Interfascicular Cambium - A strand of cambium present between two adjacent vascular bundles. The formation of interfascicular cambium constitutes the first stage in the normal secondary thickening of a stem.

Intrazonal Soil - Used for describing the order of soil types according to soil classification USDA revised system of Thorp and Smith (1949). This reflects some local factor, other than the climate of the zone e.g., nature of parent rock, possesses dominant influence on the soil development. Suborders halomorphic, hydromorphic and calcimorphic have been recognized in this order, each having many soil groups.

Introgression (introgressive hybridization). Used for describing the phenomenon of incorporation of genes of one species or subspecies into other related species or subspecies. It takes place as a result of successful hybridization and successful backcrossing of hybrids with one of the parental population

.Intron - Said of a non-coding polynucleotide sequence situated between two coding regions (exons) of DNA. It is present in eukaryotic cell but not in bacterial cell. Length and number of introns in a gene varies. In some genes, introns are absent.

Introrse - Used for anthers that release their pollen towards the centre of the flower thus promote self-pollination. Compare extrorse.

Intrusive Growth (Interpositional growth). Said of the Growth that is exhibited by certain cells when they intrude between other cells that are either growing at a slower rate or not growing at all.

Intussusception - Used for describing the insertion of cellulose into spaces within the wall of an elongating cell. Water enters the cell vacuole, creates outwards pressure on the wall and this stretches the wall due to which cellulose gets incorporation resulting in increase in area of the wall. Next step is generally the thickening of wall by apposition.

Inulin - Said of a storage polysaccharide made up of fructose residues which are linked by (2-1) glycosidic bonds and about 30-40 residues in length. The chain is usually headed by glucose residue and is synthesized from sucrose precursors.

Inversion (chromosomal inversion). Used for structural change in chromosome which involves detachment of a piece of chromosome and its reattachment to the same chromosome having ends of detached portion in opposite direction of the original thereby causing an inversion in the sequence of gene in that segment of chromosome. Inversion may be pericentric or paracentric.

In Vitro - Said of experiments on biological processes which are carried out in laboratory apparatus, outside their natural place in the living organism.

In Vivo - Said of experiments inverstigating biological processes which are carried out in the living organism.

Involucre - (1) Used for the whorl or bracts around or beneath a condensed inflorescence such as a capitulum or umbel which is resembling and carrying out the function of calyx.
(2) Said of the sheath surrounding a group of archegonia or antheridia in certain liverworts such as Sphaero arpus.
(3) Said of the tubular up growth of thallus which is surrounding the base of bryophytein in members of Anthocerotae.

Iron Bacteria - Used for describing the bacteria of order Chalamydobacteriales which are rod-shaped gram negative and aggregate into filamentous structures that sometimes may get branched. The filaments are surrounded by a sheath of organic matter which is impregnated with ferric and managanese oxides. Bacteria derive energy by bringing about oxidation of organic substrate as well as ferrous compounds into ferric oxides, probably deriving additional energy from these reactions.

Irritability (Intensity). Said of any organism or responding to some external stimulus. In plants, response takes the form of various types of moments which are caused by bending of growing region.

Isidium - Said of an outgrowth from a lichen plant having both algal and fungal cells that may detach and form a new lichen thallus vegetative. Isidia may be coral-like or bud-like.

Isochromosome - Said of a metacentric chromosome in which two arms are genetically identical to each other.

Isoenzyme - Said of any of the multiple forms of an enzyme each of which possesses different characteristic kinetic properties. Such enzymes are usually obtained by different combinations of the same two or more subunits.

Isoflavones - Used for the isomers of flavones which are having B Group of flavonoid nucleus attached to third rather than second carbon atom of central C3 group. They are particularly common in Leguminosae. Some have oestrogen-like activity.

Isogamy - Said of the fusion between gametes which are identical in morphology and exhibit equal motility in sexual reproduction.

Isokont - Said of flagella in which all flagella present in a cell are of equal length and usually of some morphological form.

Isolating Mechanism - Any of the mechanisms which is serving as a reproductive barrier for gene flow between all the members of a population. Such mechanisms may arise in many ways e.g., allopatry, different flowering times or different pollinators etc.

Isomerase - Any enzyme calaysing the conversion of a molecule of substrate from its one isomeric form to other form.

Isomorphic - (1) Said of organism or organ that exists in only one form. Compare dimorphism, polymorphism.
(2) Said of (homologous) alternation between morphologically identical gametophytic and sporophytic generations so as to complete the sexual cycle. Compare heteromorphic.

Isonome Method - Used for a technique of studying plant distribution in a particular area which is having obvious variations in species composition. This study is due by laying continuous quadrates in the chosen area and recording abundance of each species in quadrat. Abundance values of each different species are then made to plot on separate pieces of squared paper and isonomes are constructed by joining up roughly equal abundance values.

Isoquinoline Alkaloids - Said of a group of alkaloids which are having structure based on isoquinoline nucleus and derived from amino acid tyrosine and phenylanine e.g., morphine or curare.

Isotonic - Said of the two solutions having same osmotic pressure. Compare hypertonic, hypotonic.

Isotopic Tracer (tracer). Said of any stable or radioactive isotope which is used to label a metabolite by incorporating it in place of normally incorporated isotope. Consequently it could be followed in the metabolic pathway in the living organism e.g., ¹C isotope is incorporated into CO2 taken up by a plant in photosynthesis is to know the photosynthetic pathway.

Isotype - Said of duplicate specimen of holotype of a taxon (See also holotype) which is collected at the same time and from the same place as the holotype and deposited usually in some place other than that of holotype.

Isthymus - The narrow centra1 portion which is connecting two halves of a placoderm desmid.