The secondary growth in root also takes place by the activity of the cambium and cork cambium. It is a usual feature of dicotyledonous and gymnospermous roots, where it generally starts at a very early stage, so much so that it is difficult to get the roots without secondary growth in most of the cases.
1. Activity of Cambium:
Certain of the cells of conjunctive tissue just beneath the phloem become meristematic and form strips of cambium. The number of strips produced depends upon thus number of phloem bundles present. In a diarch root two, in triarch root three and in tetrarch root four such strips are formed. These strips exertend both ways in between phloem and xylem and ultimately unite with the pericyclic cells lying just outside the protoxylem.
The pericyclic cells divide tangentially and produce two layers of which the cells of inner layer also become meristematic and unite with the strips of cambia and thus, a continuous wavy band of cambium is produced extending down the phloem and over the xylem (Fig. 13.6). It becomes active and forms new cells.
It divides by periclinal divisions and then by anticlinal divisions for increase in circumference. The strip of cambium below the phloem becomes active earlier and the activity is much faster on the inner side. Because of this, phloem and cambium strip below it are pushed outward and the wavy band of cambium now becomes circular to form a cambium ring. Now the entire of the cambium becomes active.
The cells formed on the inner side get differentiated into secondary xylem. It consists of comparatively large vessels, tracheids, a little wood fibres and well evolved 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 can remain intact up to the last or the crushed. The annual rings like the stem are not visible in roots. The pith is entirely crushed, or if some part is left the cells become thick walled.
The secondary vascular tissues produced by the activity of cambium do not form a continuous ring but are interrupted by the bands of radially-elongated, parenchymatous cells, known as primary medullary rays. These are formed above each primary xylem patch and extend up to the phloem. Sometimes, other smaller medullar rays can also develop from other parts of cambium and may be known as secondary medullary rays. The number of rays goes on increasing with the increase in the size of the vascular cylinder.
The secondary xylem cells vary in quantity if different roots, they have only tracheids in gymnosperms, only vessels in willow and both tracheids and vessels in most of the plants. In some storage roots storage parenchyma develops in the secondary xylem. The cells of secondary xylem are arranged in definite rows when first produced become irregularly disposed due to differential enlargement of various tracheid elements.
Like the stem, in roots of perennial trees, shrubs and woody climbers also, the xylem elements produced in the bringing 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 produced.
2. Activity of Cork Cambium:
The secondary tissues produced by the activity of cambium exert a pressure on the outer tissue. To withstand this pressure, the cells of pericycle become meristematic and function as the phellogen or cork cambium. The cells of pericycle divide tangentially. Similar to stem, here also it produces 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 certain cases, the cork cambium may be formed from the phloem cells. In this case, the pericycle also produces the part of bark. Subsequent barks have only cork. Lenticels can also be formed here and there. When the bark is removed, the new cork cambium layer is formed from the parenchyma produced by the previous cork cambium.
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