As water evaporates from the leaves (see transpiration) water is drawn across the leaf by osmosis to replace it, drawing water out of the xylem. This process creates turgor pressure in the xylem vessels, which, along with rings of lignin that reinforce side walls and present them from collpasing, provides support to the plant. Tiny holes in the walls of the xylem vessels, called pits, allow water to cross from one tube to another.
Ferns and conifers do not have xylem vessels. Instead they have similar cells called tracheids, which do not lose their end walls, so water has to travel through the pits, which slows the flow. The lignin makes the walls of vessels and tracheids strong and rigid, an important support as the plant grows bigger. Xylem tissue also contains non-conducting fibers, dead cells thickened with lignin for extra support.
Xylem is of two kinds: primary, formed by differentiation from procambium and consisting of protoxylem and metaxylem, and secondary, additional xylem produced by activity of cambium. In mature woody plants, xylem makes up the bulk of vascular tissue itself and of the entire structure of stems and roots.
Related category• BOTANY
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