Secondary Growth in Dicotyledonous Root (With Diagram)!
The roots of some herbaceous dicotyledons and of all gymnosperms and woody dicotyledons show secondary increase in thickness, whereas most of the monocotyledonous roots, like those stems, are entirely primary.
The secondary tissues formed in the dicotyledonous roots are fundamentally similar to those of the stem, but the process is initiated in a different way (Figs. 645 to 647).
The dicotyledonous roots have limited number of radially arranged vascular bundles with exarch xylem. Pith is usually absent. A few parenchyma cells beneath each phloem group become meristematic and thus form strips of cambium, the number of strips being equal to the number of phloem groups present.
Cambial cells go on dividing and produce secondary tissues. The cells of the uniseriate pericycle against the protoxylem group now divide and form a few layers. The first- formed cambium now extends both ways and reaches the innermost derivatives of the xylem groups.
As a rule the cambial cells produce much more xylem than phloem; and because the first-formed cambium started producing secondary xylem much earlier and produces secondary xylem much more rapidly, the wavy cambium cylinder is ultimately rendered circular.
As already stated, the secondary vascular tissues are fundamentally similar to those of the stem. They form a continuous cylinder and the primary xylem gets completely embedded in it.
At this stage the root structure is revealed only by the radially arranged exarch primary xylem located at the central region, the strands of secondary vascular tissues being collaterally arranged like those of the stem (Fig. 647). The sieve elements of the primary phloem often get crushed.
The cambial cells originating from the pericycle against protoxylem groups function as ray initials and produce broad bands of vascular rays. These rays running between xylem and phloem through the cambium are characteristic of the roots. They are also called main medullary rays.
Periderm is formed in the outer region. Phellogen arises in the outer cells of the pericycle (Figs. 646 & 647). It produces phellem or cork cells on the outer side, and probably some phelloderm on the inner.
The pressure caused by formation of secondary tissues inside ruptures the cortex with endodermis, which is ultimately sloughed off. Lenticels may be formed. They usually occur in pairs as transversely elongated rough areas, one on each side of a lateral root.