IAA Indole-acetic acid, a natural growth hormone found in plants.

Iapetus Ocean The late Precambrian and early Palaeozoic ocean that lay between Baltica and Laurentia. The oceanic crust and upper mantle of the Iapetus Ocean floor is presumed to have been subducted during the early Palaeozoic, and the ocean to have disappeared completely by the latest Silurian-early Devonian (about 400 million years ago). The ancient suture is thought to extend from WSW to ENE across the Solway Firth and Borders Region, Scotland. The Caledonian orogenic belt, which removed all trace of the Iapetus Ocean, extends all along the border between the two ancient cratons, affecting the areas now known as Norway, eastern Greenland, Scotland, northern England, Wales, Ireland, eastern Canada, and the eastern USA.

ichnofossil (Gr. ikhnos: track; fossilis: dug out) See trace fossil.

ideotype n. The ideal architectural plant type.

idioblast (Gr. idios: in a proper or particular way; blasti, blastos: bud, sprout) n. Plant cells containing oil, gum, calcium, or other products, and appearing to help provide mechanical support; a cell whihc differs greatly from the surrounding cells or tissue.

illegitimate adj. Refers to a name published validly, but not satisfying one or more articles of the Code of botanical nomenclature or the Code of Nomenclature for cultivated Plants.

imbricate (L. imbricarus: tiled) adj. Having parts overlapping each other like tiles or shingles on a roof. Overlapping, either vertically or spirally, where the lower piece covers the base of the next higher, or laterally, as in the aestivation of a calyx or corolla, where at least one piece must be wholly external and one internal. Lying lapped over each other in regular order. Of perianth parts, having the edges overlapping in the bud. Cf. valvate.

imbricated (L. imbricarus: tiled) adj. Overlapping, as shingles on a roof. Also imbricate.

imbricately adv. See imbricate.

imbricative adj. See imbricate.

immarginate adj. Without a border. Cf. elimbate.

immersed (L. immersus: plunged in, immersed) adj. Growing under water; submerged or below the surface; referring to a capsule or perianth exceeded by the blades or awns of the perichaetial leaves. Cf. emergent, exserted.

imparipinnate (L. impar, imparis: unequal, odd; pinna; feather) adj. Having an uneven number of pinnae, by virtue of having one terminal pinna; odd-pinnate; unequally pinnate. Cf. paripinnate.

imperfect (L. imperfectus: unachieved, incomplete) adj. With either stamens or pistils, but not both; diclinous; unisexual; incomplete, especially in reference to a peristome.

imperfect flower A flower containing stamen and pistil organs required for pollination but lacking sepals or petals or both of these organs.

imperfectly adv. See imperfect.

imperfectness n. See imperfect.

imperforate adj., n. Having no opening; not perforated; not separated by rows of perforations: imperforate sheets of stamps; (Medicine) lacking a normal opening: an imperforate anus. An imperforate stamp.

impetigo n. A contagious bacterial skin infection, usually of children, that is characterized by the eruption of superficial pustules and the formation of thick yellow crusts, commonly on the face.

impetigenous adj. See impetigo.

implicate (L. implicatus: folded in, entangled) adj. Twisted together; intertwined.

impressed (impressus: stamped on) adj. Situated below the surface, as in some leaf veins; bearing marks or depressions which appear to be stamped in, as in some seeds.

inaequilateral (L. in-: prefix having a negative force; Šquilateralis: equilateral) See inequilateral.

inaperturate (L. in-: prefix having privative force; apertura: opening) adj. Lacking an aperture; generally describing a pollen grain or spore without apertures; e.g. Populus ( Salicaceae), Taxus (Taxaceae).

inbred line A line produced by continued inbreeding. In plant breeding a nearly homozygous line usually originating by continued self-fertilization, accompanied by selection. A relatively true-breeding strain that results from at least five successive generations of controlled fertilization or of backcrossing to a recurrent parent with selection, or its equivalent, for specific characteristics.

