Theepidermis (from theGreekἐπιδερμίς, meaning "over-skin") is a single layer of cells that covers theleaves,flowers,roots andstems ofplants. It forms a boundary between the plant and the external environment. The epidermis serves several functions: it protects against water loss, regulatesgas exchange, secretes metabolic compounds, and (especially in roots) absorbs water and mineral nutrients. The epidermis of most leaves showsdorsoventral anatomy: the upper (adaxial) and lower (abaxial) surfaces have somewhat different construction and may serve different functions. Woody stems and some other stem structures such aspotatotubers produce a secondary covering called theperiderm that replaces the epidermis as the protective covering.
The epidermis is the outermost cell layer of the primary plant body. In some older works the cells of the leaf epidermis have been regarded as specializedparenchyma cells,[1] but the established modern preference has long been to classify the epidermis asdermal tissue,[2] whereas parenchyma is classified asground tissue.[3] The epidermis is the main component of the dermal tissue system of leaves (diagrammed below), and also stems, roots, flowers, fruits, and seeds; it is usuallytransparent (epidermal cells have fewer chloroplasts or lack them completely, except for the guard cells.)
The cells of the epidermis are structurally and functionally variable. Most plants have an epidermis that is a single cell layer thick. Some plants likeFicus elastica andPeperomia, which have a periclinal cellular division within the protoderm of the leaves, have an epidermis with multiple cell layers. Epidermal cells are tightly linked to each other and provide mechanical strength and protection to the plant. Particularly, wavypavement cells are suggested to play a pivotal role in preventing or guiding cracks in the epidermis.[4] The walls of the epidermal cells of the above-ground parts of plants containcutin, and are covered with acuticle. The cuticle reduces water loss to the atmosphere, it is sometimes covered withwax in smooth sheets, granules, plates, tubes, or filaments. The wax layers give some plants a whitish or bluish surface color. Surface wax acts as a moisture barrier and protects the plant from intense sunlight and wind.[5]
The epidermal tissue includes several differentiated cell types: epidermal cells,guard cells, subsidiary cells, and epidermal hairs (trichomes). The epidermal cells are the most numerous, largest, and least specialized. These are typically more elongated in the leaves ofmonocots than in those ofdicots.
Trichomes or hairs grow out from the epidermis in many species. In the root epidermis, epidermal hairs termedroot hairs are common and are specialized for the absorption of water and mineral nutrients.
In plants withsecondary growth, the epidermis of roots and stems is usually replaced by a periderm through the action of acork cambium.
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The leaf and stem epidermis is covered with pores calledstomata (sing; stoma), part of astoma complex consisting of a pore surrounded on each side by chloroplast-containingguard cells, and two to foursubsidiary cells that lack chloroplasts. The stomata complex regulates the exchange of gases and water vapor between the outside air and the interior of the leaf. Typically, the stomata are more numerous over the abaxial (lower) epidermis of the leaf than the (adaxial) upper epidermis. An exception is floating leaves where most or all stomata are on the upper surface. Vertical leaves, such as those of manygrasses, often have roughly equal numbers of stomata on both surfaces. The stoma is bounded by two guard cells. The guard cells differ from the epidermal cells in the following aspects:
At night, the sugar is used up and water leaves the guard cells, so they become flaccid and the stomatal pore closes. In this way, they reduce the amount of water vapor escaping from the leaf.
The plant epidermis consists of three main cell types:pavement cells,guard cells and their subsidiary cells that surround thestomata andtrichomes, otherwise known as leaf hairs. The epidermis of petals also form a variation of trichomes calledconical cells.[6]
Trichomes develop at a distinct phase duringleaf development, under the control of two major trichome specificationgenes:TTG andGL1. The process may be controlled by theplant hormonesgibberellins, and even if not completely controlled, gibberellins certainly have an effect on the development of the leaf hairs.GL1 causesendoreplication, the replication ofDNA without subsequentcell division as well as cell expansion.GL1 turns on the expression of a second gene for trichome formation,GL2, which controls the final stages of trichome formation causing the cellular outgrowth.
Arabidopsis thaliana uses the products ofinhibitory genes to control the patterning of trichomes, such asTTG andTRY. The products of these genes will diffuse into thelateral cells, preventing them from forming trichomes and in the case ofTRY promoting the formation of pavement cells.
Expression of the geneMIXTA, or itsanalogue in other species, later in the process ofcellular differentiation will cause the formation of conical cells over trichomes.MIXTA is atranscription factor.
Stomatal patterning is a much more controlled process, as the stoma affects the plant's water retention andrespiration capabilities. As a consequence of these important functions, differentiation of cells to form stomata is also subject to environmental conditions to a much greater degree than other epidermal cell types.
Stomata are pores in the plant epidermis that are surrounded by two guard cells, which control the opening and closing of the aperture. These guard cells are in turn surrounded bysubsidiary cells which provide a supporting role for the guard cells.
Stomata begin as stomatal meristemoids.[clarification needed] The process differs betweendicots andmonocots. Spacing is thought to be essentiallyrandom in dicots thoughmutants do show it is under some form of genetic control, but it is more controlled in monocots, where stomata arise from specificasymmetric divisions of protoderm cells. The smaller of the two cells produced becomes the guard mother cells. Adjacent epidermal cells will also divide asymmetrically to form the subsidiary cells.
Because stomata play such an important role in the plants' survival, collecting information on their differentiation is difficult by the traditional means of genetic manipulation, as stomatal mutants tend to be unable to survive. Thus the control of the process is not well understood. Some genes have been identified.TMM is thought to control the timing of stomatal initiation specification andFLP is thought to be involved in preventing the further division of the guard cells once they are formed.
Environmental conditions affect the development of stomata, in particular, theirdensity on the leaf surface. It is thought that plant hormones, such asethylene andcytokines, control the stomatal developmental response to the environmental conditions. Accumulation of these hormones appears to cause increased stomatal density such as when the plants are kept in closed environments.
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