Complex permanent tissue consists of a number of
dissimilar cells which perform a common function. It forms vascular or
conducting tissue in the plant. They are of two types: Xylem & Phloem.
Xylem is responsible for conduction of mineral
salt and water from roots to upper region and also provides mechanical support
to the plant body. Xylem is also called as wood and forms the bulk of roots and
stem of vascular plants. Xylem is composed of three types of cells some of
which are living and some non-living.
Tracheary Elements: These
participate in conduction of sap (water and minerals). They are divided into
two types – tracheids and vessels.
Vessels: Vessels are elongated tubular structures with a wide lumen. It is formed
by a series of cylindrical cells which are joined end-to-end and have perforated
walls. The perforated end walls are called perforation plates. Those with a
single large opening are called simple perforation plates whereas the ones with
several openings are called as multiple perforation plates. The vessels are
dead, thick walled and lignified. On the basis of how the deposition of lignin
occurs on vessel wall, the vessels are called as annular, spiral, reticular,
sclariform and pitted(most commonly found). Vessels are present in the xylem of
angiosperm and are absent in gymnosperm and pteridophyte.
Tracheids: Tracheids are elongated, thick walled, dead tubular cells with lignified
walls. Based on the lignification, tracheids are also classified as annular,
spiral, reticulate, sclariform and pitted. The tracheids are smaller than
vessels and form chief conducting element of xylem in gymnosperm and
pteridophyte where vessels are absent. Only few tracheids are present in
angiosperm in association with vessels.
Wood Fibre/ Xylem Fibres: They are
sclerenchyma fibres. They are distinguished from tracheids on basis of their
comparatively thicker walls and obliterated central lumens. They are mostly
abundant in secondary xylem and their main function is to provide mechanical
strength to the xylem and the entire plant body. They also help vessel and tracheids in
conduction of sap. These fibres are of two types: libriform fibres have thick
cell walls, simple pits and obliterated central lumens; fibre tracheids are
intermediary between fibres and tracheids with thin cell walls.
Wood Parenchyma: are parenchymatous cells present in the xylem. It is the only
living component of the xylem. The cells are thin or thick walled with simple pits.
These cells store food material in the form of
starch or fat or such other materials as tannin. They assist vessels and
tracheids in the lateral conduction of sap.
Xylem can be distinguished into two types on basis of
the time of origin and internal structure – protoxylem and metaxylem.
Protoxylem as the name suggests is the first or easily
formed xylem. It consists of small tracheids and vessels with annular or spiral
lignification. The thickening of cell walls starts before the xylem cells are
Metaxylem is formed at a later stage. It consists of
large tracheids and vessels and the lignification occurs in reticulate,
sclariform and pitted manner. Only after the completion of elongation of xylem
cells, the thickening of cell wall occurs.