inbred pure lines Involves inbreeding annual seed-propagated material; homogeneous, homozygous isolated by selection of desired recombinants or segregates in F2 to F7 generations of crosses between parental pure lines (generally monogenotypic, can be blended to form multilines); examples are tomato, lettuce, soybean, pea, cowpea, snapbean, field bean, Arabian coffee, pepper (Capsicum), eggplant, okra, lentil, and papaya (`Solo').

inbred-variety cross The F1 cross of an inbred line with a variety.

inbreeding n. The mating of individuals more closely related than individuals mating at random.

inbreder n. See inbreeding, self-pollination.

inbreeding coefficient A quantitative measure of the intensity of inbreeding.

incanescent (L. in: into; canescere: to whiten, to turn white) adj. Of a plant having a whitish or grayish pubescence; becoming white. See canescent.

incanous (L. in: into; canus: white, of a bright white) adj. With a whitish pubescence; hairy, hoary; grey or silver colored; covered with dense, fine grayish-white trichomes.

incarnate (L. incarnatus: made flesh) adj. Flesh-colored or crimson.

incised (L. incisus: cut, nicked, notched) adj. With sharp angles between the lobes; having deeply cleft margins; cut deeply, sharply and often irregularly (an intermediate condition between toothed and lobed); appearing regularly slashed; cut into sharp divisions separated by narrow sinuses.

inclined (L. inclinis: tilted, bent) adj. Rising upward at a moderate angle, as capsules that are between the erect and horizontal positions; bent down.

included (L. incluses: shut in, enclosed) adj. Not projecting beyond an enclosing part, as stamens contained within a corolla; not exserted; enclosed; not protruding beyond surrounding structures. Cf. immersed.

included phloem In secondary vascular tissue, phloem islands surrounded by xylem.

incompatibility n. The phenomenon in which pollen from one flower fails to fertilize other flowers on the same plant (self-incompatibility) or on other genetically similar plants. This genetically determined mechanism prevents self-fertilization (breeding between likes) and promotes cross-fertilization (breeding between individuals with different genetic compositions). See also fertilization; pollination.

incomplete (L. in-prefix having a privative force; completus: finished, thorough) adj. Lacking an expected part or series of parts, as in a flower lacking one of the floral whorls (i.e. sepals, petals, stamens, or pistils).

incomplete dominance The condition that arises when neither allele controlling a characteristic is dominant and the aspect displayed by the organism results from the partial influence of both alleles. For example, a snapdragon plant with alleles for red and for white flowers produces pink flowers. Cf. co-dominance.

incomplete flower Lacking one or more whorls (sepals, petals, stamens, carpels) of the complete flower.

incrassate (L. incrassus: made thick) adj, vt. Thickened or swollen, becoming thick or thicker, especially toward a tip or margin; of a pollen grain, with thickened margins around the apertures; with thickened cell walls. Syn. incrassated.

incrassated adj. See incrassate.

incrassation n. See incrassate.

incrassative adj. See incrassate.

incubous (L. incubare: to lie upon) adj. Of leaves overlapping with the upper part of each leaf covering the base of the leaf above it. Cf. succubous.

incumbent (L. incumbere: to lie upon, to lean on) adj. Of the orientation of an embryo, with the cotyledons lying face to face and folded downwards beside the radicle; of anthers, lying against the inner face of the filament.

incumbent cotyledons Cotyledons lying against the radicle along the back of one of the cotyledons. Cf. accumbent cotyledons.

incurved (L. incurvus: curved) adj. Bent or curved inwards or upwards; of leaf margins, curved towards the adaxial surface; curved toward the base or apex. Cf. recurved.

indeciduate (L. in-: prefix having a privative force; deciduus: falling down) adj. Having permanent leaves.

indeciduous (L. in-: prefix having a privative force; deciduus: falling down) adj. Not deciduous, as leaves; Of trees, evergreen.

