Cross-section of a flax plant stem:
1. Pith,
2. Protoxylem,
3. Xylem I,
4. Phloem I,
5. Sclerenchyma (bast fibre),
6. Cortex,
7. Epidermis

The epidermis is the outer single-layered group of cells covering a plant, especially the leaf and young tissues of a vascular plant including stems and roots. Epidermis and periderm are the dermal tissues in vascular plants. The epidermis forms the boundary between the plant and the external world. The epidermis serves several functions: protection against water loss, regulation of gas exchange, secretion of metabolic compounds, and (especially in roots) absorption of water and mineral nutrients. The epidermis of most leaves shows dorsoventral anatomy: the upper (adaxial) and lower (abaxial) surfaces have somewhat different construction and may serve different functions. For other uses, see Flax (disambiguation). ... The centre dark spot (about 1 mm diameter) in this yew wood is the pith Elderberry shoot cut longitudinally to show the broad, solid pith (rough-textured, white) inside the wood (smooth, yellow-tinged). ... In vascular plants, xylem is one of the two types of transport tissue, phloem being the other one. ... In vascular plants, phloem is the living tissue that carries organic nutrients, particularly sucrose, a sugar, to all parts of the plant where needed. ... Sclerenchyma is a supporting tissue. ... Bast fibre (fiber) or skin fibre is fibre collected from the Phloem (the inner bark or the skin) or bast surrounding the stem of a certain mainly dicotyledonic plant. ... Look up cortex in Wiktionary, the free dictionary. ... For other uses, see Plant (disambiguation). ... Look up foliage in Wiktionary, the free dictionary. ... Divisions Non-seed-bearing plants †Rhyniophyta †Zosterophyllophyta Lycopodiophyta †Trimerophytophyta Pteridophyta Ophioglossophyta Superdivision Spermatophyta †Pteridospermatophyta Pinophyta Cycadophyta Ginkgophyta Gnetophyta Magnoliophyta The vascular plants, tracheophytes or higher plants are plants in the kingdom Plantae that have specialized tissues for conducting water, minerals, and photosynthetic products through the plant. ... Stem showing internode and nodes plus leaf petiole and new stem rising from node. ... For other uses, see Root (disambiguation). ... It has been suggested that this article or section be merged into Bark. ...

The epidermis is usually transparent (epidermal cells lack chloroplasts) and coated on the outer side with a waxy cuticle that prevents water loss. The cuticle may be thinner on the lower leaf epidermis than on the upper epidermis; and is thicker on leaves from dry climates as compared with those from wet climates. Transparent glass ball In optics, transparency is the property of allowing light to pass. ... Plant cuticles are a protective waxy covering produced only by the epidermal cells (Kolattukudy, 1996) of leaves, young shoots and all other aerial plant organs. ...

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 of monocots than in those of dicots. Orders Base Monocots: Acorus Alismatales Asparagales Dioscoreales Liliales Pandanales Family Petrosaviaceae Commelinids: Arecales Commelinales Poales Zingiberales Family Dasypogonaceae Monocotyledons or monocots are a group of flowering plants usually ranked as a class and once called the Monocotyledoneae. ... Orders see text Dicotyledons or dicots are flowering plants whose seed contains two embryonic leaves or cotyledons. ...

Trichomes or hairs grow out from the epidermis in many species. In root epidermis, epidermal hairs, termed root hairs are common and are specialized for absorption of water and mineral nutrients. Trichomes, from the Greek meaning growth of hair, are fine outgrowths or appendages on plants and protists. ...

In plants with secondary growth, the epidermis of roots and stems is usually replaced by a periderm through the action of a cork cambium. In vascular plants, secondary growth or, perhaps more accurately, secondary thickening is the result of the activity of the vascular cambium. ... Cork cambium is a tissue found in many vascular plants as part of the periderm. ...

Guard cells

Stoma in a tomato leaf (microscope image)

Main article: Stoma

The leaf and stem epidermis is covered with pores called stomata (sing., stoma), part of a stoma complex consisting of a pore surrounded on each side by chloroplast-containing guard cells, and two to four subsidiary cells that lack chloroplasts. The stoma 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 many grasses, often have roughly equal numbers of stomata on both surfaces. The number of stomata varies from about 1,000 to over 100,000 per square centimeter of leaf surface. For other uses, see Tomato (disambiguation). ... Stoma of a leaf under a microscope. ... A pore, in general, is some form of opening, usually very small. ... This is not about surgically created bowel openings; see stoma (medicine) In botany, a stoma (also stomate; plural stomata) is a tiny opening or pore, found mostly on the undersurface of a plant leaf, and used for gas exchange. ... Subfamilies There are 7 subfamilies: Subfamily Arundinoideae Subfamily Bambusoideae Subfamily Centothecoideae Subfamily Chloridoideae Subfamily Panicoideae Subfamily Pooideae Subfamily Stipoideae The true grasses are monocotyledonous plants (Class Liliopsida) in the Family Poaceae, also known as Gramineae. ...

The stoma is bounded by two guard cells. The guard cells differ from the epidermal cells in the following aspects:

• The guard cells are bean-shaped in surface view, while the epidermal cells are irregular in shape
• The guard cells contain chloroplasts, so they can manufacture food by photosynthesis (The epidermal cells do not contain chloroplasts)
• Guard Cells are the only epidermal cells that can make sugar. According to one theory, in sunlight the concentration of potassium ions (K+) increases in the guard cells. This, together with the sugars formed, lowers the water potential in the guard cells. As a result, water from other cells enter the guard cells by osmosis so they swell and become turgid. Because the guard cells have a thicker cellulose wall on one side of the cell, i.e. the side around the stomatal pore, the swollen guard cells become curved and pull the stomata open.

