tip of the anther. Opening by pores, as in a poppy capsule.

porogamy n. Fertilisation during which the pollen tube penetrates the ovule by way of the micropyle.

pororate adj. Said of a compound aperture in a pollen grain in which both the inner and outer parts are more or less circular in surface view.

porose (Gr. poros: passage) adj. With pores.

porphyra (Gr. porphyra: purple) n. Red seaweed used to make laverbread.

Porphyra (Gr. porphyra: purple) n. A genus (of red or purple alagae) of the phylum Rhodophyta.

porphyrin (Gr. porphyra: purple) n. Any of a group of organic pigments characterized by the possession of a cyclic group of four linked nitrogen-containing rings (a tetrapyrrole nucleus), the nitrogen atoms of which are often coordinated to metal ions. Porphyrins differ in the nature of their side-chain groups. They include the chlorophylls, which contain magnesium; and haem, which contains iron and forms the prosthetic group of haemoglobin, myoglobin, and the cytochromes.

porrect (L. porrectus: spread, stretched) adj. Extended especially forward; projecting; resembling a parrot beak. Stretched out or forth.

posterior (L. posterior: behind, last, posterior) adj. Of floral parts, on the side of the flower nearest to the axis, as the upper lip of a bilabiate corolla; in transversely inserted leaves, the side or surface oriented toward the axis. Cf. anterior.

postgenital adj. Said of of connation or adnation of parts that were initially free from one another.

postical adj. Of the ventral surface of a stem; of the leaf margin oriented towards the base of a longitudinal or obliquely inserted leaf. Cf. proximal, antical.

postpartum n. Period immediately following parturition.

potamocolous adj. Growing in rivers.

potential Hydrogen See pH.

potherb n. A plant whose leaves, stems, or flowers are cooked and eaten or used as seasoning.

pottioid adj. Having lingulate leaves with a strong costa, isodiametric papillose cells above, and typically a sheathing base with cancellinae; as in Tortula, Encalypta and Calymperaceae.

poultice n. A soft moist mass of bread, meal, clay, or other adhesive substance, usually heated, spread on cloth, and applied to warm, moisten, or stimulate an aching or inflamed part of the body. Also called cataplasm.

poulticed See poultice.

praemorse (L. præmorsus: bitten [at the end]) adj. Appearing bitten off at the end, having the end irregularly truncate. Syn. praemorsous, premorse.

praemorsous adj. See praemorse.

pre- (L. præ: devant, en avant) prefix. Meaning before.

Precambrian n. A name now used only informally to describe the Priscoan, Archaean, and Proterozoic, which together comprise the longest period of geologic time that began with the consolidation of the Earth's crust and ended approximately 4000 million years later with the beginning of the Cambrian Period around 570 million years ago. The rocks of this period of geologic time are usually altered and few fossils with hard parts or skeletons have been found within them. Precambrian rocks outcrop extensively in shield areas such as northern Canada and the Baltic Sea.

precocious (L. præcox, præcocis: early-fruiting, early-flowering, precocious) adj. Developing or appearing very early; with the flowers developing before the leaves; with the leaves developing before the flowers.

preformation n. Used to refer to leaves in an innovation that develop from primordia that were evident in the bud that gave rise to that same innovation.

prehensile (L. prehensus: seized, grabbed) adj. Adapted for grasping, as in a tendril.

premorse See praemorse.

prepotency n. The capacity of a parent to impress characteristics on its offspring so they are more alike than usual.

prevernal (L. præ: devant, en avant; vernalis: spring) adj. Pre-spring; flowering in early spring.

prickle n. A hard, pointed outgrowth from the surface of a plant, small, weak, and spine-like, arising from the epidermis, involving several layers of cells but not containing a vein and which may cover the surface of a plant. It contains cortical and vascular tissue and is not regarded as an epidermal outgrowth, usually more slender than a thorn. This is the correct term for rose 'thorns'. Cf. spine, thorn.

primary (L. primarius: the first one [in rank]) adj. First, as the first division of a leaf which is more than once compound.

primary endosperm Haploid gametophytic tissue persisting in the mature seed and forming reserve tissue for the young sporophyte.

primary forest A tract of trees with minimal impact from humans.

primary growth The growth in length, controlled by the apical meristem.

primary leaf The initial leaf (underleaf) on a juvenile plant; it generally differs morphologically from the later produced leaves (underleaves).

primary pinnae The leaflet of first subdivsion of a fern frond or compound leaf.

primary phloem The phloem derived directly from the growth of an apical meristem.

primary production The quantity of new organic matter created by photosynthesis.

primary stem The main stem; in some mosses a long creeping, rhizome-like stem with scattered scale leaves and rhizoids from which secondary stems arise; e.g. the Pterobryaceae.

primary thickening meristem A variant or part of an apical meristem with extensive primary division localised in the cells of a peripheral, mantle-like zone and forming cells in radial series like a lateral meristem.

