Editing Plant cell (section)

==Types of plant cells and tissues==

Plant cells differentiate from undifferentiated [[meristem]]atic cells (analogous to the stem cells of animals) to form the major classes of cells and tissues of [[root]]s, [[plant stem|stems]], [[leaf|leaves]], [[flower]]s, and reproductive structures, each of which may be composed of several cell types.

===Parenchyma===
[[Ground tissue#Parenchyma|Parenchyma cells]] are living cells that have functions ranging from storage and support to [[photosynthesis]] ([[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 of [[gas exchange]], but others are among the least specialized cells in plant tissue, and may remain [[totipotent]], capable of dividing to produce new populations of undifferentiated cells, throughout their lives.<ref name=Haberlandt>{{cite journal |first=Haberlandt |last=G. |date=1902 |title=Kulturversuche mit isolierten Pflanzenzellen |journal=Mathematisch-naturwissenschaftliche |publisher=Akademie der Wissenschaften in Wien Sitzungsberichte |volume=111 |issue=1 |pages=69–92 }}</ref> 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 as [[nectar]] [[secretion]], or the manufacture of [[secondary metabolite|secondary products]] that discourage [[herbivory]]. Parenchyma cells that contain many chloroplasts and are concerned primarily with photosynthesis are called [[chlorenchyma]] cells. Chlorenchyma cells are parenchyma cells involved in photosynthesis. <ref>{{Cite book|last=Mauseth|first=James D.|url=https://www.worldcat.org/oclc/1122454203|title=Botany : An Introduction to Plant Biology|date=2021|isbn=978-1-284-15737-6|edition=Second|location=Burlington, MA|language=English|oclc=1122454203}}</ref> Others, such as the majority of the parenchyma cells in [[potato]] [[tubers]] and the [[seed]] [[cotyledons]] of [[legumes]], have a storage function.

=== Collenchyma ===
[[Ground tissue#Collenchyma|Collenchyma cells]] are alive at maturity and have thickened cellulose cell walls.<ref name="Cutter" /> These cells mature from meristem derivatives that initially resemble parenchyma, but differences quickly become apparent. Plastids do not develop, and the secretory apparatus (ER and Golgi) 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.<ref name="Cutter" /> [[Pectin]] and [[hemicellulose]] are the dominant constituents of collenchyma cell walls of [[dicotyledon]] [[angiosperm]]s, which may contain as little as 20% of cellulose in ''[[Petasites]]''.<ref name="Roelofsen">{{cite book|last=Roelofsen|first=PA|title=The plant cell wall|date=1959|publisher=Gebrüder Borntraeger|location=Berlin|asin=B0007J57W0}}</ref> 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.

{{Plain image with caption|Plant cell types.svg|Cross section of a leaf showing various plant cell types|550px|right}}

===Sclerenchyma===
[[Ground tissue#Sclerenchyma|Sclerenchyma]] is a tissue composed of two types of cells, [[sclereid]]s and [[fibres]] that have thickened, [[lignin|lignified]] secondary walls<ref name=Cutter>{{cite book|first=EG |last=Cutter |date=1977 |title=Plant Anatomy Part 1. Cells and Tissues |publisher=Edward Arnold |location=London |isbn=0713126388 }}</ref>{{rp|78}} laid down inside of the [[primary 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 the cytoplasm is missing, leaving an empty central cavity. [[Sclereid]]s 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. [[Fibre]]s 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 the [[xylem]]) or of carbon compounds (as in the [[phloem]]), but it is likely that they evolved as modifications of xylem and phloem initials in early land plants.

[[Image:Arabidopsis-epiderm-conidiospore-hyaloperonospora-parasitica.jpg|thumb|right|cells of ''[[Arabidopsis thaliana]]'' [[Epidermis (botany)|epidermis]]]]

===Xylem===
[[Xylem]] is a complex vascular tissue composed of water-conducting [[tracheid]]s or [[vessel elements]], together with fibres and parenchyma cells. Tracheids<ref name=Tyree>MT Tyree; MH Zimmermann (2003) Xylem structure and the ascent of sap, 2nd edition, Springer-Verlag, New York USA</ref> 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 the [[Silurian]] period more than 425 million years ago (see ''[[Cooksonia (plant)|Cooksonia]]''). The possession of xylem tracheids defines the [[vascular 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 called [[pit (botany)|bordered pits]] characterise the gymnosperms. The [[fern]]s and other [[pteridophyte]]s and the [[gymnosperm]]s have only xylem [[tracheid]]s, while the [[angiosperms|flowering plants]] also have [[vessel element|xylem vessel]]s. 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 their [[sporophyte]]s have a water-conducting tissue known as the hydrome that is composed of elongated cells of simpler construction.

===Phloem===
[[Phloem]] is a specialised tissue for food transport in higher plants, mainly transporting [[sucrose]] along pressure gradients generated by osmosis, a process called [[Phloem#Function|translocation]]. Phloem is a complex tissue, consisting of two main cell types, the [[sieve tube element|sieve tubes]] and the intimately associated [[companion cell]]s, together with parenchyma cells, phloem fibres and sclereids.<ref name=Cutter/>{{rp|171}} Sieve tubes are joined end-to-end with perforated end-plates between known as ''[[sieve plate]]s'', which allow transport of photosynthate between the sieve elements. The sieve tube elements lack [[cell nucleus|nuclei]] and [[ribosome]]s, and their metabolism and functions are regulated by the adjacent nucleate companion cells. The companion cells, connected to the sieve tubes via [[plasmodesmata]], are responsible for loading the phloem with [[sugar]]s. The [[bryophyte]]s lack phloem, but [[moss]] [[sporophyte]]s have a simpler tissue with analogous function known as the leptome.

[[File:Epidermal Cells of Plant Leaf..jpg|thumb|This is an electron micrograph of the epidermal cells of a Brassica chinensis leaf. The stomates are also visible.]]

===Epidermis===
The [[Epidermis (botany)|plant 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 of [[Gas exchange#Plants|gas exchange]] between the plant and the atmosphere, glandular and clothing hairs or [[trichome]]s, and the [[root hair]]s of primary roots. In the shoot epidermis of most plants, only the [[stomata|guard 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 the ''tunica'' (L1 and L2 layers) that covers the plant [[meristem|shoot apex]],<ref name="Cutter"/> whereas the cortex and vascular tissues arise from innermost layer of the shoot apex known as the ''corpus'' (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 a [[plant cuticle|cuticle]] made of [[polyester]] [[cutin]] or polymer [[Cutan (polymer)|cutan]] (or both), with a superficial layer of [[epicuticular wax]]es. The epidermal cells of the primary shoot are thought to be the only plant cells with the biochemical capacity to synthesize cutin.<ref name="Kolattukudy 1996">Kolattukudy, PE (1996) Biosynthetic pathways of cutin and waxes, and their sensitivity to [[environmental stresses]]. In: Plant Cuticles. Ed. by G. Kerstiens,  BIOS Scientific publishers Ltd., Oxford, pp 83–108</ref>