Xylem, a network of vessels in plants that carry water and
dissolved minerals from the roots to other parts of the plant and also provides
physical support. The xylem tissue consists of various special cells that
contain water, known as tracheal elements. Together with phloem (tissue that
carries sugar from the leaves to other parts of the plant), xylem is found in
all vascular plants, including seedless club moss, ferns, horsetail, and all
angiosperms (flowering plants) and gymnosperms (plants with seeds that are not
seeds closed in the ovary).
The xylem tracheal elements consist of cells known as
tracheids and vessel members, both of which are usually narrow, hollow, and
elongated. Tracheids are less specialized than other vessel members and are the
only type of water-producing cells in most gymnosperms and seedless vascular
plants. Water that moves from the tracheid to the tracheid must pass through a
thin modified primary cell wall known as a hole membrane, which serves to
prevent the passing of damaging air bubbles.
The formation of xylem begins when root and bud cells divide
which grow actively (apical meristems) giving rise to primary xylem. In woody
plants, secondary xylem is the main part of a mature stem or root and is formed
when the plant expands in circumference and builds new xylem rings around the
original primary xylem tissue.
When this happens, the primary xylem cells die and lose
conduction function, forming a hard skeleton that only functions to support
plants. Thus, in the branches and branches of large older trees, only the outer
secondary xylem (sapwood) functions in water conduction, while the inside
consists of primary xylem which is dead but structurally strong. In temperate
or cold climates, the age of a tree can be determined by counting the number of
annual xylem rings formed at the base of the trunk (cut transversely).