dactyl n, In lichens, a hollow, nodular to cylindrical protuberance, somewhat resembling a swollen isidium, bounded by a cortex, often opening at the apex to expose the medulla.

dactyloid adj. Finger-like. Also dactylose.

dactylose See dactyloid.

daedaleous adj. Truncate, but raggedly so.

dale n. a vale or valley.

dark reaction Those chemical reactions in photosynthesis not requiring the presence of light and involving the reduction of carbon dioxide to form sugar. Cf. Calvin cycle.

darwinism n. The theory of evolution as deduced by Charles Robert Darwin (1809 - 1882) and outlined in his book On the Origin of Species (1859). The essence of Darwinism is that evolution is driven by natural selection, a process in which environmental factors favour the survival of organisms that possess certain advantageous features. If such organisms survive long enough to breed, they pass on the genes that control these advantageous features to their offspring. Darwin did not cast his theory in terms of genetics, the precise mechanisms of which were not elucidated until after his death. The modern synthesis of natural selection and genetic processes is termed neo-Darwinism.

dasyphyllous (Gr. dasys: hairy; phyllon: leaf) adj. With hairy or woolly leaves.

Davisian cycle Orderly series of stages through which land-forms were believed to pass from their initiation following uplift to their final planation by erosion. The main stages were those of youth, when hillslopes were steep and river profiles irregular; of maturity, when river profiles were smoothly concave-up and incision had markedly slowed; and of old age, when the landscape was reduced to a gently undulating surface or peneplain. This framework for land-form studies has now largely dropped out of use.

day-neutral plant See day neutral plants.

day neutral plants Plants that flower regardless of day length.

dbh An abbreviation for the phrase 'diameter at breast height', a measurement of the thickness of a tree trunk about four feet from the ground.

DBH See dbh.

DDT 'dichloro-diphenyl-trichlorethane'. An insecticide extensively used during the period after World War II that is now outlawed in the united States and most of the industrialized world.

dealbate adj. With white powder on the surface; whitish.

deca- (Gr. deka: ten) prefix. Meaning ten.

decamerous (Gr. deka: ten; meros: part) adj. With parts arranged in sets or multiples of ten.

decandrous (Gr. deka: ten; anęr, andros: male) adj. With ten stamens.

decantherous (Gr. deka: ten; anthęros: flowered) adj. With ten anthers.

decapetalous (Gr. deka: ten; petalon: leaf) adj. With ten petals.

decaphyllous (Gr. deka: ten; phyllon: leaf) adj. Ten-leaved.

decaploid adj, n. Having ten sets of chromosomes.

decarboxylated iridoids A kind of route II iridoid derived from epi-deoxyloganic acid via decarboxylation at the Cll position.

decasepalous (Gr. deka: ten; skepas: covering, shelter; [pe]talon: leaf) adj. With ten sepals.

decaspermal See decaspermous.

decaspermous (Gr. deka: ten; sperma: seed) adj. With ten seeds. Syn. decaspermal.

decidulignosa n. Communities of deciduous trees and shrubs.

deciduous (L. deciduus: falling down) adj. Describing plants in which all the leaves are shed at the end of each growing season, usually the autumn in temperate regions or at the beginning of a dry season in the tropics. This seasonal leaf fall helps the plant retain water that would otherwise be lost by transpiration from the leaves. The majority of temperate flowering trees and shrubs (such as oaks and roses) are deciduous, as are a few conifers such as the larch. Cf. caducous, fugacious, evergreen.

declinate (L. declinatus: bent, inclined) adj. Bent, curved downwards or forwards. Syn. declined.

declined See declinate.

decolorate adj. Discolored, colorless.

decomposer n. Any of various organisms, usually a bacterium or fungus, that breaks down organic substances making the nutrients available again.

decompound (L. de: prefix that indicates intensity...: componere: to put together) adj. More than once compound; divided into coumpound divisions.

decorticate adj. Lacking a cortex.

decreaser n. A species that gradually disappears under continued grazing. Cf. increaser, population density, cover.

decrescent adj. Becoming gradually less.

decumbence See decumbent.

decumbency See decumbent.

decumbent (L. decumbere: to lie down) adj. Spreading horizontally but then growing upwards; trailing on the ground and rising at the tip, as some stems; reclining, but with the tip ascending.

decumbently See decumbent.

decurrence n. Runnind down. See decurrent.

decurrency n. Runnind down. See decurrent.

decurrent (L. decurrere: to run down) adj. Extending downwards beyond the point of insertion, e.g. of a lamina extending downwards to form a flange along the petiole; applied usually to leaves in which the blade is apparently prolonged downward as two wings along the petiole or along the stem; with basal leaf margins extending down the stem past the leaf insertion as ridges or narrow wings; e.g., Coleochaetium, Morinia, Scapania, Plagiothecium.

decurrently See decurrent.

decurved (L. de: from top to bottom; curcus: curved) adj. Curved downwards.

decussate (L. decussare: to cross in a X shape) adj. Arranged along the stem in pairs, e.g. pairs of opposite leaves, etc., with successive pairs borne at right angle to each other.

decussate branching A type of branching in which the appendages are attached in pairs either alternately (alternate decussate) or at right angles (opposite decussate) to one another; the result are four ranks or rows of appendages.

decussately See decussate.

decussation See decussate.

dedifferentiation n. The reversion of a mature, or differentiated cell to an undifferentiated state with resumption of meristematic activity.

deficiency n. The absence or deletion of a segment of chromosome.

definite (L. definite: in a distinct, precise way) adj. Of a constant number; of stamens, twice as many as the petals or sepals, or less; of an inflorescence, ending in a flower or an aborted floral bud. Syn. determinate.

definiteness See definite.

deflected adj. Bent downward at a sharp angle.