indefinite (L. in-: prefix having a privative force; definite: in a precise way) adj. Variable in number; numerous; of stamens, more than twice as many as the petals or sepals; of an inflorescence, not terminating in a flower (i.e. having a continuing, terminal growing point).

indefinite inflorescence See racemose inflorescence.

indehiscence n. See indehiscent.

indehiscent (L. in-: prefix having a privative force; dehiscere: to open, gape, crack) adj. Describing a fruit or fruiting body that does not open to release its seeds or spores when ripe, instead, release occurs when the fruit wall decays or, if eaten by an animal, is digested; lacking distinct opening mechanism, as spores shed by irregular rupture or breakdown of capsule wall, e.g. Archidium. Cf. dehiscent.

independence n. The relationship between variables when the variation of each is uninfluenced by that of others, that is, correlation is zero.

indeterminate (L. in-: prefix having a privative force; determinare: to bound, limit) adj. Of growth, the condition in which the terminal bud persists and produces successive lateral branches (monopodial branching). Describes an inflorescence in which the lower or outer flowers bloom first, allowing indefinite elongation of the main axis. iN lichens, with margins not well defined.

index fossil A geographically widespread fossil, that is diagnostic of a particular time period and therefore useful in correlating the age of rock formations from different geographic areas.

indigenous (L. indigenus: native) adj. Native; originating or occurring naturally in the place specified; not introduced.

indole n. A crystalline alkaloid compound (C8H7N) that is a decomposition product of proteins containing tryptophan, that can be made synthetically, and that is used in perfumes.

indole alkaloids Alkaloids that contain an indole ring, derived from phenylalanine or tryptophane.

indument (L. indumentum: clothes, garment) n. The epidermal coverings of a plant, collectively.

indumenta See indumentum.


indumentum (L. indumentum: clothes, garment) n. (pl. indumenta or indumentums) The epidermal appendages, e.g. hairs or scales, collectively; the covering of hairs on part of a plant, especially when dense; a general term without precise connotation.

induplicate (L. inL in; duplicare: to double) adj. Folded inwards so that the outer faces of the margins are in contact. With the petals or sepals edge to edge along their entire length, the margins rolled inward. Syn. induplicative.

induplication n. See induplicate.

induplicative See induplicate.

indurate adj. Hardened or stiffened. Also indurated.

indurated (L. induratus: hardened) adj. Hardened, callous, inured. Also indurate.

indusia See indusium.

indusial adj. See indusium.

indusiate (Lat. indusium: woman's shirt) adj. With an indusium.

indusium (Lat. indusium: woman's shirt) n. (pl. indusia) The thin, scale-like, sometimes kidney-shaped tissue covering the immature sorus (fruit-cluster) of certain ferns that protects the developing sporangia; It withers when the sorus ripens to expose the sporangia. A collection of hairs enclosing the stigma of some flowers. The pollen-cup of Goodeniaceae.

induviŠ (L. induviŠ: clothes) n. The persistent portions of a calyx or corolla; also, leaves which do not disarticulate from the stem, and hence remain for a long time.

induviate Covered with induviŠ, as the upper part of the trunk of a palm tree. See marcescent.

inequilateral (L. in-: prefix having a negative force; Šquilateralis: equilateral) adj. With sides of unequal shape and length.

inerm See inermous.

inermis See inermous.

inermous (L. inermis: without weapons) Unarmed, with no prickles, thorns, teeth, etc., as a leaf. Syn. inerm, inermis. Cf. unarmed.

inferior (L. inferior: lower) adj. Describing a structure that is positioned below or lower than another structure in the body. For example, in flowering plants the ovary is described as inferior when it is located below the other organs of the flower, i.e. when situated below the apparent point of attachment of stamens and perianth. See epigyny. Cf. superior.

inferior ovary Opetals, and possibly below the point of attachment of all other flower parts and embedded in the floral stem.

