Trichome
Trichomes (; ) are fine outgrowths or appendages on plants, algae, lichens, and certain protists. They are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae. A covering of any kind of hair on a plant is an indumentum, and the surface bearing them is said to be pubescent. Algal trichomes Certain, usually filamentous, algae have the terminal cell produced into an elongate hair-like structure called a trichome. The same term is applied to such structures in some cyanobacteria, such as Spirulina and Oscillatoria. The trichomes of cyanobacteria may be unsheathed, as in Oscillatoria, or sheathed, as in Calothrix. These structures play an important role in preventing soil erosion, particularly in cold desert climates. The filamentous sheaths form a persistent sticky network that helps maintain soil structure. Plant trichomes Plant trichomes have many different features that vary between both species of plants and organs of an individual plant. These features affect the subcategories that trichomes are placed into. Some defining features include the following: Unicellular or multicellular Straight (upright with little to no branching), spiral (corkscrew-shaped) or hooked (curved apex) Presence of cytoplasm Glandular (secretory) vs. eglandular Tortuous, simple (unbranched and unicellular), peltate (scale-like), stellate (star-shaped) Adaxial vs. abaxial, referring to whether trichomes are present, respectively, on the upper surface (adaxial) or lower surface (abaxial) of a leaf or other lateral organ. In a model organism, Cistus salviifolius, there are more adaxial trichomes present on this plant because this surface suffers from more ultraviolet (UV), solar irradiance light stress than the abaxial surface. Trichomes can protect the plant from a large range of detriments, such as UV light, insects, transpiration, and freeze intolerance. Aerial surface hairs Trichomes on plants are epidermal outgrowths of various kinds. The terms emergences or prickles refer to outgrowths that involve more than the epidermis. This distinction is not always easily applied (see Wait-a-minute tree). Also, there are nontrichomatous epidermal cells that protrude from the surface, such as root hairs. A common type of trichome is a hair. Plant hairs may be unicellular or multicellular, and branched or unbranched. Multicellular hairs may have one or several layers of cells. Branched hairs can be dendritic (tree-like) as in kangaroo paw (Anigozanthos), tufted, or stellate (star-shaped), as in Arabidopsis thaliana. Another common type of trichome is the scale or peltate hair, that has a plate or shield-shaped cluster of cells attached directly to the surface or borne on a stalk of some kind. Common examples are the leaf scales of bromeliads such as the pineapple, Rhododendron and sea buckthorn (Hippophae rhamnoides). Any of the various types of hairs may be , producing some kind of secretion, such as the essential oils produced by mints and many other members of the family Lamiaceae. Many terms are used to describe the surface appearance of plant organs, such as stems and leaves, referring to the presence, form and appearance of trichomes. Examples include: glabrous, glabrate – lacking hairs or trichomes; surface smooth hirsute – coarsely hairy hispid