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 vapour escaping from the leaf.

Cell differentiation in the epidermis

The plant epidermis consists of three main cell types: pavement cells, guard cells and their subsidiary cells that surround the stomata and trichomes, otherwise known as leaf hairs. The epidermis of petals also form a variation of trichomes called conical cells. These cells all develop from the pavement cells, which make up the majority of the plants surface cells. In short, cellular differentiation of the epidermal cells is controlled by two major factors: genetics and environmental conditions. For surgically created body openings, see stoma (medicine). ... This is not about surgically created bowel openings; see stoma (medicine) In botany, a stoma (also stomate; plural stomata) is a tiny opening or pore, found mostly on the undersurface of a plant leaf, and used for gas exchange. ... Trichomes, from the Greek meaning growth of hair, are fine outgrowths or appendages on plants and protists. ... This article is about the general scientific term. ...

Trichomes develop at a distinct phase during the actual leaf development, under the control of two major trichome specification genes: TTG and GL1. The process may be controlled by the plant hormones gibberellins, and even if not completely controlled, gibberellins certainly have an effect on the development of the leaf hairs. GL1 causes endoreplication, the replication of DNA without subsequent cell 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. Look up foliage in Wiktionary, the free dictionary. ... For other uses, see Gene (disambiguation). ... Plant hormones (also known as plant growth regulators (PGRs) and phytohormones) are chemicals that regulate a plants growth. ... GA1 GA3 ent-Gibberellane ent-Kauren Gibberellins (GAs) are plant hormones involved in promotion of stem elongation, mobilization of food reserves in seeds and other processes. ... The structure of part of a DNA double helix Deoxyribonucleic acid, or DNA, is a nucleic acid molecule that contains the genetic instructions used in the development and functioning of all known living organisms. ... This does not adequately cite its references or sources. ...

Arabidopsis uses the products of inhibitory genes to control the patterning of trichomes, such as TTG and TRY. The products of these genes will diffuse into the lateral cells, preventing them from forming trichomes and in the case of TRY promoting the formation of pavement cells. Species See text. ... An inhibitor is a type of effector that decreases or prevents a chemical reaction. ... The term lateral can refer to: an anatomical definition of direction. ...

As previously mentioned, conical cells are a form of trichome that occurs on the petals of flowers. Expression of the gene MIXTA, or its analogue in other species, later in the process of cellular differentiation will cause the formation of conical cells over trichomes. MIXTA is a transcription factor. It has been suggested that Corolla be merged into this article or section. ... For other uses, see Flower (disambiguation). ... For the Analog Science Fiction and Science Fact publication, see Astounding Magazine. ... Differentiation can mean the following: In biology: cellular differentiation; evolutionary differentiation; In mathematics: see: derivative In cosmogony: planetary differentiation Differentiation (geology); Differentiation (logic); Differentiation (marketing). ... In molecular biology, a transcription factor is a protein that binds DNA at a specific promoter or enhancer region or site, where it regulates transcription. ...

Stomatal pattering is a much more controlled process, as the stoma effect the plants water retention and respiration 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 then other epidermal cell types. Plant respiration is the oxidation of certain substrates by enzymes, leading to a release of carbon dioxide. ...

Stomata are holes 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 by subsidiary cells which provide a supporting role for the guard cells.

Stomata begin as stomatal meristemoids. The process varies between dicots and monocots. Spacing is thought to be essentially random in dicots though mutants do show it is under some form of genetic control, but it is more controlled in monocots, where stomata arise from specific asymmetric divisions of protodermal 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. Orders see text Dicotyledons or dicots are flowering plants whose seed contains two embryonic leaves or cotyledons. ... Orders Base Monocots: Acorus Alismatales Asparagales Dioscoreales Liliales Pandanales Family Petrosaviaceae Commelinids: Arecales Commelinales Poales Zingiberales Family Dasypogonaceae Monocotyledons or monocots are a group of flowering plants usually ranked as a class and once called the Monocotyledoneae. ... Random redirects here. ... This article is about biological mutants. ...

Because stomata play such an important role in the plants survival, collecting information of there differentiation if 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 and FLP is thought to be involved in preventing further division of the guard cells once they are formed.

Environmental conditions affect the development of stomata, in particular their density on the leaf surface. It is thought that plant hormones, such as ethylene and cytokines, control the stomata’s 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. For other uses, see Density (disambiguation). ... Ethylene (or IUPAC name ethene) is the chemical compound with the formula C2H4. ... Cytokines are small protein molecules that are the core of communication between immune system cells, and even between immune system cells and cells belonging to other tissue types. ...

Stomatal cells only occur on the leaf epidermis, and it is thought that inhibitory signals must occur on other parts of the plants epidermis to prevent stomatal formation there. These signals could be hormonal, or perhaps gene products transmitted from underlying tissues via the plasmodesmata. Plasmodesmata (Singular, plasmodesma) are small cell junctions in a plant cell which connect the cytoplasm of adjacent plant cells, forming a circulatory and communication system connecting the cells in plant tissue. ...

References

• Plants and their structure
• Plant tissue systems

See also

• Bark
• Cork cambium
• Periderm