primary wall The first visible part of the cell wall deposited during extension growth of the cell and made up of cellulose fibrils, hemicelluloses, pectins, etc.

primine (L. primus: the first one, the first part of) adj. The outer integument layer of an ovule. Cf. secundine.

primitive (L. primitivus: the first one [in date], first-born) adj. Describes a character state that is present in the common ancestor of a clade. A primitive character state is inferred to be the original condition of that character within the clade under consideration. For example, 'presence of hair' is a primitive character state for all mammals, whereas the 'hairlessness' of whales is a derived state for one subclade within the Mammalia.

primocane (L. primus: the first one, the first part of; canna: stick, cane, rush) n. The floricane, but in the first year of growth before it is capable of flowering. Cf. floricane.

primordia See primordium.

primordial (L. primordium: origin, beginning) adj., n. Primitive, initial, first formed. A first principle or element. Originally or earliest formed in the growth of an individual or organ; as, a primordial leaf, a primordial cell. Primordial utricle: the interior lining of a young vegetable cell. See primordium.

primordial utricle See primordial.

primordium (L. primordium: origin, beginning) n. (pl. primordia) A group of cells that represents the initial stages in development of a plant organ. Root and shoot primordia are present in a young plant embryo while leaf primordia (or leaf buttresses) are seen as small bulges just below the shoot apex.

Priscoan n. The first of the three subdivisions of the Precambrian, lasting from the formation of the Earth to 4000 million years ago and followed by the Archaean.

prismatic (Gr. prisma: prism) adj. With sharp, definite angles and flat sides, like a prism; brilliant.

pro hybr. See pro hybrida.

pro hybrida Means 'as a hybrid'. Used in citations to show that a plant now known as a species started as a hybrid. Abbreviated 'pro hybr.'

pro- (L. pro, pro-: in front, before, for, instead of) prefix. Meaning in front, before.

probosci See probosci.

proboscidate (L. proboscis: [elephant's] trunk, muffle) adj. Beaked; having a proboscis; similar to corniculate.

proboscides See proboscis.

proboscis n. (pl. proboscides, proboscises, probosci) A specialized leaf or leaf extension consisting mainly of costa, that may bear gemmae, as in Calymperaceae and in Streptopogon. The elongate, protruding mouth parts of certain insects, adapted for sucking or piercing.

proboscises See proboscis.

probract (L. pro: in front; bractea: thin metal plate) n. A small, leaf-like structure at the base of an inflorescence in Cucurbitaceae, usually arising opposite a tendril.

procambia See procambium.

procambium L. pro: before; cambiare: to exchange) n. (pl. procambia) A plant tissue formed by the apical meristems of shoots and roots. It consists of cells elongated parallel to the long axis of the plant. The procambium subsequently gives rise to the primary vascular tissue.

procaryote See prokaryote.

procaryotic See prokaryotic.

process (L. processus: a going forward, progress) n. An outgrowth or appendage

procumbent (L. procumbere: to bend forward, to fall on the earth) adj. Trailing or lying flat but not rooting at the nodes.

production (L. productio: lenthening, prolongation) n. The weight of new organic material formed over a period of time, plus any losses during that time period. Losses may be due to respiration, excretion, secretion, injury, death, or grazing.

productivity (L. productus: led or brought forward) n. Amount of production over a given period of time. Expressed as a rate such as g/m2 per day, kg/ha per year, etc.

pro-embryo See proembryo.

proembryo n. A stage in the early development of the sporophyte between the zygote and the embryo, multicellular, but before the appearance of any organ rudiments.

progesterone n. A C-21 steroid hormone involved in the female menstrual cycle, pregnancy (supports gestation) and embryogenesis of humans and other species. Progesterone belongs to a class of hormones called progestogens, and is the major naturally occurring human progestogen.

projected (L. projectus: thrown forward) adj. Extending outward.

Prokaryotae (Gr. pro: before; karyon: nucleus) n. One of the five kingdom of the taxonomy of organisms.

prokaryote (Gr. pro: before; karyon: nucleus) n. Any organism in which the genetic material is not enclosed in a cell nucleus. Prokaryotes consist exclusively of bacteria, i.e. archaebacteria and eubacteria, which some authorities place together in the kingdom Bacteria (or Prokaryotae); others classify them in separate domains. It is believed that eukaryotic cells (see eukaryote) probably evolved as symbiotic associations of prokaryotes (see endosymbiont theory). Also procaryote.

prokaryotic (Gr. pro: before; karyon: nucleus) adj. Describing a type of cell (a prokaryote) characterized by the lack of a distinct membrane-bound nucleus (2) and organelles such as mitochondria. Prokaryotic cells comprise the kingdom Monera, which includes the bacteria. Cf. eukaryotic. Also procaryotic.

prolate adj. Longer than wide. Cf. oblate.

prolepsis n. The development of an axillary bud only after a period of rest.