inflated (L. inflatus: swolen) adj. Swollen or expanded; hollow or swelled out with air; bladdery; e.g. the lobules of certain species of Frullania or Lejeunea or alar cells of Sematophyllum. Cf. explanate.

inflect v. To bend in.

inflected adj. Bent, flexed.

inflexed (L. inflexus: bent, curved) adj. Bent inwards or downwards, toward the axis; bent sharply upwards or forwards. Cf. deflexed, incurved, involute, inrolled, reflexed.

inflorescence (L. inflorescere: to become covered with flowers) n. The arrangement of a group of flowers borne on the same main stalk. In a racemose (or indefinite) inflorescence the tip of the main stem continues to grow and flowers arise below it. Examples are the raceme (e.g. foxglove, lily-of-the-valley: Convallaria majalis), the spike (e.g. wheat, heather: Calluna vulgaris), the catkin or ament (e.g. the male flowers of birch: Betula and hazel), which is a kind of spike that often hangs down from the stem, the spadix (e.g. kack-in-the-plupit: Arisema triphyllum). Flat-topped racemose inflorescences include the umbel, the simple umbel of milkweed: Asclepias syriaca, or the compound umber of water parsnip: Sium cicutaefolium (see Umbelliferae), the capitulum, or head, or anthodium (e.g. daisy, dandelion: Taraxacum officinalis), and the corymb (e.g. candytuft, red chokeberry: Aronia obtusifolia). A cymose (or definite) inflorescence, or cyme, is one in which a flower is produced at the tip of the main stem, which then ceases to grow, e.g. chickweed: genus Cerastium. Growth is continued either by one lateral bud, to produce a monochasium (e.g. buttercup), or by a pair of buds, giving a dichasium (e.g. stitchwort).

inflorescence axis The main axis of an inflorescence bearing inflorescence branches or flowers.

inflorescence bract An often more or less reduced leaf subtending a branch of the inflorescence or borne on the inflorescence axis below any branches or flowers.

influent n. A plant or animal that has an important effect on the biotic balance in a community.

infra- (L. infra: below) prefix. Meaning below or beneath.

infra-axillary (L. infra: below; axilla: armpit) adj. Below the axil.

infrageneric adj. Refers to any taxon of a rank below that of genus, such as species, variety, cultivar, etc.

inframedial (L. infra: below; mediale: middle) adj. Below the middle

infraspecific (L. infra: below; specificus: specific) adj. Of lower taxonomic rank than species.

infra-specifically See infraspecifically.

infraspecifically adv. See infraspecific. Also infra-specifically.

infrastaminal (L. infra: below; stamen: thread) adj. Below the stamens.

infrastipular (L. infra: below; stipula: stalk, straw, culm, stubble) adj. Below the stipules.

infructescence (L. in: into; fructus, fruit) n. The inflorescence in a fruiting stage; collective fruits; the grouping or arrangement in which fruits are borne on a plant.

infundibular (L. infundibulum: funnel) See infundibuliform.

infundibulate (L. infundibulum: funnel) See infundibuliform.

infundibuliform (L. infundibulum: funnel; forma: shape) adj. Funnel-shaped Syn. infundibular, infundibulate.

ingroup n. In a cladistic analysis, the set of taxa which are hypothesized to be more closely related to each other than any are to the outgroup.

initial adj. Of a meristematic, undifferentiated cell at its earliest stage of specialization; continued division and differentiation of its derivatives produce particular structures or organs; e.g., rhizoid initial, leaf initial.

innate adj. Borne at the apex; sunken, immersed.

innocuous (L. in-: prefix having a negativeforce; nocuus: harmfull, injurious) adj. Harmless; lacking thorns or spines.

innovation (L. renewal) n. A short, basal offset from the base of a stem; a new shoot; in acrocarpic mosses, a branch formed after the formation of sex organs; in mosses subfloral innovations are produced at the base of a gynoecium; in liverworts, between bracts and perianth.

