epiderm

Epiderm is a term used in biology to denote the outermost tissue layer of an organism, particularly in reference to the epidermis of animals and the epidermis of plants. Etymologically it comes from Greek epi- “upon” and derma “skin.” In modern usage, epiderm is less common than epidermis or epidermal and is more often found in historical texts or in compound terms.

In animals, the epidermis is a stratified epithelium forming the outermost layer of skin and mucous membranes. It is mainly composed of keratinocytes that arise in the basal layer (stratum basale) and migrate outward through the layers spinosum, granulosum, and, in thick skin, lucidum, culminating in the cornified stratum corneum. The epidermis provides barrier protection against mechanical, chemical, and biological insults, participates in immune defense via Langerhans cells, and helps regulate water loss. It rests on a vascularized dermis that supplies nutrients and houses appendages such as hair follicles and glands.

In plants, the epidermis is a single-cell-thick protective layer covering the aerial portions of the plant and, in some species, roots. Epidermal cells are typically flattened and may secrete a waxy cuticle that reduces water loss. Guard cells regulate gas exchange through stomata; some epidermal cells form trichomes that deter herbivores or reduce transpiration. The plant epidermis arises from the protoderm during embryogenesis and is often replaced by periderm in mature woody tissues.

Terminology notes: while "epiderm" appears in some texts, contemporary biology tends to use "epidermis" or “epidermal” forms. The term remains more common in historical literature or specific compound terms.