Theground tissue of plants includes all tissues that are neitherdermal norvascular. It can be divided into three types based on the nature of the cell walls. This tissue system is present between the dermal tissue and forms the main bulk of the plant body.
Parenchyma is a versatile ground tissue that generally constitutes the "filler" tissue in soft parts of plants. It forms, among other things, thecortex (outer region) andpith (central region) of stems, the cortex of roots, themesophyll of leaves, the pulp of fruits, and theendosperm ofseeds. Parenchyma cells are often living cells and may remainmeristematic, meaning that they are capable ofcell division if stimulated. They have thin and flexiblecellulosecell walls and are generallypolyhedral when close-packed, but can be roughly spherical when isolated from their neighbors. Parenchyma cells are generally large. They have largecentral vacuoles, which allow the cells to store and regulateions, waste products, andwater. Tissue specialised for food storage is commonly formed of parenchyma cells.
Parenchyma cells have a variety of functions:
The shape of parenchyma cells varies with their function. In the spongymesophyll of a leaf, parenchyma cells range from near-spherical and loosely arranged with large intercellular spaces,[2] to branched orstellate, mutually interconnected with their neighbours at the ends of their arms to form a three-dimensional network, like in the red kidney beanPhaseolus vulgaris and othermesophytes.[3] These cells, along with theepidermalguard cells of thestoma, form a system of air spaces and chambers that regulate the exchange of gases. In some works, the cells of the leaf epidermis are regarded as specialised parenchymal cells,[4] but the modern preference has long been to classify the epidermis asplant dermal tissue, and parenchyma as ground tissue.[5]
Shapes of parenchyma:
Collenchyma tissue is composed of elongated cells with irregularly thickenedwalls. They provide structural support, particularly in growingshoots andleaves (as seen, for example, the resilient strands in stalks ofcelery). Collenchyma cells are usually living, and have only a thickprimary cell wall[6] made up of cellulose and pectin. Cell wall thickness is strongly affected by mechanical stress upon the plant. The walls of collenchyma in shaken plants (to mimic the effects of wind etc.), may be 40–100% thicker than those not shaken.
There are four main types of collenchyma:
Collenchyma cells are most often found adjacent to outer growing tissues such as thevascular cambium and are known for increasing structural support and integrity.
The first use of "collenchyma" (/kəˈlɛŋkɪmə,kɒ-/[7][8]) was byLink (1837) who used it to describe the sticky substance onBletia (Orchidaceae) pollen. Complaining about Link's excessive nomenclature,Schleiden (1839) stated mockingly that the term "collenchyma" could have more easily been used to describe elongated sub-epidermal cells with unevenly thickened cell walls.[9]
Sclerenchyma is the tissue which makes the plant hard and stiff. Sclerenchyma is the supporting tissue inplants. Two types of sclerenchyma cells exist: fibers cellular andsclereids. Theircell walls consist ofcellulose,hemicellulose, andlignin. Sclerenchyma cells are the principal supporting cells in plant tissues that have ceased elongation. Sclerenchyma fibers are of great economic importance, since they constitute the source material for many fabrics (e.g.flax,hemp,jute, andramie).
Unlike the collenchyma, mature sclerenchyma is composed of dead cells with extremely thick cell walls (secondary walls) that make up to 90% of the whole cell volume. The termsclerenchyma is derived from the Greek σκληρός (sklērós), meaning "hard." It is the hard, thick walls that make sclerenchyma cells important strengthening and supporting elements in plant parts that have ceased elongation. The difference between sclereids is not always clear: transitions do exist, sometimes even within the same plant.
Fibers orbast are generally long, slender, so-called prosenchymatous cells, usually occurring in strands or bundles. Such bundles or the totality of a stem's bundles are colloquially called fibers. Their high load-bearing capacity and the ease with which they can be processed has since antiquity made them the source material for a number of things, likeropes,fabrics andmattresses. The fibers offlax (Linum usitatissimum) have been known inEurope andEgypt for more than 3,000 years, those ofhemp (Cannabis sativa) inChina for just as long. These fibers, and those ofjute (Corchorus capsularis) andramie (Boehmeria nivea, anettle), are extremely soft and elastic and are especially well suited for the processing totextiles. Their principal cell wall material iscellulose.
Contrasting are hard fibers that are mostly found inmonocots. Typical examples are the fiber of manygrasses,Agave sisalana (sisal),Yucca orPhormium tenax,Musa textilis and others. Their cell walls contain, besides cellulose, a high proportion oflignin. The load-bearing capacity ofPhormium tenax is as high as 20–25 kg/mm², the same as that of goodsteel wire (25 kg/ mm²), but the fibre tears as soon as too great a strain is placed upon it, while the wire distorts and does not tear before a strain of 80 kg/mm². The thickening of a cell wall has been studied inLinum.[citation needed] Starting at the centre of the fiber, the thickening layers of the secondary wall are deposited one after the other. Growth at both tips of the cell leads to simultaneous elongation. During development the layers of secondary material seem like tubes, of which the outer one is always longer and older than the next. After completion of growth, the missing parts are supplemented, so that the wall is evenly thickened up to the tips of the fibers.
Fibers usually originate frommeristematic tissues.Cambium andprocambium are their main centers of production. They are usually associated with thexylem andphloem of the vascular bundles. The fibers of the xylem are alwayslignified, while those of the phloem arecellulosic. Reliable evidence for the fibre cells' evolutionary origin fromtracheids exists.[10] During evolution the strength of the tracheid cell walls was enhanced, the ability to conduct water was lost and the size of the pits was reduced. Fibers that do not belong to the xylem are bast (outside the ring of cambium) and such fibers that are arranged in characteristic patterns at different sites of the shoot.The term "sclerenchyma" (originallySclerenchyma) was introduced byMettenius in 1865.[11]
 | This sectiondoes notcite anysources. Please helpimprove this section byadding citations to reliable sources. Unsourced material may be challenged andremoved.(September 2015) (Learn how and when to remove this message) |
Sclereids are the reduced form of sclerenchyma cells with highly thickened, lignified walls.
They are small bundles of sclerenchyma tissue inplants that form durable layers, such as the cores ofapples and the gritty texture ofpears (Pyrus communis). Sclereids are variable in shape. The cells can be isodiametric, prosenchymatic, forked or elaborately branched. They can be grouped into bundles, can form complete tubes located at the periphery or can occur as single cells or small groups of cells withinparenchyma tissues. But compared with most fibres, sclereids are relatively short. Characteristic examples arebrachysclereids or the stone cells (called stone cells because of their hardness) of pears andquinces (Cydonia oblonga) and those of the shoot of thewax plant (Hoya carnosa). The cell walls fill nearly all the cell's volume. A layering of the walls and the existence of branched pits is clearly visible. Branched pits such as these are called ramiform pits. The shell of many seeds like those of nuts as well as the stones ofdrupes likecherries andplums are made up from sclereids.
These structures are used to protect other cells.
| Subdisciplines | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Plant groups | |||||||||||
| Plant anatomy |
| ||||||||||
| Plant physiology Materials | |||||||||||
| Plant growth and habit | |||||||||||
| Reproduction | |||||||||||
| Plant taxonomy | |||||||||||
| Practice | |||||||||||
| |||||||||||