inodorous (L. in-: prefix having a negativeforce; odor, odoris: smell, odor) adj. Without an odor.

inolizidine alkaloid A. polyhydroxy alkaloid.

inoperculate adj. Lacking an operculum. Cf. cleistocarpous.

inquilinism (L. inquilinus: tenant) n. An association between members of two different species in which one, the inquiline, lives on or in the other (the host), or inside the host's home, obtaining shelter and in some instances taking some of the host's food. For example, certain mosquitoes live and breed in the fluid contained in the pitchers of pitcher plants, benefiting from the protection afforded by the pitcher and also making use of nutrients from prey trapped by the plant. The nest of many social insects harbour inquilines, which have evolved ingenious strategies to gain food or avoid being attacked, as evidenced by the staphylinid beetle Atemeles pubicollis. As a larva this lives in ant colonies, adopting the 'begging' posture of ant larvae in order to receive food from adult ants.

inrolled adj. Curled or rolled inward. Cf. involute, incurved, inflexed.

insectivore (L. insecta: insect; vorare: to devour) n. An insectivorous animal or plant.

insectivorous (L. insecta: insect; vorare: to devour) adj. Catching, and ostensibly feeding on, insects.

insectivorous plant See carnivorous plant.

inserted (L. insertus: put inside, introduced) adj. Especially of the parts of a flower, attached to, arising from, or growing out of some part.

insertion (L. insertus: put inside, introduced) n. The manner of place of attachment, as of an organ; attachment of a part or organ, with special reference to the site or manner of such attachment.

insertional (L. insertus: put inside, introduced) adj. See insertion.

insipid (L. insipidus: flat, bland) adj. Lacking taste or flavor

intectate adj. Of a pollen grain lacking a tectum, but with sculpturing.

integrin n. Adhesive protein of the extracellular matrix in animals.

integument (L. in: into; tegumen: all that can be used to cover) n. The outer protective covering of a plant ovule. It is perforated by a small pore, the micropyle. Usually two integuments are present in angiosperms and one in gymnosperms. After fertilization the integuments form the testa (seed coat) of the seed. A natural outer covering, as a skin, husk, rind, etc.

integumentary (L. in: into; tegumen: all that can be used to cover) adj. Of, pertaining to, or like an integument.

inter- prefix. Between, among.

intercalary adj. Inserted between; e.g., meristems with growth that is not apical or basal, but occurs some distance between.

intercalary inflorescence An inflorescence that either arises in an internodal position, or one that was originally terminal but ceases to be so when vegetative growth subsequently resumes from the stem apex.

intercalary meristem An actively-growing region of primary tissue somewhat removed from the apical meristem and occurring between regions of more or less differentiated tissues, e.g. at the base of a monocot leaf, in some internodal regions, etc.

intercellular (L. inter: between; cellula: cell [of a monk]) adj. Lying between cells, as intercellular space in plant tissue.

intercostal (L. inter: between; costa: rib, side) adj. Situated between the ribs or nerves.

interfascicular cambium A lateral meristem developing from tissues in the region between the vascular bundles, and, with the fascicular cambium, making up the vascular cambium.

interfascicular region Tissues in the region between the vascular bundles (fascicles).

interference (L. inter: between; ferire: to strike) n. The overall influence of one plant or groups of plants on another, and encompasses allelopathy or competition, or both of these processes. The effect of one crossover influencing the probability than another will occur in the immediate vicinity.

interfertile (L. inter: between; fertilis: fruitful) adj. Able to interbreed. Capable of interbreeding.

interfertility n. See interfertile.

interfoliaceous adj. At the same node with opposite or whorled leaves, but occupying a position between their places of attachment.

internal phloem Primary phloem that is found adaxially or internally to the xylem in the stem, in the leaf manifest as a bicollateral vascular bundle.

internodal adj. Pertaining to sections of stem between leaves or buds.