deflexed (L. deflexus: bent down adj. Bent abruptly downwards or outwards. Deflected. (Of leaves, petals, etc.) bent sharply outwards and downwards. Cf. inflexed.

deflorate (L. defloratus: withered) adj. Past the flowering time.

defoliate adj. Denuded of leaves; with leaves removed or lost.

defoliation (L. defoliare: to strip of its leaves) n. The shedding of leaves.

dehisce (L. dehiscere: to open, gape, crack) v. To split apart and discharge seeds or spores; to split along a natural line, to discharge contents by so splitting.

dehiscence (L. dehiscere: to open, gape, crack) n. The spontaneous and often violent opening of a fruit, seed pod, or anther to release and disperse the seeds or pollen. Examples are the splitting of laburnum pods and primrose capsules; such structures are described as dehiscent. cf indehiscent.

dehiscent (L. dehiscere: to open, gape, crack) adj. Breaking open at maturity to release the contents; the term is commonly applied to anthers or fruits (seed pods). Cf. indehiscent.

dehiscing See dehisce.

deliquesce (L. deliquescere: to melt, to become liquid) v. To branch into many subdivisions and lacking a main axis. To liquefy or become soft with age, as some fungi.

deliquescence See deliquesce.

deliquescent (L. deliquescere: to melt, to become liquid) adj. With an irregular pattern of branching without a well defined central axis from bottom to top. Becoming soft with age. See deliquesce.

delphinidin n. An anthocyanin.

deltate (Gr. the letter delta) adj. Broadly triangular with an obtuse apex. See deltoid.

deltoid (Gr. the letter delta; eidô: to ressemble) adj. Shaped like the Greek letter delta; triangular in outline with the sides of about equal length, i.e. like an equilateral triangle; a low triangle attached at the middle of the wide part, rather than at the point; e.g. the stipe leaves of Hypnodendron dendroides. Syn. deltoidal.

deltoidal See deltoid.

dematiaceous adj. Fungi with brown or black pigment in the cell wall, thus appearing brown or black micro- and macroscopically.

deme n. One or more populations of a taxon; a population that will interbreed.

demersed adj. Constantly under water.

Demospongiae n. A class of sponges that first appears in the Cambrian. The soft tissue of the skeleton is supported by siliceous spicules, each spicule consisting of either a single ray (monaxon) or four rays (tetraxon) diverging at 60° or 120°. Most fossil species are represented by their spicules only, but they can usually be referred to modern families. Phylum Porifera.

demulcent adj. Serving to soothe or soften. n. A soothing, usually mucilaginous or oily substance, such as glycerin or lanolin, used especially to relieve pain in inflamed or irritated mucous membranes. See demulcents.

demulcents n. Herbal medicines that can break up phlegm or other mucus; sometimes used externally to sooth and soften skin.

dendriform (Gr. dendron: tree; L. forma: shape) adj. With a tree-like form. Syn. dendroid.

dendritic (Gr. dendron: tree) adj. with a branching pattern similar to that in a tree, as in some hairs in the Cruciferae (Brassicaceae).

dendrochronology n. The science of dating events and cyclic weather patterns by studying growth rings in trees and lumber.

dendroid (Gr. dendron: tree; eidô: to look like) adj. Tee-like, in shape but not in form; branched above a distinct trunk-like stipe; e.g. Climacium, Pterobryon, Hypnodendron. Cf. frondose. See dendriform.

dendrology (Gr. dendron: tree; logos: word) n. The scientific study of trees.

dendrophilous adj. Tree-loving; dwelling on or among trees.

denitrification n. The loss or removal of nitrogen or nitrogen compounds; specifically, a reduction of nitrates or nitrites caused by bacteria that usually results in the escape of nitrogen into the air.

dense adj. Congested, describing the disposition of flowers in an inflorescence (cf. open).

Dennstaedtiaceae n. A family of the phylum Filicinophyta.

density-independent factor An influence that is effective regardless of density of a population, like the influence of a drought. Some scientists dispute the existence of such a factor.

dentate (L. dentatus: with teeth) adj. Toothed, with large saw-like teeth on the margin pointing outward, not forward.

dentation n. The teeth along the margin of an organ.

denticle (L. denticulus: little tooth) n. A small tooth or toothlike projecting point.

denticulate (L. denticulus: little tooth) adj. Having small teeth; finely dentate, finely toothed.

dentiform (L. dens, dentis: tooth; forma: shape) adj. Tooth-shaped. Syn. dentoid.

dentoid (L. dens, dentis: tooth; Gr. eidô: to look like) See dentiform.

denudate (L. denudatus: denuded) adj. Stripped bare; denuded.

denuded (L. denudatus: denuded) adj. Stripped of leaves, made bare, or left naked; e.g. the bare costa left by erosion of lamina or stem by erosion or loss of leaves. Cf. defoliate. Also denudate.

deoperculate adj. Applies to a capsule after its lid has detached, has fallen.

deoxyanthocyanin n. A red-colored flavonoid, anthocyanin have no oxygen in the 3 position.

deoxyloganic acid The precursor of iridoids.

deoxyribonucleic acid See DNA.

depauperate (L. de: prefix that indicates intensity; pauperatus: impauverished) adj. Stunted or poorly developed, usually due to adverse environmental conditions.

dependence n. An association between organisms in which the dependent one receives benefits but gives no benefit to the other, characteristic of such plants as mistletoe, Phoradendron, living on an oak, Quercus; not reciprocal. Cf. symbiosis, competition, coaction, parasite.

dependent adj. Hanging downward because of excess weight of flowers or fruit.

deplanate adj. Flat.

depressed (L. depressus: pressed from top to bottom) adj. Flattened (as viewed form above) as if pressed down from the top or end. Cf. compressed, complanate.

depurative adj., n. Purifying the blood or the humors; depuratory. A depurative remedy or agent; or a disease which is believed to be depurative.

derived (L. derivare: to divert) adj. Describes a character state that is present in one or more subclades, but not all, of a clade under consideration. A derived character state is inferred to be a modified version of the primitive condition of that character, and to have arisen later in the evolution of the clade. 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.

derived trait Same as apomorphy.

dermal (Gr. derma: skin) adj. Of or pertaining to the epidermis.

dermatitis n. Inflamation of the skin, that becomes itchy and may develop blisters.

dermatogen (Gr. derma, dermatos: skin; gennaô: to produce) n. A thin layer of meristem in embryos and growing ends of stems and roots, which gives rise to the epidermis.

dermatogenic adj. That has a deleterious effect on the skin.

