Raffi at 05:21, 6 April 2007

2007-04-06T05:21:09Z

New page

The types of '''ground tissue''' found in [[plants]] develops from ''ground tissue'' [[meristem]] and consists of three simple tissues:

* Parenchyma (have retained their [[protoplasm]])
* Collenchyma (have retained their protoplasm)
* Sclerenchyma (have lost their protoplasm in mature stage, i.e. are 'dead')

==Parenchyma==
Parenchyma is the most common ground tissue, it forms e.g. the [[cortex (botany)|cortex]] and [[pith]] of stems, the cortex of roots, the [[mesophyll]] (photosynthetic cells), the pulp of fruits, the [[endosperm]] of seeds, and the photosynthetic areas of a leaf. Parenchyma cells are capable of cell division even after maturation (i.e. they are still meristematic). They have thin, but flexible, [[cell wall]]s, and are generally cube-shaped and are loosely packed. They have large [[central vacuole]]s, which allows the cells to store nutrients and water.

Parenchyma cells have a variety of functions;
*photosynthesis (may then be called Chlorenchyma /Mesophyll cells),
*gas exchange ([[Aerenchyma]]),
*storage,
*secretion (e.g. Epithelial cells lining the inside of resin ducts)
*healing
*other specialised functions.
Parenchyma differs depending on the varied functions they perform. The epidermal parenchyma cells of a leaf are barrel shaped and have no [[chloroplast]]. The parenchymatous layer just beneath the epidermis in the stem have chloroplasts. They take part in [[photosynthesis]] and are known as [[chlorenchyma]] in the leaf, just beneath the epidermis large parenchymatous cells with chloroplast are closely arranged,and are known as palisade tissues.palisade tissue along with the loosely arranged spongy tissue have major role in photosynthesis. There are many air cavities between the aerenchyma cells of water plants like lotus. These cells are known as parenchyma which help the plants to float in water. Parenchymma cells are arranged at the center of some types of stem, which perform the function of storing food. In certain types of roots, parenchyma cells also store food.

==Collenchyma==
Collenchyma tissue is composed of elongated cells with unevenly thickened [[cell wall|walls]]. They provide structural support, particularly in growing [[shoots]] and [[leaves]]. Collenchyma tissue composes, for example, the resilient strands in stalks of [[celery]]. Its growth is strongly affected by mechanical stress upon the plant. The walls of collenchyma in shaken (to mimic the effects of wind etc) plants may be 40%-100% thicker than those not shaken. The name collenchyma derives from the Greek word "kolla", meaning "glue", which refers to the thick, glistening appearance of the walls in fresh tissues.

There are three principal types of collenchyma;
*Angular collenchyma (thickened at intercellular contact points)
*Tangential collenchyma (cells arranged into ordered rows and thickened at the tangential face of the cell wall)
*Lacunar collenchyma (have intercellular space and thickening proximal to the intercellular space)

==Sclerenchyma==
Sclerenchyma is a supporting tissue. Two groups of sclerenchyma cells exist: [[fibres]] and sclereids. Their walls consist of [[cellulose]] and/or [[lignin]]. Sclerenchyma cells are the principal supporting cells in [[plant]] tissues that have ceased elongation. Sclerenchyma fibres are of great economical importance, since they constitute the source material for many fabrics ([[flax]], [[hemp]], [[jute]], [[ramie]]).

Unlike the collenchyma, mature sclerenchyma is composed of dead cells with extremely thick [[cell wall]]s (secondary walls) that make up to 90% of the whole cell volume. The term "sclerenchyma" is derived from the Greek "scleros", meaning "hard". It is their hard, thick walls that make sclerenchyma cells important strengthening and supporting elements in plant parts that have ceased elongation. The difference between fibres and sclereids is not always clear. Transitions do exist, sometimes even within one and the same plant.

Fibres 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 fibres. 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, like [[rope]]s, [[Textile|fabric]]s or [[mattress]]es. The fibres of [[flax]] (Linum usitatissimum) have been known in [[Europe]] and [[Egypt]] for more than 3000 years, those of [[hemp]] (Cannabis sativa) in [[China]] for just as long. These fibres, and those of [[jute]] (Corchorus capsularis) and [[ramie]] (Boehmeria nivea, a [[nettle]]), are extremely soft and elastic and are especially well suited for the processing to [[textiles]]. Their principal cell wall material is [[cellulose]].

Contrasting are hard fibres that are mostly found in [[monocots]]. Typical examples are the fibres of many Gramineae, [[Agave]]s (sisal: Agave sisalana), [[lilies]] (Yucca or Phormium tenax), Musa textilis and others. Their cell walls harbour, besides cellulose, a high proportion of [[lignin]]. The load-bearing capacity of [[Phormium tenax]] is as high as 20-25 kg/mm2, the same as that of good [[steel]] wire (25 kg/ mm2), but the fibre tears as soon as too great a strain is placed upon it, while the wire distorts and tears not before a strain of 80 kg/mm2. The thickening of a cell wall has been studied in Linum. Starting at the centre of the fibre are the thickening layers of the secondary wall 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 fibres.

Fibres stem usually form [[meristematic]] tissues. [[Cambium]] and [[procambium]] are their main centers of production. They are often associated with the [[xylem]] of the vascular bundles. The fibres of the xylem are always lignified. Reliable evidence for the fibre cells' evolutionary origin of tracheids exists. During evolution the strength of the cell walls was enhanced, the ability to conduct water was lost and the size of the pits reduced. Fibres that do not belong to the xylem are bast (outside the ring of cambium) and such fibres that are arranged in characteristic patterns at different sites of the shoot.

Sclereids are small bundles of sclerenchyma tissue in [[plants]] that form durable layers, such as the cores of [[apples]] and the gritty texture of [[pears]]. Sclereids are variable in shape. The cells can be isodiametric, prosenchymatic, forked or fantastically 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 within [[parenchyma]] tissues. But compared with most [[fibres]] [[sclereid]]s are relatively short. Characteristic examples are the stone cells (called stone cells because of their hardness) of [[pears]] (Pyrus communis) and [[quince]]s (Cydonia oblonga) and those of the shoot of the [[wax-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 of drupes like [[cherry|cherries]] or [[plum]]s are made up from sclereids.

==References==
*Moore, Randy; Clark, W. Dennis; and Vodopich, Darrell S. (1998). ''Botany'' (3rd ed.). McGraw-Hill. ISBN 0-697-28623-1.

{{Biological tissue}}

[[Category:Plant anatomy]]
[[Category:Plant cells]]
[[Category:Tissues]]

Ground tissue - Revision history

Raffi at 05:21, 6 April 2007