Trichome: Meaning (information, definition, explanation, facts)

Trichomes, from the Greek meaning "growth of hair", are fine outgrowths or appendages on plants and protists. These are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae.

Algal trichomes

Certain—usually filamentous—algae, have the terminal cell produced into an elongate "hair-like" structure called a trichome.

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-bit climber). Also, there are nontrichomatous epidermal cells that protrude from the surface.

A common type of trichome is a hair. Plant hairs may be unicellular or multicellular, branched or unbranched. Multicellular hairs may have one or several layers of cells. Branched hairs can be dendritic (tree-like), tufted, or stellate (star-shaped).

A common type of trichome is the scale or peltate hair: a plate or shield-shaped cluster of cells attached directly to the surface or borne on a stalk of some kind.

Any of the various types of hairs may be glandular. The cannabis plant produces most of its oils in trichomes.

In describing the surface appearance of plant organs, such as stems and leaves, many terms are used in reference to the presence, form, and appearance of trichomes. The most basic terms used are glabrous—lacking hairs— and pubescent—having hairs. Details are provided by:

• glabrous, glabrate – lacking hairs or trichomes; surface smooth.
• hirsute – coarsely hairy
• hispid – having bristly hairs
• downy – having an almost wool-like covering of long hairs
• pilose – pubescent with long, straight, soft, spreading or erect hairs
• puberulent – minutely pubescent; having fine, short, usually curly, hairs
• pubescent – bearing hairs or trichomes of any type
• strigillose – minutely strigose
• strigose – having straight hairs all pointing in more or less the same direction as along a margin or midrib.
• villosulous – minutely villous
• villous – having long, soft hairs, often curved, but not matted

Hairs on plants are extremely variable in their presence across species, location on plant organs, density (even within a species), and therefore functionality. However, several basic functions or advantages of having surface hairs can be listed. It is likely that in many cases, hairs interfere with the feeding of at least some small herbivores and, depending upon stiffness and irritability to the "palate", large herbivores as well. Hairs on plants growing in areas subject to frost keep the frost away from the living surface cells. In windy locations, hairs break-up the flow of air across the plant surface, reducing evaportation. Dense coatings of hairs reflect solar radiation, protecting the more delicate tissues underneath in hot, dry, open habitats. And in locations where much of the available moisture comes from cloud drip, hairs appear to enhance this process.

Root hairs

Root hairs are tubular outgrowths of the epidermal cells of a plant root. That is, root hairs are lateral extensions of a single cell and only rarely branched. Root hairs vary between 5 and 17 micrometres in diameter, and 80 to 1,500 micrometres in length (Dittmar, cited in Esau, 1965). The ability of the root epidermis to absorb water from the surrounding soil is the same with or without root hairs, but the root hairs enlarge and extend the water absorbing surface.

• Esau, K. 1965. Plant Anatomy, 2nd Edition. John Wiley & Sons. 767 pp.