1. (in zoology) The outermost layer of cells of the body of an animal. In invertebrates the epidermis is normally only one cell thick and is covered by an impermeable cuticle. In vertebrates the epidermis is the thinner of the two layers of skin (compare dermis). It consists of a basal layer of actively dividing cells (see Malpighian layer), covered by layers of cells that become impregnated with keratin (see keratinization). The outermost layers of epidermal cells (the stratum corneum) form a water-resistant protective layer. The epidermis may bear a variety of specialized structures (e.g. feathers, hairs).
2. (in botany) The outermost layer of cells covering a plant. It is overlaid by a cuticle and its functions are principally to protect the plant from injury and to reduce water loss. Some epidermal cells are modified to form guard cells (see stoma) or hairs of various types (see piliferous layer). In woody plants the functions of the shoot epidermis are taken over by the periderm tissues (see cork cambium) and in mature roots the epidermis is sloughed off and replaced by the hypodermis.
ep·i·der·mis / ˌepiˈdərmis/ • n. Biol. the outer layer of cells covering an organism, in particular: ∎ Zool. & Anat. the surface epithelium of the skin of an animal, overlying the dermis. ∎ Bot. the outer layer of tissue in a plant, except where it is replaced by periderm.DERIVATIVES: ep·i·der·mal / -ˈdərməl/ adj.ep·i·der·mic adj.ep·i·der·moid adj.