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 unspecialised 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.