- Xylem vessels: Xylem vessels comprise a vertical chain of lengthened, dead cells known as vessel elements. The cells are arranged end to endand the cross-walls dissolve completely or have simple or complex perforation plates between successive cells. The secondary walls of vessels are impregnated with lignin and are thickened unevenly. The walls of the vessels may be thickened in different ways, e.g. annular, spiral and pitted thickening may be observed.
- Xylem tracheids: A tracheids is an elongated cell, the contents of which are non-living. The cell walls are thickened, impregnated with ligninand the lumen is smaller. As in the case of vessels, there is a differentiation between annular, spiral and pitted tracheids again caused by the type of thickening of the secondary walls. Tracheids have no perforation plates.
- Xylem fibres and xylem parenchyma bear a strong resemblance to normal fibres and parenchyma. Xylem fibres are sometimes separated by thin cross walls and the walls of xylem parenchyma are sometimes thicker than those of normal parenchyma.
Functions:
- xylem is an important strengthening tissue,
- xylem vessels and tracheids transport water and mineral salts,
- starch is sometimes stored in the xylem fibres and xylem parenchyma.
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A line drawing of the different xylem cells.
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