Page 22 - LN-ANATOMY OF FLOWERING PLANT

and the secondary phloem in the radial direction. These are secondary medullary
rays.
Formation of Annual Rings
In tropical areas, the growth of secondary xylem is continuous. In others, yearly
growth is quite distinct and appears in the form of annual rings. The transition from
spring wood to autumn wood is gradual. After autumn wood and before spring wood
of next year, there is no growth.
Therefore, change over from autumn wood to spring wood is sudden. The light
coloured spring wood and its next dark coloured autumn wood constitutes an annual
ring or growth ring.
It represents the total secondary xylem or wood formed in one year. Hence, by
counting the number of annual rings, the age of a plant can be determined. This is
done with the help of an instrument called increment borer. Besides giving the age of
the plant, the annual rings can also provide information of the climatic conditions
prevailing in the past.
Spring Wood and Autumn Wood
The activity of cambium is under the control of many physiological and
environmental factors. In temperate regions, the climatic conditions are variable
through the year.
In springs, cambium is very active and produces a large number of xylary elements
having vessels with wider cavities. The wood formed in this season is called spring
wood or early wood.
In autumn, the cambium is less active and forms few xylary elements that have
narrow vessles. Thus, the wood formed is called autumn wood or late wood.

In old trees, the considerable region of secondary xylem is dark brown due to the
accumulation of organic materials like tannins, resins, nils, gums, aromatic
substances and essential oils in the central or innermost layers of the stem.
These substances make it hard, durable and resistant to the attacks of
microorganisms and insects. This region comprises dead elements with highly
lignified walls and is called heartwood.
The heartwood does not conduct water, but it gives mechanical support to the stem.
The peripheral region of the secondary xylem, is lighter in colour and is known as the
sapwood, which is involved in the conduction of water and minerals from root to
leaf.