proliferous (L. proles, prolis: offspring; ferre: to bear) adj. Bearing supplementary structures such as buds or flowers, either in an abnormal manner or in a manner that is normal but from adventitious tissue. In Conostylis, having erect or spreading, elongating stems which are capable of rooting at the nodes but rarely do so. Bearing plantlets or bulblets, usually from the leaves.

prominent (L. prominens: standing out, protruding) adj. Standing out from the surrounding surface, as raised veins on the surface of a leaf.

prominulous adj. Slightly raised above the adjoining tissue.

promycelia See promycelium.

promycelial adj. See promycelium.

promycelium (Gr. pro: before; mykęs: fungus) n. (pl. promycelia) A short filament produced in the germination of a spore, which bears small spores and then dies.

propagula See propagulum.

propagule See propagulum.

propagulum (L. propago: layer, runner, cutting) n. (pl. propagula) Any structure having the capacity to give rise to a new plant, whether through sexual or asexual (vegetative) reproduction. This includes seeds, spores and any part of the vegetative body capable of independent growth (as runnera or sucker) if detached from the parent. Also propagule. By extension, the term also refers to colonizing individuals that found, outside of their place of birth, a new population.

pro parte Means 'in part'. Used in citations to show that a taxon as used by one author shows only a portion of what was intended by the original author. Abbreviated 'p.p.'

prophase n. The stages in mitosis or meiosis from the appearance of chromosomes to metaphase.

prophyll (Gr. pro: in front, before; phyllon: leaf) n. A leaf formed at the base of a shoot, usually smaller than those formed subsequently. The bracteole under a single flower or pedicel. The showy first bract borne on the peduncle and, occasionally, on the inflorescence branches of some monocots. One of the paired bracteoles subtending the flowers of some Juncus species. Syn. prophyllum.

prophyllate (Gr. pro: in front, before; phyllon: leaf) adj. With prophylls.

prophyllum See prophyll.

prop root Adventitious roots arising from lower nodes and prividing support to a stem.

prorate adj. Having papillae or mammillae borne at the tips of cells, or formed by projecting cell ends, e.g. Philonotis. Cf. scindula, scindulose.

prosenchyma (Gr. pro: in front, before; en; in; khymos: sap, juice) n. The tissue characteristic of the woody and blast portions of plants consisting typically of long, narrow cells with pointed ends; a tissue made up of narrow elongate cells with tapered overlapping end walls. Cf. parenchyma.

prosenchymatous adj. See prosenchyma.

prosoplectenchyma n. Tissue consisting of cells with thickened walls and longish lumina and in which hyphal elements are recognisable as hyphae.

pro sp. See 'pro specie'.

pro specie Means 'as a species'. Used in citations to show that a plant which had been accepted as a species is now designated as a hybrid. Abbreviated 'pro sp.'

prostaglandin n. Any of a group of organic compounds derived from essential fatty acids and causing a range of physiological effects in animals. Prostaglandins have been detected in most body tissues. They act at very low concentrations to cause the contraction of smooth muscle; natural and synthetic prostaglandins are used to induce abortion or labour in humans and domestic animals. Two prostaglandin derivatives have antagonistic effects on blood circulation: thromboxane A2 causes blood clotting while prostacyclin causes blood vessels to dilate. Prostaglandins are also involved in inflammation, being released from affected tissues. See also aspirin.

prostome n. A rudimentary structure outside, and usually adhering to, the main peristome teeth; e.g. the Pterobryaceae. Also preperistome.

prostrate (L. prostratus: lying, thrown down) adj. Growing on the ground, trailing, lying flat on the ground.

protandric See protandrous.

protandrous (Gr. pro: before; anęr, andros: male) adj. Having the male sex organs of a flower (stamens) maturing before the female ones (carpels) thereby ensuring that self-fertilization does not occur; of a flower, shedding the pollen before the stigma is receptive; in bryophytes, applied to monoicous bryophytes that mature and release spermatids prior to the maturation of the archegonia on the same plant. Cf. protogynous, protandry. Also protandric.

protandrously adv. See protandrous.

protandry (Gr. pro: before; anęr, andros: male) n.The condition in which the male reproductive organs (stamens) of a flower mature before the female ones (carpels), thereby ensuring that self-fertilization does not occur. Examples of protandrous flowers are ivy and rosebay willowherb. Cf. protogyny, homogamy. See also dichogamy. Syn. proterandry.

protein (Gr. prôtos: first) n. A class of polypeptide molecule with a wide variety of types and functions. Proteins are important components of animal diets and provide a source of amino acids for building new proteins. Proteins are also important constituents of plasma membranes; all enzymes and some hormones are proteins. Many seeds (e.g. beans) store proteins, making them important constituents of vegetarian diets.

proteinaceous adj. See protein.

proteinic adj. See protein.

proteinous adj. See protein.

proteolysis n. The hydrolytic breakdown of proteins into simpler, soluble substances such as peptides and amino acids, as occurs during digestion.

proteolytic adj. Relating to, characterized by, or promoting proteolysis.

proteolytically adv. See proteolytic.

proterandric See proterandrous.