internodally adv. See internodal.

internode (L. inter: between; nodus: knot) n. The portion of a stem between two nodes; the section of stem between leaves or buds; the portion of a stem between the level of insertion of two successive leaves or leaf pairs (or branches of an inflorescence).

interpetiolar (L. inter: between; petiolus: petiole) adj. Of stipules, between the petioles of two opposite leaves. Cf. intrapetiolar.

interrupted (L. interruptus: broken) adj. Of an inflorescence, having the flowers unevenly distributed along the axis, with conspicuous gaps. Also describing the fertile frond of some ferns with the clusters of sporangia similarly arranged.

interruptedly pinnate Pinnate with leaflets of various sizes intermixed

intertidal adj. Of or being the region between the high tide mark and the low tide mark.

interspecific adj. Refers to hybrids between two separate species of the same genus.

interstaminal corona The fleshy lobes, often connate into a tube, attached to the base of the staminal column in the interstaminal areas of members of the family Asclepiadaceae. Cf. gynostegial corona.

interthecial adj. Saif of hyphae, lying between asci. See hamathecium.

intervenous (L. inter: between; vena: vein) adj. Pertaining to the spaces between veins.

intine (L. intus: inside) n. The inner layer of the two-layered wall of a spore, especially of a pollen grain.

intolerant )L. intolerans: who cannnot withstand) adj. Not surviving well under a dense forest canopy.

intra- (L. intra: inside, within) prefix. Meaning within.

intracalycine (L. intra: inside, within; calyx: cup) adj. Within the calyx, as in the membrane within the calyx of some members of the Gentianaceae

intramarginal (L. intra: inside, within; margo, marginis: edge) adj. Situated inside but close to the margin, e.g. of a vein in a leaf.

intramarginal vein A continuous more or less looping vein that runs admedially to or inside the leaf blade margin and which joins the secondary veins, a composite vein usually formed by linking parts of secondary veins, as in brochidodromous venation.

intrapetiolar (L. intra: inside, within; margo, petiolus: petiole adj. Between a petiole and the subtending stem. Cf. interpetiolar.

intrastaminal (L. intra: inside, within; stamen) adj. In between the stamens.

intravaginal squamule Same as colleter.

intraxylary phloem Same as internal phloem.

intricate (L. intricatus: tangled) adj. Tangled together, interwoven, entangled. Syn. intricated.

intricated adj. See intricate.

intricately adv. See intricate.

intricateness n. See intricate.

introduced (L. introducere: to bring in, to lead into) adj. Not indigenous; not native to the area in which it now occurs. Said of species that have been released (planted) that may or may not have become naturalised. Cf. adventive.

introgression n. Infiltration of the genes of one species into the gene pool of another through repeated backcrossing of an interspecific hybrid with one of its parents; the formation of a range of intermediate plants by hybridization among parents and progeny.

introgressive hybridization Hybridization followed by recrossing with the parental species in such a way that certain features of one species become transferred to the other species without impairment of taxonomic integrity.

intron n. The part of a gene sequence that is transcribed but not translated.

introrse (L. introrsum: towards the inside) adj. Turned inward, toward the axis; opening inward. Of anthers, dehiscing towards the centre of the flower. Cf. extrorse.

intumescence n. A swelling up; the state of being swollen; a swollen mass.

intumescent (L. intumescere: to swell) adj. See intumescence. Syn. tumid.

intussusception n. In cell wall formation, growth by addition of new wall material within previously formed wall.

inulin n. A fructan that acts as a storage polysaccharide, yields fructose on hydrolysis.

invaginated (L. in: into: vagina: sheath) adj. Sheathed; infolded.

invalid adj. Refers to scientific names published with incomplete information or in an invalid publication.

invasive adj. Marked by a tendency to spread.

inversion n. A rearrangement of a chromosome segment so that its genes are in reversed linear order. A rearrangement of a group of genes in a chromosome in such a way that their linear order is reversed.

