Xylem is a complex tissue composed of xylem vessels, xylem tracheids, xylem fibres
and xylem parenchyma.
Xylem vessels
: Xylem vessels comprise a vertical chain of lengthened,
dead cells known as vessel elements. The cells are arranged end to
end and 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
lignin and 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.
