2019-03-01 Biology Times

(singke) #1
Biology Times March 19

Annual rings:
In temperate countries due to changes in the
seasonal activities different types of secondary
xylem are formed. In the spring season, plants
show rapid growth. All the living activities,
especially photosynthesis proceed at a rapid
speed. The cells need more water and therefore
the xylem vessels formed in the spring are less
dense but more broad. They have less of wood
fibres. In summer, the plants need comparatively
less water and the process of photosynthesis is
slowed down.
The xylem vessels formed are more and less
broad. There walls are also much more lignified
and xylem fibres are rather more. The wood
formed in the spring is known as springwood and
the wood formed in summer is called summer
wood or autumn wood. The spring wood and
autumn wood together form concentric ring.
It is one annual ring formed in one year; every
year one annual ring develops. Each annual
ring indicates one year’s growth and hence by
counting the number of rings in a cross section of
wood we can say the approximate age of the plant
and that study is called dendrochronology.

As  the  growth proceeds    the  wood    in  the     centre 
become dark and nonfunctional to form the heart
wood (duramen) and the wood surrounding
it is light coloured and function as sap wood
(alburnum). The vessels of the heartwood are
filled up with tannins, gums, resins, essential oils
etc.,. It only acts as a mechanical support to the
stem. The vessels of sapwood are functional and
help in the conduction of water and mineral salts
from the roots to the leaves.
Tyloses: In several plants, the xylem and ray
parenchyma cells produce ballon like projections
into their cavities. These projections are called
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