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.
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