Derryan See Atokan.

descending (L. descendere: to go down) adj. Directed downward at a moderate angle.

deserta n. Various types of plants found in areas that are low in available water like arid deserts, saline deserts, permafrost deserts, tundra, strand vegetation, and rocky mountainsides.

Desmoinesian n. A series in the Pennsylvanian of N. America, underlain by the Atokan, followed by the Missourian, and roughly contemporaneous with the Podolskian and Myachkovskian Stages of the Moscovian Series.

determinate (L. determinare: to bound, limit) adj. Of growth or branching, with a bud or flower terminating the growth of the main axis; describes an inflorescence in which the terminal flower blooms first, halting further elongation of the main axis. Having a distinct, defined form. See definite.

determinate growth A type of growth in which the axis ceases growing, usually after the apical meristem differentiates into a reproductive organ, such as a flower or a cone.

detoxify v. To counteract or destroy the toxic properties of. To remove the effects of poison from. To treat (an individual) for alcohol or drug dependence, usually under a medically supervised program designed to rid the body of intoxicating or addictive substances.

Deuteromycota (Gr. deuterios: of ower quality, secondary; mykęs, mykętos: fungi) n. A taxon, usually given the rank of phylum, that is used in some classifications to include all fungi in which sexual reproduction is apparently absent. These fungi, described as 'imperfect fungi' or 'Fungi Imperfecti', are usually regarded as ascomycotes or basidiomycotes that have lost the ability to produce asci or basidia, respectively, and in some cases it may be possible to identify close sexual relatives, or even sexual stages of the same species, in either of these groups. For example, Penicillium, traditionally classified as a deuteromycote, is now known to have a sexual stage in the form of the ascomycote fungus Talaromyces.

deutzioside n. A route I secoiridoid.

Devonian n. The fourth of the six periods of the Palaeozoic Era and the first of the Upper Palaeozoic Sub-era. It began about 408.5 million ago and ended about 362.5 Ma ago. In Europe there are both marine and continental facies present, the latter being commonly known as the Old Red Sandstone. Although originally described from the type area in Devon, the marine Devonian is subdivided stratigraphically into stages established in the exceptionally fossiliferous deposits of the Ardennes in Belgium. These stages are the Gedinnian (408.5 to 401 million years ago), Siegennian (401 to 394 million years ago), and Emsian (394 to 387 million years ago) of the Lower Devonian, the Eifelian (387 to 380 million years ago) and Givetian (380 to 374 million years ago) of the Middle Devonian, and the Frasnian (374 to 367 million years ago) and Famennian (367 to 362.5 million years ago) of the Upper Devonian. The subdivision of the marine deposits is based on lithologies and the presence of an abundant invertebrate fauna including goniatites and spiriferid brachiopods. The continental Old Red Sandstone deposits contain a fauna of jawless fish and plants belonging to the primitive psilophyte group. As a result of the Caledonian orogeny of late Silurian times, much of the British Isles was covered with continental red-bed facies.

dextrinoid adj. Of spores that turn reddish-brown upon treatment with Melzer's iodine solution.

dextrorse (L. dextrorsus: to the right) adj. Turned to the right or spirally arranged to the right, or clockwise when seen from the apex as in the leaves on some stems. Cf sinistrorse.

D horizon The soil layer below the effect of weathering, made up of undifferentiated and unconsolidated parent materials, located immediately below the C horizon. Cf. A horizon, B horizon.

di- (Gr. dis: twice) prefix. Meaning two or twice.

dia- (Gr. dia: twice) prefix. With the meaning of separation.

diacetylmorphine See heroin.

diacytic adj. Said of stomata, with two subsidiary cells surrounding the guard cells, their radial walls at right angles to the long axis of the guard cells.

diadelphous (Gr. dis: twice; adelphos, brother) adj. Having the stamens united by their filaments into two groups, often unequal, or all but one united in a group and one free.

diadromous (Gr. dia: through; dromos: race) adj. Having a fanlike venation.

diagnosis n. A list of characters in systematics that distinguishes one taxon from another.

dialycarpel (Gr. dialyô: to separate; karpos: fruit) n. An ovary or fruit with separate carpels.

dialycarpic See dialycarpous.

dialycarpous (Gr. dialyô: to separate; karpos: fruit) adj. With separate carpels. Syn. dialycarpic

dialypetalous (Gr. dialyô: to separate; petalon: leaf) adj. With separate petals; Syn. polypetalous, apopetalous.

dialyphyllous (Gr. dialyô: to separate; phyllon, leaf) adj. With the leaves distinct.

dialysepalous (Gr. dialyô: to separate; skepas: covering, shelter; [pe]talon: leaf) adj. With separate sepals.

diameter at breast height The width of the trunk of a standing tree, measured at 4.5 feet (1.3 meters) above ground surface.

diameter breast height See diameter at breast height.

diandrous (Gr. dis: twice; anęr, andros: male) adj. With two stamens.

diaphanous (Gr. dia: through; phainô, to show) adj. Extremely thin and transparent.

diaphoretic adj., n. Producing or increasing perspiration. A medicine or other agent that produces perspiration.

diarrhea n. Excessive and frequent evacuation of watery feces, usually indicating gastrointestinal distress or disorder.