inversion of configuration Inversion of configuration occurs when a (+)-stereoisomer is changed into the corresponding (-)-stereoisomer (or vice versa), causing the substance to rotate the plane of plane-polarized light in the opposite direction. The discovery of Walden inversion was helpful in identifying the common mechanism of nucleophilic substitution.

invertebrate n. Any animal lacking a backbone or notochord; e.g. an insect, a mollusk; the term is not used as a scientific classification.

inverted (L. invertere: to turn upside down) adj. Positioned opposite the typical direction; reversed.

investing (L. investire: to surface, to fill) adj. Covering or surrounding.

involucel (diminutive of L. involucrum: cover, wrap) n. A secondary involucre, as the bracts subtending the secondary umbels in the Umbelliferae (Apiaceae).

involucelate (diminutive of L. involucrum: cover, wrap) adj. With an involucel. Syn. involucellate, involucellated, involucelated.

involucelated See. involucelate.

involucella See involucellum.

involucellate See. involucelate.

involucellated See. involucelate.

involucellum n. (pl. involucella) An extensive sheath-like haustorial collar; e.g. Jackiella.

involucra See involucrum.

involucral (L. involucrum: cover, wrap) adj. Of or pertaining to an involucre.

involucral bract A sheath of foliar origin, derived by a fusion of the bracts surrounding a perianth.

involucrate (L. involucrum: cover, wrap) adj. With an involucre.

involucre (L. involucrum: cover, wrap) n. A protective structure in some flowering plants and bryophytes. In flowering plants it consists of a ring of bracts arising beneath the flower cluster of those species with a capitulum (i.e. members of the dandelion family) or an umbel (i.e. members of the carrot family). In mosses and liverworts the involucre is a projection of tissue from the thallus that arches over the developing archegonium. In Hymenophyllaceae, a layer of tissue enveloping the sporangia. Syn. involucrum.

involucrellum n. The tissue forming the upper part of a perithecium, distinct from and surrounding the exciple.

involucrum n. (pl. involucra) See involucre.

involute (L. involutus: rolled up) adj. Rolled upwards. Of leaves having the edges rolled inwards at each side, toward the adaxial side. Cf. revolute, inrolled, incurved.

involution (L. involutus: rolled up) n. A rolling inwards of leaves; a rolling in or folding in upon itself; a part so formed.

ion (Gr. i˘n: going) n. An atom or small molecule which carries a positive or negative charge.

iridescent (L. iris, iridis: rainbow) adj. Displaying many colors, as in a rainbow.

iridoid See iridoids.

iridoids n. A subclass of terpenoids, monoterpenoids with lactone substitutions, usually in glycosidic form, in nearly all deoxyloganic acid or epi-deoxyloganic acid is an intermediary.

irradiation n. The exposure of plants or plant parts to X-rays or other radiations to increase mutation rates.

irregular (prefix ir-: without; L. regular: used as a rule) adj. Not uniform; Of a flower having the member of some or all of its floral circles or whorls (especially sepals and petals) differing from one another in size, or shape, or extent of union. See alos zygomorphic.

irregular flower Describing a flower showing inequality in the size, form, or union of its similar parts. Describing a flower bilaterally symmetrical; said of a flower in which all parts are not similar in size and arrangement on the receptacle. Cf. regular, zygomorphic.

isabelline adj. Dirty brownish grey, yellowish or tawny.

isadelphous (Gr. isos: equal; adelphŕ: sister) adj. With diadelphous stamens of equal number in each bundle.

isandrous (Gr. isos: equal; anŕr, andros: male) adj. With the number of stamens equal to each other and equal to the number of perianth parts.

isanthous (Gr. isos: equal; anthos: flower) adj. Having regular flowers. See regular.

isidia See isidium.

isidiate adj. With isidia.

isidioid adj. Having the form of an isidium.

