Protection tissues are located in the more external part of the plant organs and are usually in contact with the environment. There are two protection tissues: epidermis is found in organs with primary growth and the periderm in organs with secondary growth.
During primary growth, epidermis is the protection tissue of stem, root, leaf, flower and seeds. It is the most external part of the plant, but it is not present in the apical tip of roots, under the calyptra, and it is not differentiated in aerial meristems. Epidermis is laid out by the external layer, known as protoderm, of apical meristem. Although epidermis is protection tissue, it develops other essential functions such as respiration, transpiration, gas exchange, storing, secretion, repels herbivores, attract pollinating insects, water absorption by the roots, and more.
Epidermis is commonly formed by a single row of cells. Some exceptions are found with stratified organization, as is the case of certain aerial roots or oleander leaves. Stratified epidermis is called velamen. Epidermis is composed by the epidermal cell proper and by other less abundant cell types.
Epidermal cell proper is not highly specialized. They are tightly joined, without almost no intercellular spaces. Morphology and size are variable depending of the structure where they are located. For example, they are elongated in stem, but start-like in same leaves. They contain plastids, but not chloroplasts, a large vacuole, a well-developed endoplasmic reticulum and Golgi apparatus. Generally, the cell wall is only primary cell wall, although with variable thickness. A secondary cell wall is observed in just a few cases, such as seeds. Lignin is no frequent in this secondary cell wall, but it can be found in some gymnosperm leaves. Epidermal cells show primary pit fields and plasmodesmata.
In the aerial parts, epidermal cells synthesize and secrete a lipidic impermeable substance called cutin, which is deposited outside of the cell wall to form a continuous layer called cuticle. In the root epidermis, as well as in the root hairs, the secreted substance is suberin. The cuticle thickness of a epidermal cell varies depending on the function and cellular localization. Over the cuticle, other lipid substances such as certain waxes are deposited, which can be in crystalline form or dissolved as oils. In the free wall of the epidermal cell there are microchannels allowing communication between cytoplasm and cuticle, that allow the secretion of substances.
Among epidermal cells proper, there are other cell types that sometimes are used as taxonomic character. Thus, some epidermal cells specialize in storing water, as do bulliform cells in the leaves of grasses and other monocotyledon plants. They are much bigger than the rest of the epidermal cells, because of their high water content and scarce cuticle. They seem to be involved in the folding and unfolding mechanism of leaf during transpiration.
Stomata are found in the epidermis. Stomatal guard cells are specialized epidermal cells organized to allow an opening, known as ostiole, by which the internal tissue of plant and the external environment are communicated. There is an air chamber under the ostiole called substomatal chamber. Both structures, along with the guard cells, form what is typically called a stoma. The guard cells are usually kidney-shaped, have chloroplasts and a non-uniformly thickened cell wall that make possible turgidity to change cell morphology, and thus increase or decrease the diameter of ostiole.
Trichomes are elongated epidermal cells, with major axis usually perpendicular to the epidermal surface. They can be protective or glandular (see next section). The protective trichomes can be unicellular or multicellular. They protect against intense light and create a surface boundary air layer over the epidermis that allows a less fluctuating environment. These trichomes are particularly abundant in young plant structures, and may disappear when they become adults.
Periderm is found in those parts of the roots and shoots with secondary growth. It is produced by cork cambium, also known as phellogen meristem. Phellogen is secondary meristem coming from dedifferentiation of parenchyma or collenchyma cells located beneath the epidermis, and it may lead to a continuous or discontinuous meristem. Phellogen cells divide, with a periclinal plane, resulting rows of cells distributed unevenly toward surface or toward the interior of the plant organ. The outermost layers are more numerous and their cells contain suberin and die forming cork. Inward cells are alive and arranged in a stacks, forming the phelloderm.
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Atlas of Plant and Animal Histology
Dep. of Functional Biology and Health Sciences.
Faculty of Biology.
University of Vigo