Trichome

Bud and scape of a Stylidium species, displaying the trichomes that can trap and kill insects.

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 1 Algal trichomes 2 Plant trichomes 2.1 Aerial surface hairs 2.2 Root hairs 3 Trichomes and cannabis 3.1 Types 4 See also 5 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

Epiderm of Arabidopsis thaliana with a trichome.

Flower bud of a Capsicum pubescens plant, with lots of trichomes.

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.

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.

Trichomes and cannabis

See also: Kief

The evolution of the cannabis plant\'s unique trichome is debated. Most likely, the cannabis plant developed such large and numerous trichomes because it is a wind pollinated plant, and trichomes of THC are very sticky, thus aiding in pollen collection. This can be observed most effectively in female cannabis plants that have been deprived of male pollen. In order to increase her chances of fertilization via windborne pollen, the female cannabis plant devotes her energy into calyx and trichome production, creating larger, stickier buds. 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. There are two layers that make up the trichome and only one of them is psychoactive. The disc cell is on the bottom and it carries the phenols and turpenes, which are not psychoactive. And on top of that is the secretory cavity, which contains the psychoactive cannabinoids. Between these two layers is the disc cell, which is a fibrous mat that separates the two. Phenols and turpenes are precursors to cannabinoids. They become different cannabinoids when they travel through the disc cell in following order CBC to CBG to CBD to THC.

Types

The Capitate-Stalked trichomes of the cannabis plant.

• 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 convex 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?

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