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'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.

Contents

Algal trichomes

Plant trichomes

Aerial surface hairs

Root hairs

Trichomes and cannabis

Types

See also

References

Algal trichomes

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

Plant trichomes

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'.
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 evaporation. 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, the rhizoids of many vascular plants, are tubular outgrowths of 'trichoblasts', the hair-forming cells on the epidermis of a plant root. That is, root hairs are lateral extensions of a single cell and only rarely branched. Just prior to the root hair development, there is a point of elevated phosphorylase activity.
Root hairs vary between 5 and 17 micrometres in diameter, and 80 to 1,500 micrometres in length (Dittmar, cited in Esau, 1965).
Root hairs can survive for 2 to 3 weeks and then die off. At the same time new root hairs are continually being formed at the top of the root. This way, the root hair coverage stays the same.
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.
It is therefore understandable that repotting must be done with care, because the root hairs are being pulled off for the most part. This is why planting out, leaves the plant withered for some time.
There are also plants lacking root hairs, mostly water plants or marsh plants, such as duckweeds (family Lemnaceae), and gymnosperms.

Trichomes and cannabis

Trichomes evolved on cannabis to prevent herbivory/parasitism by small insects. In the wild, insects landing on a cannabis leaf become adhered to the trichomes causing death or lost time (see sundew) and in addition will absorb the cannabinoids and, so intoxicated, experience increased rates of predation and loss of will to escape. However, the intercession of humans in the natural selection process has rapidly accelerated the favoring of genotypes that produce copious amounts of THC-laden trichomes. In addition, THC has very high UV-B absorption properties, thus cannabis evolution may have favored the evolution of genotypes that produced these THC laden capitate-stalked trichomes as a built in 'sun-screen' for protection against UV-B light rays. The dried trichomes of cannabis used for smoking purposes is called kief.

Types

★ 'Bulbous': The bulbous type is the smallest (15 to 30 micrometre). From one to four cells make up the "foot" and "stalk," and one to four cells make up the "head" of the gland. Head cells secrete a resin - presumably cannabinoids, and related compounds which accumulate between the head cells and the cuticle. When the gland matures, a nipple-like protrusion may form on the membrane from the pressure of the accumulating resin. The bulbous glands are found scattered about the surfaces of the above-ground plant parts.

★ 'Capitate-Sessile': The second type of gland is much larger and is more numerous than the bulbous glands. They are called capitate, which means having a globular-shaped head. On immature plants, the heads lie flush, appearing not to have a stalk and are called capitate sessile. They actually have a stalk that is one cell high, although it may not be visible beneath the globular head. The head is composed of usually eight, but up to 16 cells, that form a rosette. These cells secrete cannabinoids, and related compounds which accumulate between the rosette and its outer membrane. This gives it a spherical shape. The gland measures from 25 to 100 micrometres across.

★ 'Capitate-Stalked': Cannabinoids are most abundant in the capitate-stalked gland which consists of a tier of secretory disc cells subtending a large non-cellular secretory cavity. During flowering the capitate glands that appear on the newly formed plant parts take on a third form. Some of the glands are raised to a height of 150 to 500 micrometres when their stalks elongate. These capitate-stalked glands appear during flowering and form their densest cover on the female flower bracts. They are also highly concentrated on the small leaves that accompany the flowers. The male flowers have stalked glands on the sepals, but they are smaller and less concentrated than on the female bracts. Male flowers form a row of very large capitate glands along the opposite sides of anthers.

See also

★ Seta

References

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

★ What are Trichomes?