diaspore (Gr. diaspora: scattering) n. A disseminule, especially one that undergoes dispersal; any structure that becomes detached from the parent plant and gives rise to a new individual. Cf. brood body, spore, gemma, propagulum. Syn. disseminule.

diatom (Gr. diatomę: cutting in two) n. A large and diverse division of photosynthetic protoctistans comprising a single class (Bacillariophyceae). The characteristic feature of the group is an extracellular wall consisting of two halves (valves) which contains silica. Diatoms are abundant in fresh and marine waters and are estimated to account for up to one-fifth of global primary productivity. Fossil deposits of diatom remains from the floors of Tertiary and Quaternary lakes are called diatomite.

diatomaceous earth A porous, light-colored material composed of the skeletons of diatoms. See also kieselguhr. Syn. diatomite.

diatomite See kieselguhr.

diatropism (Gr. dia: across; tropos: direction) n. The tendency of some plant organs to take a transverse position to the line of action of an outside stimulus.

dibrachiate adj. Refers to branches that spread widely.

dicarpellary (Gr. dis: twice; karpos: fruit) See bicarpellate.

dicarpellate (Gr. dis: twice; karpos: fruit) See bicarpellate.

dichasiu See dichasium.

dichasium (Gr. dikhasis: division by half, from dis: twice; kazo: separate) n. (pl. dichasia) A cymose inflorescence with opposite branching below the flower which terminates each axis. A cymose inflorescence in which each axis produces two opposite or subopposite lateral axes. Cf. monochasium.

dichlamydeous (Gr. dis: twice; khlamis: cloak, mantle) adj. Of a flower, having two whorls of perianth parts, i.e. calyx and corolla.

dichogamic See dichogamous.

dichogamous (Gr. dikha: in two; gamos: marriage) adj. With the pistils and stamens maturing at different times to prevent self-fertilization. Syn. dichogamic.

dichogamy (Gr. dikha: in two; gamos: marriage) n. The condition in which the male and female reproductive organs of a flower mature at different times, thereby ensuring that self-fertilization does not occur. Cf. homogamy, protandry, protogyny.

dichopodial adj. Forking; regular, unequal branching; e.g. Conocephalum.

dichotomize (Gr. dikhotomeô: to cut in two) v. To split into two equal parts.

dichotomous (Gr. dikhotomos: cut in two) adj. Forking into two more or less equal branches resulting from division of the growing point; having or consisting of a pair or pairs; paired. Describing the type of branching in plants that results when the growing point (apical bud) divides into two equal growing points, which in turn divide in a similar manner after a period of growth, and so on. Dichotomous branching is common is ferns and mosses. Cf. bifurcate, pseudodichotomous.

dichotomous branching A type of branching in which the apical meristem divides into two more-or- less equal apices; by repetition of this type of branching in various planes distinctive shoot systems may be produced; for example see decussate branching.

dichotomous key A series of choices leading to the identification of a species. (syn. floral key)

dichotomously adv. See dichotomous.

dichotomy (Gr. dikhotomia: cuttinf in two) n. Bifurcation; especially: repeated bifurcation (as of a plant's stem); a system of branching in which the main axis forks repeatedly into two branches. Branching of an ancestral line into two equal diverging branches. The division of a growing point into two halves.

diclinous (Gr. dis: twice; klinę: bed) adj. Having the stamens and the carpels in separate flowers, either in the same plant or on different plants; monoecious or diosecious; having only stamens or only pistils.

dicot n. Angiospermous plant having two cotyledons. Syn. dicotyledon.

dicotyledon (Gr. dis: twice; kotylędon: cavity, hollow) n. A flowering plant in which the embryo in the seed possesses two seed leaves (See cotyledons). Dicotyledonous plants generally have broad leaves with networks of veins, and floral parts arranged in multiples of two, four or five. Buttercups and sunflowers are examples. In some classifications the y are now commonly regarded as a class (Magnoliidae) within the phylum Magnoliopsida. Cf. monocotyledon.

Dicotyledonae (Gr. dis: twice; kotylędon: cavity, hollow) n. One of the two classes of flowering plants (see Anthophyta), distinguished by having two seed leaves (cotyledons) within the seed. The dicotyledons usually have leaf veins in the form of a net, a ring of vascular bundles in the stem, and flower parts in fours or fives or multiples of these. Dicotyledons include many food plants (e.g. potatoes, peas, beans), ornamentals (e.g. roses, ivies, honeysuckles), and hardwood trees (e.g. oaks, limes, beeches). Cf. Monocotyledonae.

dicotyledonous (Gr. dis: twice; kotylędon: cavity, hollow) adj. With two cotyledonons.

dicranoid Having a haplolepidous peristome of l6 forked teeth each having ventral trabeculae more strongly thickened than the dorsal, as in Dicranum, Fissidens, Leucobryum.

dictyostele (Gr. dictyon: net; stęlę: stele) n. A dictyostele is essentially a divided amphiphloic siphonostele; the stele consists of individual bundles of vascular tissue; these contain a central strand of xylem surrounded by phloem with an endodermis surrounding the bundle; the bundles are arranged in one circle around the axis. See also stele.

dicyclic (Gr. dis: twice; kyklikos: circular) adj. With two whorls

didymous (Gr. didymos: double) adj. Borne, developping or occurring in pairs; twinned; of anthers, having two lobes, with scarcely any tissue connecting them; divided in two. Cf. dichotomous.

didynamous (Gr. dis: twice; dynamis: power) adj. Of stamens, four in number, two being distinctly longer than the other two.

dieback n. The dying of the outer portions of a plant due to disease or weather damage; death of part or all of the woody portion of a plant.

diecious See dioecious.

dieciously See diecious.

differential species A species with high fidelity to a particular community that can be used to distinguish vegetation units. Also differentiating species.

differentiating species See differential species.

