Normal Secondary growth in dicot root and Origin of lateral roots
Secondary growth in dicot root:-
> The secondary growth in the root takes place due to the formation of secondary tissues by lateral meristems.
> Most of the dicot roots show secondary growth in thickness, like that of dicot stems.
> Secondary tissues formed by meristem are vascular cambium and cork cambium, respectively.
> The roots of most dicots and gymnosperms show secondary growth. The roots of extant vascular cryptogams and most monocots do not show any secondary growth; they remain entirely primary throughout their life.
> As in the stem of dicot plants, secondary growth occurs after the primary growth is over. This increases the girth of the stem and roots and not the length.
> The roots of a few monocots like Dracaena, Yucca, Agave, Aloe and Sansevieria, show anomalous secondary growth.
> Secondary growth occurs in all dicot woody plants—the root increases in girth by stelar and extrastelar cambium activity.
1. Origin and Activity of Vascular Cambium ring:-
> The vascular bundles that are present in dicot roots are radial.
> The xylem is exarch. That means the protoxylems are directed towards the periphery and the metaxylem towards the centre.
> The cambium is absent initially but develops later at the time of secondary growth.
> The pericycle cells lying outside the protoxylem and the conjunctive parenchyma cells on the lateral side of the phloem bundle become meristematic to form many cambial strips.
> During development, these strips become continuous laterally. This is due to the tangential divisions of pericycle cells opposite each protoxylem layer. Thus, a wavy continuous cambium ring is produced.
> This ring is present below the phloem but above the protoxylem and is a secondary meristem.
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> First, the strips of cambia below the phloem cut off the new secondary xylem inward.
> The vascular cambium developed from the pericycle gives rise to only ray initial cells. The development of these ray initials is slower than the formation of secondary vascular tissues. Due to this, the depressed parts of the vascular cambium move outward, and ultimately the cambium becomes circular.
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> The cambial cells divide tangentially again and again and produce secondary tissues, joined with the first formed cambium strips to form a complete but wavy continuous ring of the vascular cambium.
> The cambial cells produce more xylem elements than phloem elements and produce secondary xylem and secondary phloem like the secondary growth in stems.
> Thus, after stellar secondary growth, a solid central cylinder of wood is developed.
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2. Origin and Activity of Cork Cambium ring:-
> The cork cambium or phellogen, or secondary meristem, develops because of the tangential division of the outer cells of the pericycle.
> The activity of cork cambium is like that found in the dicot stem, so it produces cork cells or phellem towards the outside and phelloderm or secondary cortex towards the inside.
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> Suberin is a lipid-like substance secreted from the protoplast of cork cells deposited in the walls.
> As the cork cambium in the dicot root develops from the pericycle cells, all tissues lying outside the cork (endodermis, cortex) are completely sloughed off.
> These cells die due to further deposition of suberin. The epiblema dies out earlier, and the lenticels may also be formed.
Difference Between Secondary Growth in Dicot Stem and Dicot Root:-
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Origin of lateral roots:-
> The lateral roots are endogenous in origin (from a deeper layer).
> The seat of its origin is cells of pericycle, usually opposite a protoxylem group, become meristematic and go on dividing periclinal and anticlinal.
> Lateral Root showing the Vascular Connection between it and the parent Stele.
> Lateral roots are produced in a definite pattern.
> The youngest roots are closing (proximal) to the apex. The pattern of lateral root production is readily observed with tap root systems.
> The tap root can also be called the primary root while its branches are called secondary or lateral roots.
> The origin of lateral roots can be unraveled for species that have fibrous root systems. A clear pattern emerges with careful study.
> The hump penetrates into the cortex and emerges as a lateral branch. Later, the hump differentiates into 3 regions of the root apex- dermatogen, Periblem and Pleroma. Finally, the lateral root comes out. > The number of lateral roots corresponds to the number of xylem bundles.
