Epidermis (botany) 2959657 222767163 2008-07-01T00:36:14Z Work permit 4066914 Reverted edits by [[Special:Contributions/65.34.134.71|65.34.134.71]] to last version by FatalError (using [[WP:HG|Huggle]]) [[Image:Stem-histology-cross-section-tag.svg|thumb|right|250px|Cross-section of a [[flax]] plant stem:<br> 1. [[:en:pith|Pith]],<br> 2. [[:en:protoxylem|Protoxylem]],<br> 3. [[:en:xylem|Xylem]] I,<br> 4. [[:en:phloem|Phloem]] I,<br> 5. [[:en:Sclerenchyma|Sclerenchyma]] ([[:en:bast fibre|bast fibre]]),<br> 6. [[:en:cortex|Cortex]],<br> 7. [[:en:epidermis (botany)|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 [[Plant stem|stems]] and [[root]]s. 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. The epidermis is usually [[transparency (optics)|transparent]] (epidermal cells lack chloroplasts) and coated on the outer side with a waxy [[Plant cuticle|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. 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 [[monocot]]s than in those of [[dicot]]s. [[Trichome]]s 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. In plants with [[secondary growth]], the epidermis of roots and stems is usually replaced by a periderm through the action of a [[cork cambium]]. ==Guard cells== [[Image:Tomato leaf stomate 1.jpg|240px|thumb|Stoma in a [[tomato]] leaf (microscope image)]] {{Main|Stoma}} The leaf and stem epidermis is covered with [[pore]]s 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 [[Poaceae|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. 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== [[Image:Leaf_epidermis.jpg|[[Scanning electron microscope]] image of ''[[Nicotiana alata]]'' leaf's epidermis, showing [[trichome]]s (hair-like appendages) and [[stoma]]ta (eye-shaped slits, visible at full resolution)|thumb]] The plant epidermis consists of three main cell types: [[pavement cell]]s, [[guard cell]]s and their subsidiary cells that surround the [[stomata]] and [[trichome]]s, otherwise known as leaf hairs. The epidermis of petals also form a variation of trichomes called [[conical cell]]s. 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. Trichomes develop at a distinct phase during the actual [[leaf]] development, under the control of two major trichome specification [[gene]]s: ''TTG'' and ''GL1''. The process may be controlled by the [[plant hormone]]s [[gibberellin]]s, 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. ''[[Arabidopsis]]'' uses the products of [[inhibition|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. As previously mentioned, [[conical cell]]s are a form of trichome that occurs on the [[petal]]s of [[flower]]s. Expression of the gene ''MIXTA'', or its [[Analog (chemistry)|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]]. Stomatal pattering is a much more controlled process, as the stoma effect the plants water retention and [[Plant respiration|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. 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 cell]]s which provide a supporting role for the guard cells. Stomata begin as stomatal meristemoids. The process varies between [[dicot]]s and [[monocot]]s. Spacing is thought to be essentially [[random]] in dicots though [[mutant]]s do show it is under some form of genetic control, but it is more controlled in monocots, where stomata arise from specific [[asymmetric cell division|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. 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. 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]]. ==References== * [http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookPLANTANAT.html| Plants and their structure] * [http://biology.about.com/library/weekly/aa030101a.htm| Plant tissue systems] == See also == * [[Bark]] * [[Cork cambium]] * [[Periderm]] {{biological tissue}} [[Category:Plant anatomy]] [[Category:Tissues]] [[bg:Епидермис (ботаника)]] [[cs:Pokožka (rostliny)]] [[fr:Épiderme (botanique)]] [[it:Epidermide (botanica)]] [[pl:Skórka]]