Xylem and phloem are called the conducting tissues. Xylem is meant to conduct water and mineral salts upward from root to the leaf and give mechanical strength to the plant body. Except wood parenchyma all other xylem elements are lignified thick walled and dead. Phloem as a whole is meant to conduct prepared food material from leaf to the storage organs and the growing regions.
Elements of xylem are (a) Tracheids (b) Vessels or tracheae (c) wood fibres (d) wood parenchyma.
(a) Tracheids
These are elongated tube like dead cells with hard thick and lignified walls and a large cell cavity. Their ends are commonly tapering or oblique. Their walls are provided with one or more rows of bordered pits. Tracheids may be annular, spiral, scalariform or pitted. In T.S they are angular polygonal or rectangular. Tracheids occur alone in wood of ferms and gymnosperms and in wood of angiosperms. They occur associated with vessels. Being lignified and hard, archeids give strength to the plant body but their main function is conduction of water and mineral salts from root to leaf.
(b) Vessels or tracheae
Vessels are rows of elongated tube like dead cells placed end to end with their transverse or end walls dissolved. A vessel or trachea is thus is like series of water pipes forming a pipe line. Their walls are thickened in various ways and according to the mode of thickening vessels have received their names such as annular, spiral, scalariform, reticulate and pitted. Some tracheids are attached with vessels. Tracheids and vessels form main elements of wood or xylem of vascular bundle theory. They have large cell cavities which serve for conduction of water and mineral salts from roots to leaves. They are dead thick walled and lignified and serve the mechanical function of strengthening plant body.
(c) Wood Fibres
Schlerenchymatous cells associated with wood or xylem are known as wood fibres. They are in much amount in woody dicotyledons and add to the mechanical strength of the xylem and the plant body as a whole.
(d) WOOD PARENCHYMA
Parenchymatous cells associated with xylem together form wood parenchyma. The cells are alive, thin walled and abundant. Wood parenchyma assists directly or indirectly in the conduction of water upwards through vessels and tracheids. It also serves for food storage.
Scleros = hard.
Schlerenchyma consists of very long, narrow, thick walled and lignified cells pointed at both ends. They are fibre like and are called schlerenchymatous fibres or simply fibres. They have simple often oblique pits in their walls. These cells are many in number in plants and occur in patches. They are dead cells and serves as mechanical function and give strength and rigidity to plant body and thus enable it to withstand various strains. Their average length is 1 to 3 mm but in fibre yielding plants such as hemp, Indian hemp, Decan hemp, jute. Such fibres are of commercial importance. Sometimes here and there in the plant body special types of sclerenchyma may be developed. These are known as stone or sclerotic cells. Cells are thick walled and are irregular in shape or slightly elongated. Some cells occur in hard seeds, nuts and stony fruits.
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