diffract adj. Cracked into small areas, areolate.

diffuse (L. diffusus: spread) adj. Widely or loosely spreading.

diffuse porous Said of wood, with vessels scattered throughout the year's growth.

digamous adj. With both sexes in the same flower cluster.

digitaliform (L. digitellum: foxglove; forma: shape) adj. Foxglove-shaped; campanulate, but longer and less even.

digitalis (L. digitellum: foxglove) n. A preparation of the dried leaves or seeds of the foxglove (Digitalis), used historically as a heart stimulant. Modern clinically prescribed drugs derived from digitalis include digoxin and digitoxin, both of which belong to a class of drugs known as the cardiac glycosides. They are used to treat heart failure and some forms of arrhythmia because of their ability to increase the force of contraction of the heart muscle. Their toxic effects arise from their capacity to disturb the normal rhythm of the heart.

digitate (L. digitus: finger) adj. Having fingerlike divisions, as some leaves; branching from the axis or stalk like the fingers of a hand; lobed, veined, or divided from a common point like the fingers of a hand; palmate, with narrower segments.

digitation (L. digitus: finger) n. A digit-like lobe or division. A digitate formation. A digit-like process or division.

digitiform (L. digitus: finger; forma: shape) adj. Finger-like.

digonous (Gr. dis: twice; gônia: angle) adj. With two angles, as the stems of some cacti.

digynous (Gr. dis: twice; gynę: female) adj. With two pistils.

dihaploid adj., n. A haploid (2x) derived from a tetraploid (4x).

dihybrid n. A crossbreed; an organism that came about from the breeding of parents that differ in at least two characters, e.g., leaf shape and plant height. An heterozygous with respect to two genes.

dihydrochalcone See dihydrochalcones.

dihydrochalcones n. A class of colorless flavonoids derived from chalcones by reduction of the alpha,beta double bond.

dihydroflavones See flavanones.

dikaryotic adj. A cell or hyphal compartment having two genetically distinct haploid nuclei.

dilated (L. dilatare: to broaden, spread out) adj. Flattened or expanded.

dillenioid adj. Said of a leaf tooth in which the medial vein ends in a clear expanded apex, the vein often projecting further, and no laterals are involved.

dimerous (Gr. dis: twice; meros: part) adj. With parts arranged in sets or multiples of two, having two parts in a whorl, etc.

dimer n. A molecule consisting of two identical simpler molecules. A chemical compound consisting of such molecules.

dimidiate (L. dimidiare: to divide in two) adj. Divided unequally into halves, so that one half is so reduced as to appear lacking; half-formed, the other half completely, or mostly, absent, whether by accident or nature; split, e.g. a calyptra split on one side; in lichens, of an involucrellum, covering only the upper half of a perithecium. Cf. cucullate.

dimitic adj. Of a fruiting body of Basidiomycota having two types of hyphae (generative hyphae commonly with clamps, skeletal hyphae commonly thick-walled, aseptate, and of limited length).

dimorphic (Gr. dis: twice; morphę: shape) adj. Having two forms; having two shapes; exhibiting, or occurring in two distinct forms; in ferns usually referring to fertile versus sterile laminae or portions of laminae. Syn. dimorphous.

dimorphism (Gr. dis: twice; morphę: shape) The occurence of two different forms of flowers, leaves, etc., on the same plant, or on distinct plants of the same species.

dimorphous adj. See dimorphic.

Dinantian n. The Lower Carboniferous sub-System in western Europe, overlain by the Silesian and comprising the Tournaisian and Visean Series. It is dated at 362.5 to 332.9 million years ago and is roughly contemporaneous with the Carboniferous Limestone (Britain), Kinderhockian, Osagean, Meramecian, and lower Chesterian (N. America).

dinoflagellate n. A unicellular aquatic organism with two flagella, of a group variously classed as algae or as protozoa.

dinosteranes See dinosteroids.

dinosteroids n. Chemicals found in dinoflagellates, which have been useful in documenting their existence early in the fossil record.

dioecious (Gr. dyo: two; oikos: house) adj. Describing plant species that have the male (staminate) and female (pistillate) flowers borne on different, separate plants. Examples of dioecious plants are willows; with archegonia and antheridia on separate plants. Syn. diecious, dioicous. Cf. androecious, andromonoecious, gynomonoecious, hermaphroditic, monoecious, trimonoecious.

dioeciously See dioecious.

dioeciousness See dioecious.

dioecy (Gr. dyo: two; oikos: house) The fact of being dioecious. Also dioeciousness.

dioicous See dioecious.

dipetalous (Gr. dis: twice; petalon: leaf) adj. Having two petals. Syn. bipetalous.

diphyllous (Gr. dis: twice; phyllon: leaf) adj. Having two leaves.

diplecolobous adj. Of cotyledons in a seed, twice folded transversely.

diplochory n. Describes seed dispersal involving two or more modes. For instance, a bird eats a fruit and disperses seed in it's droppings and these are later carried away by insects.

diploid (Gr. diploos: double; eidos: form) adj. A condition in which the chromosomes in the nucleus of a cell exist as pairs. Each member of the pair carries information about the same genetic characters as the other. Such a pair of chromosomes is referred to as a homologous pair. One of each pair is inherited from each parent. The diploid number of chromosomes in a cell is expressed as 2n. In humans the diploid number is 46, and all body cells contain 46 chromosomes, with the exception of red blood cells, which have no nucleus. Such a diploid number is constant within a species. When gametes are formed the diploid number is reduced by half to the haploid number (n) by meiosis. When gametes fuse at fertilization the diploid condition is restored. Cf. haploid, polyploid.

diplolepideous See diplolepidous.