proterandrous adj. Protandrous. Also proterandric.

proterandrously adv. See proterandrous.

proterandrousness n. See proterandrous.

proterandry See protandry.

proteranthous (Gr. pro: before; anthos: flower) adj. With the flowers developing before the leaves.

proteranthy (Gr. pro: before; anthos: flower) n. When the flowers develop before the leaves.

proterogynous See protogynous.

proterogyny n. See protogyny.

prothalli See prothallus.

prothallia See prothallium.

prothallial adj. See prothallus.

prothallic adj. See prothallus.

prothalline adj. See prothallus.

prothallium n. (pl. prothallia) See prothallus.

prothalloid adj. See prothallus.

prothallus (Gr. pro: before; thallos: young shoot) n. (plural prothalli, prothallia) A small flattened multicellular structure that represents the independent gametophyte generation of clubmosses, horsetails, and ferns. In some of these plants a single prothallus bears both male and female sex organs. In others there are separate male and female prothalli. In lichens, a weft of fungal hyphae (white, reddish or blue-black) at the margin of the thallus, lacking photobiont, often projecting beyond the thallus onto the substratum. Also prothallium.

protist (Gr. prôtos: first) n. A primitive organism of the kingdom Protista, with both plant and animal characteristics.

Protista (Gr. prôtos: first) n. A kingdom that originally comprised all unicellular organisms (including bacteria); it was proposed by the German biologist Ernst Haeckel in 1866. Bacteria were later excluded on the grounds that their cell structure differed fundamentally from that of other organisms. The kingdom is still used in some modern classifications and is usually held to comprise all eukaryotic organisms consisting of single cells or aggregates of similar cells, including the protozoa, unicellular algae (such as diatoms), and slime moulds. Protista is now usually held to comprise unicellular eukaryotes that typically possess a flagellum, a grouping that includes only the protozoans and simple algae and fungi. However, the bacteria and other algae and fungi have sometimes also been included. Cf. Protoctista.

proto- prefix. Meaning first.

proto-Atlantic See Iapetus Ocean

protoctist See protoctistan.

Protoctista (Gr. prôtos: first; ktistęs: founder) n. One of the five major kingdoms of living organisms, including algae, protozoa, and slime moulds, all of which are eukaryotes. Protoctists typically comprise single cells or aggregations of similar cells. Cf. Protista.

protoctistan (Gr. prôtos: first; ktistęs: founder) n. A member of the kingdom Protoctista (Protista), which contains all eukaryotic single-celled organisms, including the group formerly known as Protozoa and the single-celled algae. Syn. protoctist.

protoderm n. The primary meristem of stem or root, located at the apex, giving rise to the epidermis.

protogynous (Gr. prôtos: first; gynę: woman) adj. Having the female sex organs maturing before the male; of a flower, shedding the pollen after the stigma has ceased to be receptive; applied to monoicous bryophytes that produce mature archegonia before maturation and release of spermatids on the same plant. Cf. protandrous. Also proterogynous.

protogyny (Gr. prôtos: first; gynę, female) n. Development and maturation of the female reproductive organs (carpels) of a flower before the appearance of the corresponding male products - thus inhibiting self-fertilization; metandry. Cf. protandry, homogamy, dichogamy. Syn. proterogyny.

protolog n. The original descrption of a species, genus, etc. Also protologue.

protologue See protolog.

protonema (Gr. prôtos: first; nęma: thread) n. (pl. protonemata) A primary, usually filamentous structure produced by the germination of the spore in mosses and certain related plants, and upon which the leafy plant which bears the sexual organs arises as a lateral or terminal shoot. In bryophytes, a filamentous, globose or thalloid structure resulting from spore germination and including all stages of development up to the production of one or more gametophores; the protonema is extremely variable as to the amount of chlorophyll present, the degree of obliqueness of its end walls and the degree to which it branches (Cf. chloronema and caulonema); in liverworts a globose, short thalloid or filamentous structure generally gives rise to a single gametophore; in mosses the protonema is typically filamentous although Sphagnum, Andreaea and Tetraphis have thallose protonemata; in a few taxa two morphologically distinct protonematal phases occur, a primary protonema and a secondary protonema, e.g. Jubula and Andreaea.

protonemal adj. See protonema.

protonematal adj. See protonema.

protonematoid adj. See protonema.

protophloem (Gr. prôtos: first; phloios: inner bark) The part of the primary phloem that develops first, consisting of narrow, thin-walled cells.

protoplasm (Gr. prôtos: first; plasma: something molded) n. An archaic term applied to the fundamental material comprising the structure of cells. It is now more commonly referred to as cytoplasm and nucleoplasm.

protoplast n. The organised living unit of a single cell, both protoplasmic and non-protoplasmic, but excluding the cell wall.

