BOTANY Higher Secondary Second Year - Textbooks Online

Each annual ring refers to one year’s growth. By counting the total
number of annual rings, the age of the plant can approximately be calculated.
The determination of the age of a tree by counting the annual rings is called
Dendrocronology. The section taken at the base of the trunk of American
Sequoia dendron has revealed that the tree is about 3500 years old.
Tyloses
In many dicot plants, the lumens of the xylem vessels are blocked by
many balloon like in-growths from the neighbouring parenchymatous cells.
These balloon like structures are called tyloses.
Fig. 2.22 Tyloses
Bands of
annular
thickenings
Tyloses
Xylem
parenchyma
Xylem
vessel
107
Usually these structures are
formed in secondary xylem vessels.
In fully developed tyloses starch,
crystals, resins, gums, oils, tannins
and coloured substances are found.
Sap wood and heart wood
Xylem is also known as wood.
When the secondary growth occurs
for several years, sapwood and
heartwood can be distinguished in
the secondary xylem. The outer
part of the wood, which is paler in
colour, is called sapwood or
alburnum. The centre part of the wood, which is darker in colour, is
called heartwood or duramen.
The sapwood is useful in the conduction of water. As vessels of the
heartwood is blocked by tyloses, water is not conducted through them.
The tyloses contain oils, gums, tannins, resins and other coloured substances.
Due to the presence of these substances, the heartwood becomes the hardest
part of the wood. From economic point of view the heartwood is more
useful than sapwood. The timbers from the heartwood are more durable
and more resistant to the attack of microorganisms and insects than
those from sapwood.