diplolepidous adj. Form of arthrodontous peristome; originally: having main teeth with two columns of cells up the dorsal face; strictly: having two concentric circles of teeth, with the outer circle (exostome) derived from thickening of the contiguous walls of the outer and primary peristomial layers and the inner circle (endostome) derived from thickening of the contiguous walls of the primary and inner peristomial layers. The exostome is generally more heavily thickened than the endostome; however one or both circles may be vestigial; also used to denote a member of the Diplolepidae. Cf. haplolepidous. Also diplolepideous.

diplospory n. The production of diploid spores; spores resulting from an irregular meiotic division; thought to be the most probable source of polyploidy in mosses. Cf. apogamy, apospory.

diplostemonous (Gr. diploos: double; stęmôn: thread) adj. Having two series of stamens, or twice as many stamens as petals.

diplotene n. The stage of meiosis which follows pachytene and during which the four chromatids of each bivalent move apart in two pairs but remain attached in the region of chiasmata.

dipteral See dipterous.

dipterous (Gr. dis: twice; pteron: wing) adj. With two wings; having two winglike appendages as seeds, stems, etc. Syn. dipteral.

disc (L. discus: disk) n. An enlargement or outgrowth of the receptacle around the base of the ovary; in the Compositae (Asteraceae) the central portion of the involucrate head bearing tubular or disk flowers; a plate or rim of tissue, derived from the receptacle of a flower, occurring between whorls of floral parts.

discal (L. discus: disk) adj. Disc-shaped.

disc-floret See disc floret.

disc floret One of the small tubular, actinomorphic florets which make up the central part of the flower head in Compositae, each with a pistil and stamens but generally no other conspicuous flower parts. Also: disc flower. Cf. ray floret.

disc flower See disc floret.

disc flowers See disc floret.

discifloral (L. discus: disk; florus: flowered) asj. Having flowers in which the receptacle is expanded into a conspicuous disk, as in composite plants.

disciform (L. discus: disk; forma: shape) adj. Round and flattened; flat and circular. Cf. discoid.

discoid (Gr. diskoeidęs: disk-like) adj. Having the form of a disk; discoid flower: a compound flower not radiated but with tubular florets, a as in an involucrate head of the Compositae (Asteraceae) which lacks ray flowers.

discolorous (L. dis-: incicatif separation, division; color: color) adj. Of different colours; of leaves, having the two surfaces different in colour; variegated; coloured differently on different sides. Cf. concolorous.

discomycete n. A member of the Discomycetes.

Discomycetes n. A group of Ascomycetes in which the hymenium is exposed at maturity; one in which the fruiting body is an apothecium or discocarp.

discontinuity n. A gap in the range of a taxon. Cf. distribution, disjunct.

disjunct (L. disjunctus: separated) adj. Of plants occurring in widely separated geographic areas; of a population of a species widely separated geographically or ecologically from other populations of the same species.

disjunct distribution Discontinuous distribution of a species. Distribution in which one in which two closely related taxa are widely separated geographically.

disjunction n. The separation of chromosomes at anaphase.

disjunctly adv. See disjunct.

disk See disc.

disk floret See disc floret.

disk flower See disc flowers.

disk flowers See disc flowers.

disome See monosome.

disoperation n. Interaction between organisms in which one or all are harmed, as competition of maple seedlings, Acer, results in tall weak growth.

disparity n. Distinct in morphological characters; morphological variation; cf. diversity.

dispermous (Gr. dis: twice: sperma: seed) adj. Two-seeded.

dispersal n. The mechanism of dispersing reproductive propagules like seeds or pollen.

dispersal mechanism The means by which a propagule (seed, some other structure) of a plant is removed from the vicinity of the parent plant.

dispersion n. The pattern of distribution of individuals within a population, especially in prediction of probability. In soil, the breakdown of aggregates, resulting in a single grain structure. Generally the more easily the soil disperses, the more it will erode.

disphotic zone The depth in bodies of water where light is inadequate for photosynthesis but satisfactory for animal life.

dissected adj. Deeply divided into many narrow segments.

disseminule (L. disseminare: to spread, propagate) n. Any propagative part of a plant, as a bud, seed, or spore, that is capable of disseminating the plant.

dissepiment (L. dis-: apart; sæpimentum: hedge) n. A partition (septum) within an ovary or fruit, derived by fusion of adjacent carpels.

dissilient (L. dissiliare: to leap aside) adj. Bursting open or apart at maturity, as in some ripe fruits.

distal (L. distare: to be far-off) adj. Describing the part of an organ that is farthest from the organ's point of attachment to the rest of the body; towards the apex in position. For example, hands and feet are at the distal ends of arms and legs, respectively. Cf. proximal.

distally adv. See distal.

distant (L. distare: to be distant, far-off, to stand apart) adj. Well spaced; e.g., leaves positioned with a space between adjacent leaves.

distended (L. distendere: to stretch, inflate) adj. Stretched or swollen.

distichous (Gr. dis: twice; stikhos: rows; having two rows) adj. Arranged in two rows on opposite sides of a stem and thus in the same plane; two-ranked; in the case of plants with alternate leaves, the arrangement is such that first is directly below the third; e.g. Fissidens, Distichium.

distinct (L. distinctus: differentiated, separated) adj. Separate; not attached to like parts.

distribution n. The geographic range, continuous or discontinuous, of a plant, animal, or community. Dispersal patterns of occurrence of individuals in a taxon from a certain area, such as a) random distribution, poisson distribution, normal distribution; b) nonrandom above normal distribution, contagious dispersal, over-dispersion, hyperdispersion; and c) nonrandom below normal distribution, hypodispersion, or even-spaced distribution. The terms following each letter are synonyms.

disturbed adj. Referring to habitats that have been impacted by the actions of people.

distylous adj. Refers to a flower with two styles.

distyly n. Said about a heterostylous species, having plants that have either long styles and short stamens, or vice versa, or said of plants that have styles of different length.

diterpene See diterpenes.