protostele (Gr. prôtos: first; stęlę: stele, standing block) n. When a plant's vascular tissue develops in a solid central bundle, it is said to have a protostele; it is a stele with a solid core of vascular tissue, lacking a pith, forming a solid rod with the phloem surrounding the xylem. It is the solid stele of most roots, having a central core of xylem enclosed by phloem. Protosteles are relatively simple, and have been observed in the oldest fossil plants. Consequently, they represent the first steles. See also stele.

protostelic adj. See protostele.

protoxylem (Gr. prôtos: first; xylon: wood) n. The primary xylem tissue that is formed in the expanding region of roots and shoots before the shoot or root has completed the elongation process. In the secondary walls of these vessels the lignin is deposited in rings and spirals, which allows for further elongation. The protoxylem is succeeded by metaxylem.

protozoa (Gr. prôtos: first; zôon: animal) n. A group of unicellular or acellular, usually microscopic, organisms now classified in various phyla of the kingdom Protoctista (Apicomplexa; Ciliophora; Rhizopoda; Zoomastigota). They were formerly regarded either as a phylum of simple animals or as members of the kingdom Protista. They are very widely distributed in marine, freshwater, and moist terrestrial habitats; most protozoans are saprotrophs, but some are parasites, including the agents causing malaria (Plasmodium) and sleeping sickness (Trypanosoma), and a few contain chlorophyll and carry out photosynthesis, like plants. Protozoan cells may be flexible or rigid, with an outer pellicle or protective test. In some (such as Paramecium and Trypanosoma) undulipodia (cilia or flagella) are present for locomotion; others (such as Amoeba) have pseudopodia for movement and food capture. Contractile vacuoles occur in freshwater protozoans. Reproduction is usually asexual, by binary fission, but some protozoans undergo a form of sexual reproduction. See Protozoa.

Protozoa (Gr. prôtos: first; zôon: animal) n. Formerly the phylum containing single-celled eukaryotic heterotrophs. These organisms are now included in the protoctistans.

protozoal (Gr. prôtos: first; zôon: animal) adj. Of, pertaining to, or characteristic of a protozoan.

protozoan (Gr. prôtos: first; zôon: animal) n. A simple unicellular organism. Protozoa literally means 'first animals', and it was probably from such life-forms that the animal kingdom evolved. Most living protozoans, which are a vast, diverse assemblage of one-celled non-photosynthetic organisms, are very far from simple, being themselves the product of lengthy evolution. There are at least 50,000 species of protozoans, which together with algae and the slime moulds are included in the kingdom Protoctista. Protozoans are incredibly diverse in habits. Flagellates, radiolarians, and foraminifera are abundant in plankton, while amoebas and ciliates (e.g. Paramecium) occur in fresh water. Many protozoans are parasitic (causing dysentery, sleeping sickness, and malaria), or symbiotic. In fact, examples of protozoa can be found eating almost anything, which reflects their amazing adaptability.

protuberance (L. protuberare: to swell, to protrude) n. A rounded bulge, swelling, or projection.

provascular tissue See procambium.

proximal (L. proximus: nearest) adj. Denoting the part of an organ that is nearest to the organ's point of attachment. For example, the knuckles are at the proximal end of the fingers. Cf. distal.

proximally adv. See proximal.

proximate adj. Very near or next, as in space, time, or order.

pruina n. A woolly white covering of the upper cortex of some lichens.

pruinate See pruinose.

pruinose (L. pruinosus: frost covered) adj. Having a whitish, waxy, powdery bloom on the surface; frosted in appearance. Also pruinate.

pruniform (L. prunum, pruni: plum; forma: shape) adj. Plum-shaped.

prurient (L. prurire: to itch) adj. Causing itching.

pruritus n. Severe itching, often of undamaged skin.

psammophyte (Gr. psammos: sand; phyton: plant) n. A plant growing in sand or in snady soil.

pseud- See pseudo-.

pseudanthia Seepseudanthium.

pseudanthium (Gr. pseudęs: false; anthos: flower) n. (pl. pseudanthia) A compact inflorescence of many small flowers which simulates a single flower.

pseudautoicous adj. With dwarf male plants epiphytic on the female.

pseudo- (Gr. pseudęs: false) prefix. False, pretended, close or deceptive resemblance to; apparent but not genuine.

pseudobulb (Gr. pseudęs: false; L. bulbus: bulb) n. A bulbous thickening on the stems of many epiphytic orchids.

pseudocarp (Gr. pseudęs: false; karpos: fruit) n. A fruit that incorporates, in addition to the ovary wall, other parts of the flower, such as the receptacle. For example, the fleshy part of the strawberry is formed from the receptacle and the 'pips' on the surface are the true fruits; apples, pineappel, etc., are pseudocarps. See also composite fruit, pome, sorosis, syconus.

pseudocyphella n. (pl. pseudocyphellae) A tiny white dot or pore seen in large numbers on the upper and sometimes the lower thallus surfaces of many foliose lichen species, and causes by a break in the cortex.

pseudocyphellae See pseudocyphella.

pseudodichotomous adj. With a false dichotomy, branching in which two more or less equal stems or thalli are produced, but not by the equal division of an apical cell. Cf. dichotomous. bifurcate.

