Characteristic trichomes, classification and functions

1235
Robert Johnston

The trichomes are structures present in the epidermis of plants. These vary widely in their morphology and can be composed of a single cell or more. The term "trichome" includes hairs, scales, and papillae..

These epidermal extensions prevent desiccation in the plant, regulate gas exchange and act as protection against herbivores and insects. They may also have specialized cells that excrete substances to the outside or, on the contrary, have absorption functions.

Source: I, Paethon [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5)], from Wikimedia Commons

Trichomes are used as a taxonomic characteristic to identify a wide group of angiosperms, verifying the presence or absence of them in the individual or by observing the structure..

Article index

  • 1 Features
  • 2 Classification
    • 2.1 Unicellular
    • 2.2 Multicellular
    • 2.3 Other classifications
  • 3 Functions
    • 3.1 Protection against herbivores
    • 3.2 Avoid water loss
    • 3.3 Protection against high solar radiation
    • 3.4 Secretion
    • 3.5 Fixation
    • 3.6 Water absorption
  • 4 References

Characteristics

In the epidermis of most plants there are extensions called trichomes or hairs. They can be located in all structures and the duration can be very short - called ephemeral trichomes - or it can be the same as all epidermal cells.

Plants can present different types of trichomes or, in some cases, the trichome is particular to the genus or species analyzed, representing a diagnostic character that allows its classification..

For example, in the genre Adesmia belonging to the Fabaceae family, a trichome without glandular properties is observed, formed by three cells; a basal, a short central and a long. The walls are thick and taper at the tip.

Trichomes originate from a process called unequal mitosis, where the smallest cell gives rise to the trichome. In the case of trichomes formed by more than one cell, there are usually periclinal or anticlinal divisions of the cells in the plant epidermis.

Classification

Trichomes are classified according to their morphological characteristics in the following categories:

Unicellular

They are formed by a single cell that is inserted into the epidermis, projecting outwards. In turn, this group of trichomes is divided into papillaries, due to their papilla shape - in the flowers it gives them a texture and an appearance reminiscent of velvet - and into simple or rolled ones. The latter are thin and can be rolled into the apical sections.

They can also branch (in these cases it is a single cell that spreads, no cell division occurs) or take a stellate shape.

Multicellular

Trichomes can also be made up of more than one cell of epidermal origin. Like the unicellular, these trichomes are classified into subcategories depending on their morphology.

We have elongated trichomes, which are made up of several cells arranged in a row. The formation of a foot and a head can be distinguished at the tip, as in the case of the hairs responsible for secreting certain substances, where the secretory cells are located in the head.

If the cells are located one on top of the other, columns with different heights will be obtained. This cellular organization is known as woolly trichomes..

The secretory or glandular trichomes, typical of carnivorous plants, are called pellets. Here there is a cell inside the epidermis, covering this cell we find others that cover it.

Multicellular trichomes can also branch in different planes or organize themselves in a radial arrangement, resembling a star..

The trichomes mentioned are not specific to certain genera or species. A plant can have more than one type of trichome. In other words, they are not mutually exclusive..

Other classifications

In the literature there are other ways to classify trichomes. One of them is to divide them into glandular and non-glandular. Simple, bulbous and rosulate belong to the first group..

The second group, the non-glandular ones, is constituted by the solitary, fasciculate, stellate, multistallate and fused-stellate..

Features

The diversity of functions of trichomes is as diverse as the morphological forms they present. Among the most important we have:

Protection against herbivores

Trichomes appear, for example, in the form of stinging hairs that reduce the rate of predation by animals interested in the plant..

This defense mechanism extends to arthropods, especially phytophagous insects. Some insects go to plants to feed or to lay. Trichomes can prevent these actions, either by trapping the insect or making it difficult to move..

For example, in beans Phaseolus vulgaris It presents varieties with trichomes that are resistant to its predators. Similarly, in potatoes, trichomes prevent predation by beetle larvae..

Avoid loss of water

The presence of trichomes is linked to the environmental conditions that the plant must face. In extreme environments, a significant number of trichomes are usually present.

Trichomes may be present in the vicinity of the stomata, helping to avoid excessive water evaporation.

Protection against high solar radiation

Trichomes also have thermoregulation functions, keeping the temperature of the leaves relatively constant, since they increase light reflection and, as we discussed in the previous point, reduce gas exchange..

Secretion

Trichomes are capable of secreting a wide range of substances, from sugary compounds that attract potential pollinators to highly toxic substances to keep predators away..

Some carnivorous plants secrete the enzymes they need to efficiently digest their prey through trichomes. Trichomes release proteolytic substances that hydrolyze nitrogenous compounds in animals. An example of them is found in the genre Sundew Y Utricularia.

In saline environments, there are trichomes responsible for secreting saline solutions. Likewise, they can secrete essences, such as the typical scents of mint and basil..

Generally the secretions originate in the Golgi apparatus or in the endoplasmic reticulum.

Fixation

Plants that climb and adhere to certain surfaces can do so by means of trichomes, which act as hooks and provide support..

Water absorption

The extensions of the root are known as root hairs. These structures are capable of absorbing a greater amount of water, since they increase the surface of the root..

Plants that live in extreme environments with little availability of water, their roots have a high density of trichomes.

References

  1. Alberts, B., & Bray, D. (2006). Introduction to cell biology. Panamerican Medical Ed..
  2. Blanco, C. A. (2004). The blade: external morphology and anatomy. National University of the Litoral.
  3. Espíndola, C. (Ed.). (2004). Biology practices of multicellular organisms. Pontifical Javeriana University.
  4. Fahn, A. (1967). Plant anatomy. Pergamon Press New York.
  5. Fernández, J. J. L. (2001). Natural forests of Asturias. Oviedo University.
  6. Peña, J. R. A. (2011). Plant histology manual. Editorial Paraninfo.
  7. Rojas, G. V. (2011). General Botany. From the mosses to the trees. EUNED.

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