Plant cells are thecells present ingreen plants, photosyntheticeukaryotes of the kingdomPlantae. Their distinctive features include primary cell walls containing cellulose, hemicelluloses and pectin, the presence of plastids with the capability to perform photosynthesis and store starch, a largevacuole that regulates turgor pressure, the absence offlagella orcentrioles, except in the gametes, and a unique method of cell division involving the formation of a cell plate orphragmoplast that separates the new daughter cells.
Plant cells differentiate from undifferentiatedmeristematic cells (analogous to the stem cells of animals) to form the major classes of cells and tissues ofroots,stems,leaves,flowers, and reproductive structures, each of which may be composed of several cell types.
Parenchyma cells are living cells that have functions ranging from storage and support tophotosynthesis (mesophyll cells) and phloem loading (transfer cells). Apart from the xylem and phloem in their vascular bundles, leaves are composed mainly of parenchyma cells. Some parenchyma cells, as in the epidermis, are specialized for light penetration and focusing or regulation ofgas exchange, but others are among the least specialized cells in plant tissue, and may remaintotipotent, capable of dividing to produce new populations of undifferentiated cells, throughout their lives.[17] Parenchyma cells have thin, permeable primary walls enabling the transport of small molecules between them, and their cytoplasm is responsible for a wide range of biochemical functions such asnectarsecretion, or the manufacture ofsecondary products that discourageherbivory. Parenchyma cells that contain many chloroplasts and are concerned primarily with photosynthesis are calledchlorenchyma cells. Chlorenchyma cells are parenchyma cells involved in photosynthesis.[18] Others, such as the majority of the parenchyma cells inpotatotubers and theseedcotyledons oflegumes, have a storage function.
Collenchyma cells are alive at maturity and have thickened cellulose cell walls.[19] These cells mature from meristem derivatives that initially resemble parenchyma, but differences quickly become apparent. Plastids do not develop, and the secretory apparatus (ER andGolgi) proliferates to secrete additional primary wall. The wall is most commonly thickest at the corners, where three or more cells come in contact, and thinnest where only two cells come in contact, though other arrangements of the wall thickening are possible.[19]Pectin andhemicellulose are the dominant constituents of collenchyma cell walls ofdicotyledonangiosperms, which may contain as little as 20% of cellulose inPetasites.[20] Collenchyma cells are typically quite elongated, and may divide transversely to give a septate appearance. The role of this cell type is to support the plant in axes still growing in length, and to confer flexibility and tensile strength on tissues. The primary wall lacks lignin that would make it tough and rigid, so this cell type provides what could be called plastic support – support that can hold a young stem or petiole into the air, but in cells that can be stretched as the cells around them elongate. Stretchable support (without elastic snap-back) is a good way to describe what collenchyma does. Parts of the strings in celery are collenchyma.
Sclerenchyma is a tissue composed of two types of cells,sclereids andfibres that have thickened,lignified secondary walls[19]: 78 laid down inside of theprimary cell wall. The secondary walls harden the cells and make them impermeable to water. Consequently, sclereids and fibres are typically dead at functional maturity, and thecytoplasm is missing, leaving an empty central cavity.Sclereids or stone cells, (from the Greek skleros,hard) are hard, tough cells that give leaves or fruits a gritty texture. They may discourage herbivory by damaging digestive passages in small insect larval stages. Sclereids form the hard pit wall of peaches and many other fruits, providing physical protection to the developing kernel.Fibres are elongated cells with lignified secondary walls that provide load-bearing support and tensile strength to the leaves and stems of herbaceous plants. Sclerenchyma fibres are not involved in conduction, either of water and nutrients (as in thexylem) or of carbon compounds (as in thephloem), but it is likely that they evolved as modifications of xylem and phloem initials in early land plants.
Xylem is a complex vascular tissue composed of water-conductingtracheids orvessel elements, together with fibres and parenchyma cells. Tracheids[21] are elongated cells with lignified secondary thickening of the cell walls, specialised for conduction of water, and first appeared in plants during their transition to land in theSilurian period more than 425 million years ago (seeCooksonia). The possession of xylem tracheids defines thevascular plants or Tracheophytes. Tracheids are pointed, elongated xylem cells, the simplest of which have continuous primary cell walls and lignified secondary wall thickenings in the form of rings, hoops, or reticulate networks. More complex tracheids with valve-like perforations calledbordered pits characterise the gymnosperms. Theferns and otherpteridophytes and thegymnosperms have only xylemtracheids, while theflowering plants also havexylem vessels. Vessel elements are hollow xylem cells without end walls that are aligned end-to-end so as to form long continuous tubes. The bryophytes lack true xylem tissue, but theirsporophytes have a water-conducting tissue known as the hydrome that is composed of elongated cells of simpler construction.
Phloem is a specialised tissue for food transport in higher plants, mainly transportingsucrose along pressure gradients generated by osmosis, a process calledtranslocation. Phloem is a complex tissue, consisting of two main cell types, thesieve tubes and the intimately associatedcompanion cells, together with parenchyma cells, phloem fibres and sclereids.[19]: 171 Sieve tubes are joined end-to-end with perforated end-plates between known assieve plates, which allow transport of photosynthate between the sieve elements. The sieve tube elements lacknuclei andribosomes, and their metabolism and functions are regulated by the adjacent nucleate companion cells. The companion cells, connected to the sieve tubes viaplasmodesmata, are responsible for loading the phloem withsugars. Thebryophytes lack phloem, butmosssporophytes have a simpler tissue with analogous function known as the leptome.
Theplant epidermis is specialised tissue, composed of parenchyma cells, that covers the external surfaces of leaves, stems and roots. Several cell types may be present in the epidermis. Notable among these are the stomatal guard cells that control the rate ofgas exchange between the plant and the atmosphere, glandular and clothing hairs ortrichomes, and theroot hairs of primary roots. In the shoot epidermis of most plants, only theguard cells have chloroplasts. Chloroplasts contain the green pigment chlorophyll which is needed for photosynthesis. The epidermal cells of aerial organs arise from the superficial layer of cells known as thetunica (L1 and L2 layers) that covers the plantshoot apex,[19] whereas the cortex and vascular tissues arise from innermost layer of the shoot apex known as thecorpus (L3 layer). The epidermis of roots originates from the layer of cells immediately beneath the root cap. The epidermis of all aerial organs, but not roots, is covered with acuticle made ofpolyestercutin or polymercutan (or both), with a superficial layer ofepicuticular waxes. The epidermal cells of the primary shoot are thought to be the only plant cells with the biochemical capacity to synthesize cutin.[22]
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