pseudoelater (Gr. pseudęs: false; elatęr: driver) n. Moisture-sensitive cells produced in the sporangium of hornworts.

pseudoextinction (Gr. pseudęs: false; L. exstinctio: extinction) n. The apparent disappearance of a taxon. In cases of pseudoextinction, this disappearance is not due to the death of all members, but the evolution of novel features in one or more lineages, so that the new clades are not recognized as belonging to the paraphyletic ancestral group, whose members have ceased to exist. The Dinosauria, if defined so as to exclude the birds, is an example of a group that has undergone pseudoextinction.

pseudofasciculate (Gr. pseudęs: false; L. fasciculus: small bundle) adj. Closely clustered, but not actually joined into a bundle.

pseudohypha n. Hypha-like structure formed by budding yeasts, totally separated by septa without cytoplasmic exchange.

pseudoisidia See pseudoisidium.

pseudoisidium n. (pl. pseudoisidia) An elongate soredium which has become partially corticate and resembles an isidium but has the same origin as a soredium.

pseudolamina (Gr. pseudęs: false; L. lamina: plate) n. The extended apical portion of a phyllode.

pseudomonomerous (Gr. pseudęs: false; monos: single; meros: part) adj. A structure which appears to be simple, though actually derived from the fusion of separate structures, as a pistil which appears to be composed of a single carpel, though actually composed of two or more carpels.

pseudoparaphyllia See pseudoparaphyllium.

pseudoparaphyllium n. (pl. pseudoparaphyllia) Small, unistratose, filiform or foliose structure resembling paraphyllium, but restricted to the areas of the stem around branch primordia and often found in pleurocarpous mosses.

pseudoparenchyma n. A tissue resembling parenchyma but developmentally filamentous. See also plectenchyma.

pseudoperianth Tissue of thalline origin surrounding the archegonial cluster, calyptra and subsequent sporophyte, as in Pallavicinia, Marchantia.

pseudopod (Gr. pseudęs: false; pous, podos: foot) n. A temporary projection of the cytoplasm of certain cells, such as phagocytes, or of certain unicellular organisms, especially amoebas, that serves in locomotion and phagocytosis. Also pseudopodium.

pseudopodal adj. See pseudopod.

pseudopodetia See pseudopodetium.

pseudopodetium n. (pl. pseudopodetia) An upright, fruticose thallus of some lichens.

pseudopodia See pseudopodium.

pseudopodial adj. See pseudopod.

pseudopodium (Gr. pseudęs: false; pous, podos: foot) n. (pl. pseudopodia) An elongation of the gametophyte axis below the sporophyte in Sphagnum and Andreaea, serving the function of a seta; also applied to a similar extension of a stem tip bearing clusters of gemmae. A temporary projection from the cell of a protozoan, leucocyte, etc., used for feeding and locomotion. A pseudopod. Cf. proboscis. Also pseudopod.

pseudopore (Gr. pseudęs: false; poros: passage) n. A pore-like structures with a thin membrane that may be revealed by staining, as found in hyalocysts of Calymperaceae.

pseudoscape (Gr. pseudęs: false; L. scapus, shaft [of a pillar], stalk [of a plant]) n. A false scape, where not all of the leaves are truly basal in origin though, superficially, they appear to be so.

pseudostem n. The stemlike structure of grass plants prior to elongation. The pseudostem is composed of leaf sheathes and leaf blades rolled around each other. Pseudostem is also used to refer to the growth stage in winter wheat after vernalization when the plant begins to grow more erect but prior to stem elongation.

pseudostoma n. (pl. pseudostomata) In Sphagnum capsules, a vestigial stomata, consisting of two guard cells but no pore.

pseudothallose adj. Pertaining to a false thallus as a leafy liverwort gametophyte that resembles a thalloid form; e.g. Metzgeriopsis.

pseudoverticillate (Gr. pseudęs: false; L. verticillus: whorl) adj. Not actually whorled, but appearing so; where spirally-arranged leaves are congested at the end of each innovation and appear to form a whorl.

pseudovivipary (Gr. pseudęs: false; L. vivus: alive; parere: to bear) n. A condition where vegetative propagules replace some or all of the normal sexual flowers in the inflorescence

psilate adj. Said of of the surface of pollen, lacking sculpturing.

Psilophyta (Gr. psilos: bare; phyton: plant) n. A phylum of primitive tracheophyte plants that contains two extant genera - Psilotum (whisk ferns) and Tmesipteris - as well as numerous extinct forms that flourished in the Devonian period. Psilophytes have rhizoids rather than roots; the stems, which show dichotomous branching, may be naked or bear scalelike leaves.

Psilotaceae n. A family of the phylum Filicinophyta.

psoriasis n. A chronic, non-contagious disease characterized by inflamed lesions covered with silvery-white scabs of dead skin.

pterate (Gr. pteron: wing) adj. Winged, e.g. dipterate, 2-winged, tripterate, 3-winged.

