Trichome

From Wikipedia, the free encyclopedia
  (Redirected from Trichomes)
Jump to: navigation, search
Flower bud of a Capsicum pubescens plant, with many trichomes

Trichomes (/ˈtrkmz/ or /ˈtrɪkmz/), from the Greek τρίχωμα (trikhōma) meaning "hair", are fine outgrowths or appendages on plants, algae and certain protists. They are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae. A covering of any kind of hairs on a plant is an indumentum, and the surface bearing them is said to be pubescent.

Algal trichomes[edit]

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. Cyanobacteria trichomes may be unsheathed, as in Oscillatoria, or sheathed, as in Calothrix.[1] These structures play an important role in preventing soil erosion, particularly in cold desert climates.[citation needed] The filamentous sheaths form a persistent sticky network that helps maintain soil structure.

Plant trichomes[edit]

Sticky trichomes of a carnivorous plant, Drosera capensis with a trapped insect
Trichomes on Cannabis, rich in cannabinoids.
Trichomes on the surface of a Solanum scabrum leaf
Trichomes on the petiole of a Solanum quitoense leaf
Epidermis of Arabidopsis thaliana with a trichome.
An unopened Streptocarpus hybrid flower, showing star-shaped form and trichomes

Aerial surface hairs[edit]

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.

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) 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 glandular, producing some kind of secretion, such as the essential oils produced by mints and many other members of the family Lamiaceae.

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
  • articulate – simple pluricellular-uniseriate 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
  • tomentellous – minutely tomentose
  • tomentose – covered with dense, matted, woolly hairs
  • villosulous – minutely villous
  • villous – having long, soft hairs, often curved, but not matted

Hairs on plants are extremely variable in their presence across species and even within a species, such as their location on plant organs, size, density, 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 transpiration. Dense coatings of hairs reflect sunlight, protecting the more delicate tissues underneath in hot, dry, open habitats. In addition, in locations where much of the available moisture comes from fog drip, hairs appear to enhance this process.

Root hairs[edit]

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.[clarification needed] 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 two to three 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 may cause plants to wilt.

Significance for taxonomy[edit]

The type, presence and absence and location of trichomes are important diagnostic characters in plant identification and plant taxonomy.[2] In forensic examination, plants such as Cannabis sativa can be identified by microscopic examination of the trichomes.[3][4] Although trichomes are rarely found preserved in fossils, trichome bases are regularly found, and in some cases their cellular structure is an important for identification.

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

Other human uses[edit]

Bean leaves have been used historically to trap bedbugs in houses in Eastern Europe. The trichomes on the bean leaves capture the insects by impaling their feet (tarsi). The leaves would then be destroyed.[5]

Defense[edit]

Plants may use trichomes in order to deter herbivore attack via physical and/or chemical means. However, some organisms have developed mechanisms to resist the effects of trichomes. The larvae of Heliconius charithonia, for example, are able to physically free themselves from trichomes, are able to bite off trichomes, and are able to form silk blankets in order to navigate the leaves better.[6]

See also[edit]

References[edit]

  1. ^ "Identify That Alga". Retrieved September 20, 2013. 
  2. ^ Davis, P.H.; Heywood, V.H. (1963). Principles of angiosperm taxonomy. Princeton, New Jersey: Van Nostrandpage. p. 154. 
  3. ^ Bhatia, R.Y.P.; Raghavan, S.; Rao, K.V.S.; Prasad, V.N. (1973). "Forensic examination of leaf and leaf fragments in fresh and dried conditions.". Journal of the Forensic Science Society 13 (3): 183–190. doi:10.1016/S0015-7368(73)70794-5. 
  4. ^ United Nations Office on Drugs and Crime (2009). Recommended Methods for the Identification and Analysis of Cannabis and Cannabis Products (Revised and updated). New York: United Nations. pp. 30–32. ISBN 9789211482423. 
  5. ^ Szyndler, M.W.; Haynes, K.F.; Potter, M.F.; Corn, R.M.; Loudon, C. (2013). "Entrapment of bed bugs by leaf trichomes inspires microfabrication of biomimetic surfaces". Journal of The Royal Society Interface 10 (83). doi:10.1098/rsif.2013.0174. ISSN 1742-5662. 
  6. ^ Cardoso, Márcio Z. "Ecology, Behavior and Binomics: Herbivore Handling of a Plant's Trichome: The Case of Heliconius Charithonia (L.) (Lepidoptera:Nymphalidae) and Passiflora Lobata (Kilip) Hutch. (Passifloraceae)." Neotropical Entomology 37.3 (2008): 247-52. Web.
  • Esau, K. 1965. Plant Anatomy, 2nd Edition. John Wiley & Sons. 767 pp.