trichomes

Trichomes are hair- or scale-like outgrowths of the epidermis found on many land plants. They vary in size and shape, from simple unicellular hairs to multicellular structures with stalks and glandular heads. Trichomes occur on leaves, stems, flowers, and fruits, and may be glandular or non-glandular.

Non-glandular trichomes are primarily protective hairs that can be unicellular or multicellular, uniseriate or multicellular, and may be branched or unbranched. Glandular trichomes possess secretory tissue and typically have a stalk and a secretory head; some are capitate (short stalk with a single head), others peltate (shield-like heads).

Development begins in the epidermal layer, with specialized epidermal cells undergoing division and expansion to form the stalk and, for glandular types, the secretory head. Trichome initiation and development are influenced by genetics and environmental cues such as light, temperature, and herbivory.

Functions include defense against herbivores and pathogens, reduced water loss by lowering leaf boundary layer, and protection from ultraviolet radiation. They also secrete substances such as waxes, resins, oils, or mucilage. In some species, glandular trichomes produce volatile compounds and metabolites used by humans for medicines, flavors, perfumes, or industrial materials; for example, cannabis trichomes synthesize cannabinoids and terpenes.

Trichome patterns and density are used in plant taxonomy and breeding as diagnostic characters. They are also a focus of research into epidermal development and plant defense, and can influence the commercial value of crops and medicinal plants.