diterpenes n. C20 compounds made up of four isoprene units, commonly occur in resins, rarely in essential oils, include gibberellins. A terpene with four isoprene units. See terpenes.

dithecal (Gr. dis: twice; thękę: box) adj. Having two pollen sacs or cells. Syn. dithecous.

dithecal anthers Anthers lacking septi between the loculi, so that there are only two anther cells.

dithecous See dithecal.

diuretic adj., n. Tending to increase the discharge of urine. A substance or drug that tends to increase the discharge of urine.

diurient n. A medicine which increases the discharge of urine. Syn. diuretic.

diurnal (L. diurnus: daily, during daylight) adj. Of flowers, opening only during daylight hours.

diurnally See diurnal.

divaricate (L. divaricare: to spread apart) adj., vt. Widely diverging or spreading apart. To branch or spread widely apart.

divergence n. The circumstance in which seres of similar origin become less like each other as succession moves toward climax. Cf. convergence.

divergent (L. di-: apart, away; vergere: to bend) adj. Diverging or spreading; separated from one another, having tips further apart than the bases; turned in opposite directions; buds which point away from the twig; spreading away from one another, usually at a rather wide angle.

diversity (L. diversitas: diversity, variety) n. Term used to describe numbers of taxa, or variation in morphology.

diversity index The mathematical statement calculated by dividing the number of species in a specified area by the number of individuals of all of these species.

diverticulate (L. divertere, to turn away) adj. Having short offshoots approximately at right angles to axis.

divided (L. dividere: to divide) adj. Referring to the blade of an appendage when it is cut into distinct divisions to, or almost to, the midvein; cut or lobed to the base or to the midrib.

division (L. divisio, divisionis: dividing up) n. A major taxonomic rank within the Plant Kingdom ranking just below kingdom and above class. The division is usually replaced by the phylum in the Animal Kingdom; in the Plant Kingdom the term phylum is no longer generally recognised. Cf. phylum. Method of propagation for clump-forming plants through pulling apart fibrous-rooted clumps. The removal of suckers from a parent plant, for the purposes of propagation.

divisural line The line down the teeth of a peristome, through which they split.

DNA 'deoxyribonucleic acid'. The nucleic acid which carries the genetic code of an organism. It is the primary component of chromosomes.

DNA cloning See gene cloning.

doctrine of signatures According to this doctrine, if part of a plant has the shape of a part of the human body, it would be useful in treating a disease of the human part it most closely resembled. Fros insya]ance a walnut that looks somewhat as a human brain would treat brain disorders, an Hepatica with its kidney-shaped leaves would be used for kidney diseases. This is only funny folklore and is quite unscientific.

document n. In botany systematics, this term can refer to herbarium specimens and equipment, as well as to the standard reference to written materials.

documentation In botany systematics, this term refers to all the written material, specimens, equipment, and other elements used as evidence, source of information, or method of obtaining data in taxonomic studies.

documentation system In botany systematics, this term covers the total strategy for identifying, citing, referencing, substantiating and authenticating, validating, verifying, and vouchering information used for a specific purpose in a taxonomic study.

dodder n. Any of the annual parasitic wiry twining vines of the genus Cuscuta that lack chlorophyll and have tiny scales instead of leaves.

dodeca- (Gr. dôdeka: twelve) prefix. Meaning twelve,

dodecagynous (Gr. dôdeka: twelve; gynę: female) adj. With twelve pistils or styles.

dodecamerous (Gr. dôdeka: twelve; meros: part) adj. With parts arranged in sets or multiples of twelve.

dodecandrous (Gr. dôdeka: twelve; anęr, andros: male) adj. With twelve stamens.

dolabriform (L. dolabra: mattock, pickax) adj. Ax-shaped or cleaver-shaped; pick-shaped; attached at some point other than the base, usually near the middle. Cf. cuneate.

doliiform adj. Shaped like a barrel. Also doleiform.

doleiform See doliiform.

dolomite n. A mineral consisting of a calcium magnesium carbonate. Limestone or marble rich in magnesium carbonate.

dolomitic adj. See dolomite.

domain n. The primary division of living systems. There are three domains: Archaea, Bacteria, and Eucaryota.

domatia See domatium.

domatium (L. doma, domatis: roof, roofing) n. (pl. domatia) Small structures on the lower surface of a leaf (in the axils of the primary veins) in some woody dicotyledons, usually consisting of depressions, partly enclosed by leaf tissue or hairs; modified sheltering projections; e.g., the Nostoc auricles of Blasia.

dominance The intra-allelic interaction such that one allele manifests itself more or less, when heterozygous, than its alternative allele.

dominant (L. dominus: master) adj. In genetics, describes the allele that is expressed in the phenotype when two different alleles of a gene are present in the cells of an organism. For example, the height of garden peas is controlled by two alleles, 'tall' (T) and 'dwarf' (t). When both are present (Tt), i.e. when the cells are heterozygous, the plant is tall since T is dominant and t is recessive. See also incomplete dominance. In ecology describes the most conspicuously abundant and characteristic species in a community. The term is usually used of a plant species in plant ecology; for example, pine trees in a pine forest.

donor parent The parent from which one or a few genes are transferred to the recurrent parent in backcross breeding.

dormancy n. Temporary cessation of growth. A state of quiescence during the development of many plants characterized by their inability to grow, though continuing their morphological and physiological activities.

dormant (L. dormire: to sleep) adj. Temporarily inactive.

dorsal (L. dorsum: the back) adj. Pertaining to the back, to the surface turned away from the axis, as in the lower surface of a leaf; abaxial; of a lateral organ, (relating to the side) facing away from the axis, i.e. the 'back'; of a thallus, facing away from the substratum; abaxial. Cf. ventral.

dorsiferous (L. dorsum: the back; ferre: to carry) adj. Borne on the back, as the sori of many ferns.