Pteridaceae (Gr. pteris: kind of fern) n. A family of the phylum Filicinophyta.

pteridology (Gr. pteris: kind of fern; logos: word) The branch of botany dealing with ferns ans related plants, as horsetails, clubmosses, etc.

Pteridophyta (Gr. pteris: kind of fern; phyton: plant) n. In traditional classification systems, a division of the plant kingdom that included ferns, horsetails, and clubmosses, i.e. the nonseed-bearing tracheophytes. These are now classified as separate phyla: Filicinophyta (ferns), Sphenophyta (horsetails), Lycophyta (clubmosses), and Psilophyta.

pteridophyte See Pteridophytina, Pteridophyte.

Pteridophyte (Gr. pteris: kind of fern; phyton, plant) n. A major division of the plant kingdom, having clear alternation of generations with a dominant vascular sporophyte initially dependent upon the gametophyte which is very reduced. See Filicinophyta. Plant in which the sporophyte generation is the larger phase and in which the gametophyte lives an existence independent of its parent sporophyte. Pteridophytes are almost all vascular plants, and include the lycophytes, trimerophytes, sphenophytes, and ferns.

Pteridophytina (Gr. pteris: kind of fern; phyton, plant) n. Subdivision of the plant kingdom, comprising the classes Lycopsida (club mosses), Sphenopsida (horsetails), Psilopsida (but see Psilophytales), and Pteropsida (the various families of ferns). They first enter the fossil record in the Silurian. They are flowerless plants exhibiting an alternation of two distinct and dissimilar generations. The first is a non-sexual, spore-bearing, sporophyte generation. It usually appears as a relatively large plant, with stems containing vascular tissue that conducts water and dissolved solutes through the plant, and usually bears the leaves and roots. Spores are produced in sporangia that are either attached to the leaves (as in ferns) or are on specialized scales (sporophylls) grouped into cones (as in horsetails and club mosses), or in the axils of leaves on unspecialized stems (as in some club mosses). The second is a sexual, gametophyte generation, in which the plants generally are relatively small, and without differentiation of stem, leaves, or roots. These plants bear male (antheridia) and female (archegonia) sex organs, together or on separate plants. When the eggs in the archegonia are fertilized by sperms from the antheridia, an embryo results; this can grow into a new sporophyte generation.

pteridosperm (Gr. pteris: kind of fern; sperma: seed) n. An extinct group of seed plants which bore fern-like leaves.

ptero- prefix meaning winged.

pterocarpous (Gr. pteron: wing; karpos: fruit) adj. With winged fruits.

pterocaulous (Gr. pteron: wing; kaulos: stalk) adj. With winged stems.

Pterophyta (Gr. pteris: kind of fern; phyton, plant) See Filicinophyta.

Pteropsida n. In older classifications, a subdivision of tracheophytes that contained the ferns and seed plants, or a class of the Pteridophyta containing only the ferns. Syn. Filicinophyta. Class of the Pteridophytina, which comprises all living and extinct ferns. They arose in the Devonian from the trimerophyte group of psilophytes (which had developed more elaborate systems of branching, that led on to the formation of true leaves) and made an important contribution to Carboniferous floras. They are the most advanced, numerous, and varied of the pteridophytes. In most cases they have relatively large, much-divided leaves, and are still significant components of many different plant communities around the globe. Syn. Filicopsida.

pterospermous (Gr. pteron: wing; sperma: seed) adj. With winged seeds.

pteroylglutamic acid See folic acid.

pterygodont n. A type of nematodontous peristome found in the Polytrichales that has a longitudinal wing-like crest or flange on the inner surface.

ptyxis n. The pattern of folding and rolling of an individual leaf during early development.

puberulence (L. pubes, puberis: hair) n. Of having fine, short hairs

puberulent (L. pubes, puberis: hair) adj. Covered with fine, short, and nearly imperceptable down; minutely pubescent, the hairs soft, straight, erect, but scarcely visible to the unaided eye. Also puberulose, puberulous.

puberulose See puberulous.

puberulous adj. Covered with minute, soft, erect hairs. Also puberulose, puberulent.

pubescence (L. pubescere: to become covered with fine hair) n. Hairiness, short, soft down on the leaves and stems of plants.

pubescent (L. pubescere: to become covered with fine hair) adj. A general term for hairiness; covered with soft hair or down; covered

pugioniform adj. dagger-shaped.

pulse n. The edible seeds of certain pod-bearing plants, such as peas and beans. A plant yielding these seeds.

pulveraceous See pulverulent.

pulverulence n. See pulverulent.

pulverulent (L. pulvis, pulveris: dust) adj. Appearing dusty or powdery. Syn. pulveraceous.

pulverulently adv. See pulverulent.

pulvinate (L. pulvinus: cushion, pillow) adj. Cushion-like or mat-like; convex or slightly so; having a swelling at the base; used of a leafstalk. Syn. pulviniform.

pulvini See pulvinus.

