Distal - Used for describing the region of an organ which is farthest away from the organ's point of attachment. Campare proximal.

Distely - Said of the condition of stele in which vascular system is divided into two separate steles e.g., in certain species of Selaginella S. Kraussiana.

Distichous - Used for a type of alternate leaf arrangement having successive leaves on opposite side of the stem so that two vertical rows of leaves are seen on the stem which are opposite to each other e.g., grasses.

Dithiocarbamate Fungicide - Any fungicide prepared from the organic sulphur compound dithiocarbamic acid e.g., common fungicide ziram, thiram, zineb, maneb, etc.

Divergent Evolution (divergence) - Describing the evolution of different forms from a single basic structure as a result of different selection pressures acting on it. These changes may be associated with some new function that is added to it or replaces the original basic function. For example, phylloclade in plants with reduced leaves take up the function of leaves or short swollen modification of stem to serve as a perennating organ.

Division - The second highest category in the taxonomic hierarchy. Divisions are further composed of classes.
DNA Hybridization - A laboratory technique used for measuring similarity of DNAP of two species. In this technique DNA of both species, after denaturation by heat into single strands, is mixed together and incubated allowing them to recombine into hyvbrid DNAP molecules (annealing). The amount of annealing is directly proportional to the similarities of DNAP strands from two species and is measured by reheating the hybrid DNAP and recording the temperature at which strands get separated.

Dolipore Septum - A type of septum occurring in many basidiomycete fungi in which there is a narrow pore surrounded by a thickened rim and protected on both sides by caps.

Dominant - (1) Used for an allele which is expressed in the phenotype and masks expression of other allele at the same locus. (2) Describing the most abundant plant species in a community. (3) In forestry, describing the tree whose crown of leaves is more than half exposed to full illumination.

Domin Scale (1) A variation of Braun-Blanquet scale having more subdivisions of the original scale and is more accurate than that.

Dormancy - Used for describing a phase which is exhibited by seeds, spores and buds etc. (perennating organs) during which they remain inactive by deferring growth and development.

Dorsal - (1) In thallose plants, dorsal represents the upper surface that is away from the substrate.

(2) In lateral organs, the dorsal represents the side that is abaxial.

Dorsifixed - Used for an anther which gets joined to the filament for some distance along its dorsal edge, Compare  basifixed, versatile.

Double Fertilization - Used for the process in angiosperms in which one of the two male gametes entering the embryo sac undergres with the egg nucleus to form the zygote and the other one moving to the centre of embryo sac undergoes fusion either with the polar nucleus or with definitive nucleus to form a triploid primary endosperm nucleus. A form of double fertilization is also seen in gymnosperms like Ephedra in which one mile gamete undergoes fusion with the egg cell and the other undergoes fusion with the ventral canal cell but the product of second fertilization does not undergo further development.

Double Flower - A flower having more than usual number of petals. It is usually formed by transformation (by a mutation) of stamens into petals.

Double Helix - A molecule having two similar polymeric chains coiled in the same direction about the same axis e.g., in DNA or actin.

Double Recessive - A diploid individual homozygous for the same recessive allele of a factor which is revealed by the expression of recessive character in the phenotype.

Downy Mildew - A plant disease which is caused by fungi of family Peronosporaceae of order Peronosporales.

DPN - See NAD.

Drupe - Used for describing a fleshy indehiscent fruit in which the seed or seeds are surrounded by a hardened sclerenchymatous endocarp, e.g., wild cherry (prunus avium).

Druse - (sphaeroraphide). A globular mass of needle-like crystals formed which is either attached to the cell wall or free in the cytoplasm.

Dry Rot - A plant disease with disintegrating tissues and affected cells crumbling into a powdery mass. Dry rot of stored p0tatoes is caused by fungus Fusarium solani via. caerulem.

Dwarfism - Used for describing stunted growth, which is often caused by some genetic defect, often involving alteration of proportions of various body parts. In plants, it is frequently caused by a gibberellin deficiency.

Dwarf Male - (nanandrium). A fewcelled, small filamentous structure formed by germination of androspore. It is attached to oogonial wall or the wall of suffultory cell by a hold fast It occurs in nanandrous species of alga oedogonium.

Dynein - A protein having ATPase activity which makes. up the short, paired projections or arms present on the narrower microtubule of each pair of peripheral fibrils in the flagellum