Plant Tissues
Type of plant tissues using histology
All multicellular organisms possess groups of cells of similar structure and function assembled together to form tissues. Histology is the study of tissues; the latter can be defined as a group of physically linked cells and associated intercellular substances that is specialised for a particular function or functions. Plant tissues can be divided into simple tissues, vascular tissues and dermal tissues.
Simple plant tissues
A simple tissue is made up of one type of cells that form a homogeneous or uniform mass. Simple plant tissues include parenchyma, collenchyma and sclerenchyma.
Parenchyma consists of a collection of cells, which are usually roughly spherical (isodiametric) though they may be elongated. Typical parenchymatous cells are oval, spherical or polygonal. These are usually living cells with thin walls made of cellulose. They form a large part of the bulk of various organs, such as stem and root, and also occur among the xylem vessels and phloem cells. Parenchymatous cells are non-specialised and act as packing tissue between more specialised tissues. They provide support in herbaceous plants and are metabolically active. Intercellular air spaces between these cells allow exchange of gasses such as oxygen and carbon dioxide. They are also involved in food storage and transport of material through cells or cell walls.
Collenchyma consists of modified parenchymatous cells, which are elongated with oblique, slightly rounded or tapering ends. Compared to parenchymatous cells, these cells have extra cellulose deposited at their corners. Collenchyma is a mechanical tissue and provides support for those organs in which it is found. This tissue is especially important in young plants and in organs such as leaves. They are found under the skin (epidermis) of herbaceous dicotyledons (flowering plants whose seeds have two embryonic leaves). Cells of the collenchyma are living and often contain chloroplasts hence they can also manufacture sugar and starch. Therefore, along with providing mechanical support they are also vital.
Sclerenchyma consists of very long, narrow, thick and lignified (chemical compound in cell walls) cells which are usually pointed at both ends. They are fibre-like in appearance and hence are also called sclerenchymatous fibres. The mature cells in this tissue are dead and incapable of elongation so they do not mature until elongation of the living cells around them is complete.
There are two types of sclerenchyma cells, namely fibres, which are elongated cells, and sclereids or stone cells, which are roughly spherical. The sole function of sclerenchyma is to assist in providing support and mechanical strength for the plant. Sclerenchyma fibres are strong as they have lignified walls and their strength is enhanced by their arrangement. Sclereids confer firmness or rigidity on those structures they are found in. In some cases they form very resilient and solid layers as in shells of nuts.
Aerenchyma is an air channel in the roots of some plants, which allows exchange of gases between the shoot and the root. The channel of large air-filled cavities provides a low-resistance internal pathway for the exchange of gases such as oxygen and ethylene between the plant above the water and the submerged tissues. Aerenchyma form in roots subject to anoxia such as what occurs during flooding of plants and soil.
Vascular plant tissues
Vascular tissues are made up of more than one type of cells. Vascular tissues include the xylem and phloem, both of which are conductive tissues.
Xylem is composed of four types of cells which are the tracheids, vessel elements, parenchyma and fibres. Xylem, as a whole, has two major functions, the conducting of water and mineral salt from root to leaves, and to provide mechanical strength (support) to the plant body.
Tracheids are elongated and lignified, like sclerenchyma fibres they have tapering end walls that overlap with adjacent tracheids. Thus they have mechanical strength and give support to the plant. Tracheids are dead cells with empty lumens when mature.
Vessels are cylindrical, tube-like structures; they are the characteristic conducting units of the xylem. They are very long and tubular structures formed by the fusion of several cells end to end in a row. Vessels are dead at functional maturity and are perforated.
Phloem resembles xylem in possessing tubular structures modified for translocation. The phloem consists of five cell types which are sieve tube cells, companion cells, parenchyma, fibres and sclereids.
Sieve tubes are made of elongated cells placed end to end to form a tube. The end walls have pores in them forming the sieve plates. Sieve tube cells are alive at functional maturity. The main function of the sieve tubes is to carry food, especially carbohydrates.
Companion cells are parenchyma cells adjoining the sieve tube cells. They assist the sieve tube cells in the conduction of food. They are living cells with elongated nucleus.