Epinasty - a process by which the growth of branches or petioles is abnormally pointing downward. This phenomenon is caused by the more rapid growth of the upper side. Epinasty may result from either nutritional deficiencies or irregularities at the plant growth regulator level. Not to be confused with wilting, as epinastic tissues are turgid.

Epiphyte - a plant that grows upon another plant, but is neither parasitic on it nor rooted in the ground.

Episome - a genetic extrachromosomal element (e.g., the fertility factor (F) in Escherichiacoli) which replicates within a cell independently of the chromosome and is able to integrate into the host chromosome. The step of integration may be governed by a variety of factors and so the term episome has lost favour and been superseded by the wider term plasmid. Plasmids and F factors are episomes.

Epistasis - interaction between genes at different loci, e.g., one gene suppresses the effect of another gene that is situated at a different locus. Suppressed genes are said to be hypostatic. Dominance is associated with members of allelic pairs, whereas epistasis is interaction among products of non-alleles.

Epitope - a specific chemical domain on an antigen that stimulates the production of, and is recognised by, an antibody. Each epitope on a molecule such as protein elicits the synthesis of a different antibody. a.k.a. antigenic determinant.

Epizootic - a disease affecting a large number of animals simultaneously.

Equational division - mitotic-type division that is usually the second division in the meiotic sequence; somatic mitosis and the non-reductional division of meiosis. A chromosome division in which the two chromatids of each duplicated chromosome separate longitudinally, prior to being incorporated into two daughter nuclei.

Equatorial plate - the figure formed by the chromosomes in the centre (equatorial plane) of the spindle in mitosis.

Equilibrium - a state of dynamic systems in which there is no net change.

Equilibrium density gradient centrifugation - a procedure used to separate macro-molecules based on their density (mass per unit volume).

Equimolar - identical molar concentrations. See molarity; mole.

Erlenmeyer flask - a conical flat-bottomed laboratory flask with a narrow neck, designed by E. Erlen meyer. Widely used for culturing micro-organisms.

Escherichia coli - a commensal bacterium inhabiting the colon of many species. E. coli is widely used in biology, both as a simple model of cell biochemical function and as a host for molecular cloning experiments. In environmental studies, it is a key indicator of water pollution due to human sewage effluent.

Ethanol - commonly used to disinfest plant tissues, glassware utensils and working surfaces in tissue culture manipulations. The concentration used is 70% (v I v) for disinfecting and 95% (vIv) when flaming tools. Ethanol is also used to dissolve water-insoluble additions (addendums) to culture media.

Ethephon - (2-chloroethyl) phosphonic acid (ClClO3H6; f.w: 144.50) Through a spontaneous degradation of ethephon, ethylene is produced. Ethephon is a synthetic compound commonly used to treat cultured cells or unripened fruit with ethylene (q.v.).

Ethidium bromide - a fluorescent dye used to stain DNA and RNA. The dye fluoresces when exposed to UV light.

Ethylene - C2H4 a gaseous plant growth regulator regulating various aspects of vegetative growth, fruit ripening and abscission of plant parts.

Euploid - an organism or cell having a chromosome number that is an exact multiple of the monoploid (n) number. Terms used to identify different levels in an euploid series are diploid, triploid, tetraploid, and so on. cf aneuploid.