dorsifixed (L. dorsum: the back; fixus: fixed, fastened) adj. Attached at or by the back. Cf. versatile, basifixed.

dorsiventral (L. dorsum: the back; venter, ventris: belly) adj. Flattened from top to bottom; having structurally different upper and lower surfaces, as most foliage leaves; with distinct back and front (or upper and lower) surfaces, or placed with reference to the back or front. Syn. dorsoventral.

dorsoventral See dorsiventral.

double citation The occurence of two names at the end of a taxon, the first name, in parentheses, identifies the author who named the plant originally. The second name cites the author of the new name.

double cross The first generation hybrid between two single crosses. The mating of two different sets of inbred lines to produce two different single crosses which are then mated, as in double-cross hybrid corn and the like.

double fertilization A process, unique to flowering plants, in which two male gamete nuclei, which have travelled down the pollen tube, separately fuse with different female nuclei in the embryo sac. The first male nucleus fuses with the egg cell to form the zygote; the second male nucleus fuses with the two polar nuclei to form a triploid nucleus that develops into the endosperm.

double integument The two outermost layers of an ovule in angiosperms, one of which will differentiate into the seed coat; see integument.

double serrate Serrations bearing minute teeth on margins.

double-toothed adj. With each tooth bearing smaller teeth.

down n. Fine, soft feathers; soft, fine hair. An undulating, usually treeless upland plain having sparse soil. Soft feathers of some birds, especially waterfowl. An English term for a region of open treeless upland with a thin covering of soil that is used mostly for grazing sheep.

downy adj. Covered with short, fine hairs.

drepaniform (L. drepanis: swallow; forma: shape) adj. Like a sickle in shape. Syn. drepanoid. See falcate.

drepania See drepanium.

drepanium (L. drepanis: swallow) n. (pl. drepania) A floral cyme shaped like a sickle.

drift Changes in gene and genotypic frequencies in small populations due to random processes.

drip-point n. A leaf tip with an extension, acuminate, caudate, aristate, from which water drips during wet conditions. A long drooping tip on leaves, particularly those of rain forest trees. Alt. drip-tip.

drip-tip See drip-point.

drip zone The area around the base of a tree that lies below and within the circumference of the branches, etc., which forms its crown.

drooping adj. Erect or spreading at the base, then bending downwards.

dropsy n. (Formerly) edema.

drupaceous (L. drupa: ripe olive) adj. Bearing drupes; resembling a drupe or consisting of drupes.

drupe (L. drupa: ripe olive) n. A fleshy fruit that develops from either one or several fused carpels and contains one or many seeds. The seeds are enclosed by the hard protective endocarp (see pericarp) of the fruit. Thus the stone of a peach is the endocarp containing the seed. Plums, cherries, coconuts, and almonds are other examples of one-seeded drupes; holly and elder fruits are examples of many-seeded drupes. Syn. pyrenocarp. See also etaerio, berry, pyrene.

drupel See drupelet.

drupelet (L. drupa: ripe olive) n. A small drupe, usually in aggregate fruit, as in the individual segments of a raspberry or raspberry fruit. Syn. drupel.

drupeole (L. drupa: ripe olive) n. A little drupe.

drupiferous (L. drupa: ripe olive; ferre: to carry) adj. Bearing drupes.

druse n. A crystalline form of calcium oxalate that is common in plant cells, a radiating mass of crystals.

Dryopteridaceae n. A family of the phylum Filicinophyta.

duff n. The partially decayed leaves, branches, etc., on a forest floor.

dulcitol n. A hexitol.

dumose (L. dumosus: covered with bushes) adj. Full of bushes; bush-like.

dune slack A seasonal freshwater habitat that depends on water table level fluctuation.

dune-slack See dune slack.

duplex See nulliplex.

duplicate (L. duplicare: to double) adj. Folded twice.

duplicate factors Different or independent factors with the same expression; the reverse of a multiple allelomorphic series in which changes in the same gene produce different effects.

duplicate genes See duplicate factors.

duplication The occurrence of a segment of a chromosome twice in the haploid set.

duramen (L. durare: harden) n. The wood at the centre of a tree trunk or branch. It consists of dead xylem cells heavily thickened with lignin and provides structural support. Many heartwood cells contain oils, gums, and resins, which darken the wood. Cf. sapwood. Syn. heartwood.

duriherbosa n. Vegetation that has perennial roots, but whose tops die back each winter, like those of grasslands.

durilignosa n. Woody perennial vegetation with broad hard leaves, like chaparral.

Dutch elm disease A fungus spread by bark beetles that causes wilting and dieback on elms.

dwarf n. A plant that due to an inherited characteristic is shorter or slower growing than normal forms.

dyad (Gr. dyas: in two) n. A group, a combination of two.

dysentery n. An inflammatory disorder of the lower intestinal tract, usually caused by a bacterial, parasitic, or protozoan infection and resulting in pain, fever, and severe diarrhea, often accompanied by the passage of blood and mucus.

dysgenic adj. Refers to detrimental influences on the genetic properties of a community or population; tending to promote survival or reproduction of less well-adapted individuals especially at the expense of well-adapted individuals of a population; biologically defective or deficient. Also eugenics.

dysoxlin n. A limonoid.

dyspepsia (L. dyspepsia) n. Disturbed digestion; indigestion.

dysploid adj., n. A plant or species in which the chromosome number is more or less than the expected normal euploid number and when chromosomes do not show any regular pattern.

dysploidion n. A species of morphologically similar members of a dysploid series; all members are sexually reproductive (i.e. apomixis is not present).

dysploidy n. An abnormal ploidy as in the appearance of diploid (2x) or triploid (3x) individuals in a normally tetraploid (4x) population or of triploid and tetraploid ones in a normally diploid population.

dystrophic adj. Relates to or is caused by faulty nutrition. Refers to a lake with high humus material, sparse bottom fauna, and low dissolved oxygen.