Answer:
The increase in girth of stems of woody angiosperms
is called secondary growth. The tissue involved in secondary growth are the two
lateral meristems?vascular cambium and cork cambium.
(i) Vascular cambium is a vascular layer present
between xylem and phloem (vascular tissues). It is responsible for the cutting
off vascular tissues, later it forms a complete ring.
(ii) The cambial ring becomes active and begins to
cut off new cells, both towards the inner and the outer sides.
(iii) The cells cut off towards pith, mature into
secondary xylem and the cell cut of towards periphery, mature into secondary
phloem.
(iv) The cambium is generally more active on the
inner side than the outer. As a result, the amount of secondary xylem produced
is more than the secondary phloem and soon forms a compact mass.
(v) The primary and secondary phloem get gradually
crushed due to the continued formation and accumulation of secondary xylem.
(vi) The primary xylem however, remains more or
less intact, in or around the centre.
(vii) At some places, the cambium forms a narrow
band of parenchyma, which passes through the secondary xylem and the secondary
phloem in the radial directions. These are the secondary medullary rays.
(viii) The stem continues to increase in girth due
to the activity of vascular cambium.
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