Page 84 - Biology Form Two

Biology for Secondary Schools

Xylem vessel elements
Xylem vessel elements are one of the Pits
two water and mineral salts conducting
structures of the xylem tissue. Mature Vessel
xylem vessel elements are long, element Perforation plate
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cylindrical, and tube-like structures.
Their walls are made up of cellulose
and lignin. Lignin is deposited on the
cell walls of xylem vessel elements Figure 4.3: Xylem vessels

to strengthen and make them rigid. Tracheids
Therefore, the strength and rigidity of Tracheids are series of interconnected
the xylem vessel element gives it an dead, elongated, tube-like cells with
additional function of supporting the tapering ends, as shown in Figure 4.4.
plant. They are the only xylem conducting
tissues in coniferophytes and
A xylem vessel element is made up of pteridophytes. In angiospermophytes,
tiny hollow cells without end walls. both tracheids and xylem vessel elements
These cells are called vessel elements are present. Like vessels, tracheids have

and are joined end-to-end to form a a thick lignified wall and lack protoplasm
pipe-like structure or a hollow tube, that is cytoplasm, and nuclei. Both
as shown in Figure 4.3. Xylem vessel xylem vessel elements and tracheids
elements begin in the roots through the are involved in transporting of water
stem and branch up to every leaf of the and mineral salts in plants. Tracheids
plant. Xylem vessel elements have no and vessel elements differ in their shapes

cytoplasm and nuclei at maturity. Thus, and structures. Tracheids are longer than
vessel elements. They are connected
they are said to be dead. The absence of laterally to enable continuous flow of
cytoplasm and nuclei make them to be water and mineral salts while the vessel
hollow tubes, which then act as pipes. elements are connected end to end to
This enables them to transport a larger form long tubes called xylem vessels.
volume of water and mineral salts. The Tracheid end walls have holes unlike
xylem vessel elements are found in xylem vessel elements end walls that
flowering plants (angiospermophytes) have perforations. Tracheids have small
only. lumen and are narrower than xylem
vessel elements. This makes tracheids
less efficient in the conduction of water
compared to xylem vessel elements.

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