Secondary growth is the increase in thickness due to formation of secondary tissues by the activity of cambium and cork cambium. It is found in dioct root and Gymnospermous roots, where it usually starts at very early stage. It is difficult to get the roots without secondary growth in most of the cases.
(1) ACTIVITY OF CAMBIUM
Some of the cells of conjunctive tissues just beneath the phloem become meristematic and form strips of cambium. The number of strips formed depends upon number of phloem bundles present. In a diarch root two in triarch root and three in tetrarch root four such strips are formed. These strips extend between phloem and xylem and unite with pericyclic cells lying just outside protoxylum. The pericycle cells divide tangentially and form two layers of which the cells of inner layer also become meristematic and unite with the strips of cambia and thus continuous wavy band of cambium is formed extending down the phloem and over the xylem. It becomes active and form new cells. It divides by periclinical divisions and then by anticlinical divisions for increase in circumference. The phloem and cambium strip below it are pushed outward and the wavy band of cambium now becomes circular to form a cambium ring.
The cells formed on inner side get differentiated into secondary xylem. It has large vessels, tracheids, little wood fibers and well developed xylem parenchyma. The activity of cambium is so fast on the inner side that after secondary growth xylem forms the main bulk of the root and is present in the form of solid core. The primary xylem bundles may remain intact upto the last or visible in roots. The pith is completely rushed. Secondary vascular tissues formed by the activity of cambium do not form a continuous ring but are interrupted by the bands of radially elongated parenchymatous cells called primary medullar rays.
These are formed above each primary xylem patch and attend upto the phloem. Sometimes other smaller medullary rays may also develop from other parts of cambium and may call as secondary medullary rays. The number of rays goes on increasing with the increase in the size of vascular cylinder. In some storage roots storage parenchyma develops in the secondary xylem. The cells of secondary xylem are arranged in definite rows when first formed but become irregularly disposed due to differential enlargement of various tracheid elements. Like the stems, the roots of perenmial trees shrubs and woody climbers also the xylem elements formed in the beginning of each season are larger and thin walled while those formed during late are smaller in size and are thick walled. Thus annual rings are formed.
(2) ACTIVITY OF CORK CAMBIUM
The secondary tissues formed by activity of cambium exert a pressure on outer tissue. To withstand this pressure the cells of pericycle become meristematic and function as phellogen or cork cambium. The cells of pericycle divide tangentially. Similar to stem here also it forms layers of cork or phellem on the outside and secondary cortex or phelloderm on the inner side.
The bark in the case of roots includes cork, endodermis, cortex and epiblema. In some cases the cork cambium may be formed from the phloem cells. In this case the pericycle also forms the part of bark. Subsequent barks have also cork. Lenticles may also be formed here and there. When the bark is removed, new bark cambium layer is formed from the parenchyma formed by previous cork cambium.
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