pulviniform (L. pulvinus: cushion, pillow; forma: shape) See pulvinate.

pulvinule (diminutive of L. pulvinus: cushion, pillow) n. A small pulvinus at the base of a petiolule.

pulvinus (L. pulvinus: cushion, pillow) n. (pl. pulvini) A swelling or enlargement due to a group of cells at the base of a leaf or leaflet, or at the the base of a petiole or petiolule, in certain plants that, by rapidly losing water, brings about changes in the position of the leaves. In the sensitive plant (Mimosa pudica), the pulvinus is responsible for the folding of the leaves that occurs at nightfall or when the plant is touched or injured.

punctate (L. punctum: dot, sting) adj. Dotted with pits or with translucent, sunken glands or with colored dots.

puncticulate (dinimutive of L. punctum: dot, sting) adj. Minutely punctate. Minutely dotted.

punctiform adj. Dot-like.

punctuated equilibrium The theory, first proposed in 1972 by Niles Eldredge and Stephen Jay Gould, that evolution is characterized by geologically long periods of stability during which little speciation occurs, punctuated by short periods of rapid change, species undergoing most of their morphological changes shortly after breaking from their parent species. A model of evolution in which change occurs in relatively rapid bursts, followed by longer periods of stasis.

punctulate adj. Minutely dotted.

pungent (L. pungere: to sting) adj. Ending in a stiff, sharp point; having an acrid taste or smell.

puniceous (L. puniceus: blood-red, purple) adj. Crimson colored.

pupa n. (pl. pupae or pupas) The nonfeeding stage between the larva and adult in the metamorphosis of holometabolous insects, during which the larva typically undergoes complete transformation within a protective cocoon or hardened case. The third stage in the transformation of the bee.

pupal adj. See pupa

pupate v. To become a pupa. To go through a pupal stage.

purgative adj., n. Tending to cleanse or purge, especially causing evacuation of the bowels. A purgative agent or medicine; a cathartic.

purpurescent (L. purpura: the purple color) adj. Becoming purplish.

pustular See pustulose

pustulate See pustulose.

pustulate hair Hair with an enlarged base.

pustulate-isidiate adj. With isidia which become pustulate.

pustulate-sorediate adj. Developing pustules which become sorediate.

pustule (L. pustula: blister, ampulla) n. Small blister-like elevations; a pimple or blister-like swelling, hollow within, often eroding.

pustuliferous (L. pustula: blister, ampulla; ferre: to bear) See pustulose.

pustulose adj. With small blisters or pustules, often at the base of a hair. Syn. pustuliferous.

putamen (L. putamen, putaminis: what is trimmed [as useless]) n. (pl. putamina) The hard stony endocarp of some fruits. A nut shell.

putamina See putamen.

putaminous asd. See putamen.

pycnia See pycnium.

pycnial adj. See pycnium.

pycnidia See pycnidium.

pycnidial adj. See pycnidium.

pycnidium (Gr. pyknos: dense, close, frequent) n. (pl. pycnidia) In certains ascomycetes and other fungi, a globose or flask-shaped fruiting body bearing conidia on conidiophores. In lichens, a small, globose or flask-like conidioma in which conidia develop.

pycniospore (Gr. pyknos: dense, close, frequent; spora: seed) n. The spore produce in a pycnium.

pycnium (Gr. pyknos: dense, close, frequent) n. (pl. pycnia) A flask-shaped or conical sporangium of a rust fungus which develops below the epidermis of the host and bears pycniospore.

pycnoxylic (Gr. pyknos: frequent; xylon: wood) adj. Wood in which there is little or no parenchyma tissue among the xylem is called pycnoxylic, i.e. the xylem is frequent and the parenchyma rare. Conifers and flowering plants have pycnoxylic wood. Contrast with mannoxylic.

pyrene (Gr. pyręn: stone, pit) n. The stone or pit (endocarp plus seed) of a succulent fruit; a nutlet. Cf. berry, drupe.

pyrenocarp (Gr. pyręn: stone, pit; karpos: fruit) n. A perithecium; a drupe.

pyrenocarpous adj. Having a perithecioid ascoma, e.g. Verrucaria.

pyrenoid (Gr. pyręn: stone, pit; eidô: to look like) n. A spherical protein body found in the chloroplasts of many algae and the hornworts (see Anthocerophyta). Pyrenoids are associated with the storage of starch: layers of starch are often found around them; they are found in the chloroplast of hornworts and numerous algae.

pyriform (L. pirus, pyrum, pyrus: pear; forma: shape) adj. Pear-shaped; e.g the capsules of Bryum.

pyxidate (Gr. pyxis: box) adj. With a pyxis; more elongated than turbinate.

pyxidia See pyxidium.

pyxidium (Gr. pyxis: box) n. (pl. pyxidia) See pyxis.

pyxis (Gr. pyxis: box) n. (pl. pyxides) A circumscissile capsule, the top coming off as a lid; i.e. capsule which opens in a circumscissile manner. Also pyxidium.