isidioid soredia A soredia which become corticate in part.

isidium n. (pl. isidia) An asexual reproductive structure of a lichen with a distinct cortex. Usually isidia look like small pin-shaped outgrowth from the upper cortex of a lichen. Isidia may have various structures. They can be branched (coralloid) or flattened, leaf-like (phylloid).

iso- prefix. Meaning equal; in chemical names, refers to an isomer.

isoallele An allele indistinguishable except by special tests.

isobilateral (Gr. isos: equal; L. bis: twice; latus, lateris: side) See isolateral.

isocarpic (Gr. isos: equal; karpos: fruit) adj. Having carepls equal in number to the other floral parts.

isodiametric (Gr. isos: equal; diametros: diagonal, diameter) adj. About as broad as long; applied to cells about the same diameter in all directions, including square, rounded, or hexagonal; used to describe parenchyma cells that have length, width and height approximately equal.

Isoetaceae n. A family of the phylum Filicinophyta.

isoflavan n. Isomeric form of flavones with the most reduced structure of any isoflavonoids that lack hydroxy or ketone groups, i.e. there are no oxygens in the structure; they are common as phytoalexins in Fabaceae.

isoflavone See isoflavones.

isoflavones A class of flavonoids in which the B ring of flavones is attached to the 3-position (instead of the 2-position).

isoflavonoid ONe of a class of flavonoids in which the B ring of flavones is attached to the 3-position (instead of the 2-position) and that lack the 3-hydroxy group of flavonols.

isogamy n. A condition in which the sexual cells, or gametes, are of the same form and size and are usually indistinguishable from each other. Many algae and some fungi have isogamous gametes. In most sexual reproduction, as in mammals for example, the ovum is quite larger and of different appearance than the sperm cell. This condition is called anisogamy.

isogenic lines Two or more lines differing from each other genetically at one locus only. Distinguished from clones, homozygous lines, identical twins, etc., which are identical at all loci.

isogenous (Gr. isos: equal; genos: origin) adj. With the same origin.

isolateral (Gr. isos: equal; L. latus, lateris: side) adj. Having structurally similar upper and lower surfaces. Syn. isobilateral.

isolation The separation of one group from another so that mating between or among groups is prevented.

isomerous (Gr. isos: equal; meros: part) adj. With an equal number of parts, as in a flower with an equal number of members in each floral whorl.

isomorphic (Gr. isos: equal; morphŕ: shape) adj. Having a similar form, but being genetically different.

isopentenyl pyrophosphate The building unit of terpenes.

isophyllous adj. Having stem leaves and branch leaves that are similar.

isopod n. Any of various small terrestrial or acquatic crustaceans with seven pair of legs adapted for crawling.

isoprene n. A volatile unsaturated branched chain five-carbon hydrocarbon that is the basic unit of terpenes, synthesised via the mevalonic acid pathway.

isoquinoline n. A low-melting nitrogenous base C9H7N that is associated with its isomer quinoline in coal tar and that is the parent structure in many alkaloids (as narcotine and papaverine).

isoquinoline alkaloid A type of true alkaloid, formed from a precursor of 3,4-dihydroxytyramine (dopamine) linked to an aldehyde or ketone.

isosporous adj. Having spores with a unimodal distribution of size.

isothiocyanate n. An ester of isothiocyanic acid, RN=C=S, a.k.a. mustard oils, with a pungent smell and a sharp taste, derived from glucosinolates undergoing an enzymatic reaction (via the enzyme myrosinase) when plant tissue is crushed.

isotomic adj. Branching into two or more branches all of equal diameter.

isotype (Gr. isos: equal; typos: mark, image) n. A specimen which is, or is believed to be, a duplicate of the holotype, i.e. part of the same collection.

isovitexin n. A glycoflavone.

iteroparity n. The repeated production of offspring at intervals throughout the life cycle. It is usually contrasted with semelparity, where each individual reproduces only once during its life.