Xylem is a conducting tissue in plants that is
meant to conduct water and minerals upwards from the roots to the leaf. It is a
complex tissue composed of different types of cells. Xylem consists of xylem
tracheids, vessels or tracheae, xylem fibers/xylem sclerenchyma (wood fibres)
and xylem parenchyma (wood parenchyma). Secondary function of xylem is
mechanical support to the plant body.
The first formed xylem which is formed much before the cambial
activity to produce xylem cells is called primary xylem. It is composed of two
parts. The xylem formed first is with small lumen and is closest to the centre
of the stem is called protoxylem. The later formed primary xylem with large
lumen present towards the peripheral part of the primary xylem is known as
metaxylem.
Primary Xylem vs Secondary Xylem
Primary Xylem
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Secondary Xylem
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The primary xylem is derived from the
procambium of the apical meristem.
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The secondary xylem is derived from
the vascular cambium which is a lateral meristems.
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It is differentiated into protoxylem
and metaxylem.
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The secondary xylem is not
differentiated into proto and metaxylem.
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The primary xylem may be endarch,
mesarch or exarch.
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There is no such differentiation in
the secondary xylem.
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The tracheids and vessels are narrow
and long.
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The vessels and tracheids are short
and wide.
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The medullary rays are derived from
the apical meristem.
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The secondary medullary rays are
derived from ray initials of the cambium.
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The vessels of primary xylem do not contain
tyloses.
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Here the vessels contain tyloses*.
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There are no annual rings.
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The annual rings are well demarcated.
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The primary xylem is not differentiated
into sap wood and heart wood.
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There is clear cut demarcation of sap
wood and heart wood in woody trees.
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The xylem fibres are few in number or
absent.
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The xylem fibres are abundant.
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* Tylosis: Bubble like outgrowth of parenchyma cells into the lumen of a vessel through a pit cavity.