Complex Tissues
They are permanent tissues which contain more
than one type of cells. All the cells work as a unit.
The common complex permanent tissues are the
Conducting tissues. They are:
Water Conducting tissue - Xylem
Food Conducting tissue - phloem
Water conducting cells of the Xylem: (GK-xylon-
wood, Nageli)
Xylem conducts water and dissolved minerals
upward from roots into the shoots. Hence water
conducting cells, tracheids and vessel elements,
are tubular, elongated cells that are dead at
functional maturity.Tracheids & vessels take
part in transport of sap. they are called Tracheary
elements. Tracheids are in the xylem of nearly
all vascular plants. In addition to tracheids, most
angiosperms, as well as a few gymnosperms
and pteridophytes(eg:- Selaginella), (Gentum
Gymnosperm) When the living cellular contents
of a tracheid or vessel elements disintegrate, the
cell’s thickened walls remain behind, forming
a non living conduit through which water can
flow. The secondary walls of tracheids and
vessel elements are often interrupted by pits,
thinner regions where only primary walls are
present. Water can migrate laterally between
neighbouring cells through pits.
i) Tracheids are long, thin cells with tapered ends.
Water moves from cell to cell mainly through
the pits, where it does not have to cross thick
secondary walls. Tracheids constitute 90-95% of
wood in gymnosperms, but in angiosperms only
5% of th wood. Depending upon the thickenings,
tracheids are classified as
i) Annular - rings like
ii) Spiral - spiral or helix.
iii) Reticulate - network
iv) Scalariform - ladder like
v) Pitted- pits -Simple pit- Bordered pit.
A thickening called torus present on the pit
membrane of some gymnosperms for protecting
the membrane from rupturning in case of unequal
pressure on its two sides.
ii) Vessel elements are generally wider, shorter,
thinner walled, and less tapered than the
tracheids. They are aligned end to end, forming
long micropipes known as vessels. The end
walls of vessel elements have perforation plates
that enable water to flow freely through the
vessels. The lignified walls can be annular, spiral,
reticulate, scalariform & pitted.
The secondary walls of tracheids and vessel
elements are hardened with lignin. This hardening
prevents collapse under the tensions of water
transport and also provides support.