It is common in dicot roots except a few short lived herbs and submerged aquatics. In dicot roots, the meristematic tissue(cambium) is completely secondary in origin, secondary growth in dicot roots takes place by the formation of secondary tissue. Two types secondary tissue-the secondary vascular tissue and the periderm are produced during secondary growth. These are formed by the lateral meristems vascular cambium and cork cambium respectively.
Origin and activities of vascular cambium
The primary dicot roots lack cambium. The vascular cambium develops later as secondary meristem. It develops partially from conjunctive parenchyma and partially from pericycle. The conjunctive parenchyma and partially from pericycle. The conjunctive parenchyma cell lying on the inner side of the primary phloem bundles, becomes meristematic to form cambium strips. In the meantime, cells of the pericycle just outside the protoxylem also become meristematic. These join with the earlier formed cambial strips on the inner side of the primary phloem to form a wavy band of vascular cambium extending over the xylem and down phloem.
The cells of cambium derived from pericycle. functions as ray initial. These cells produce parenchyma cells both on the outer and inner sides. These multiseriate radial bands of parenchyma is called medullary rays. The portion of the cambium adjoining the inner side of phloem cuts off new cells on both the sides, but more profusely on the inner side causing the cambium and phloem to be gradually pushed otherwards. The wavy band of cambium becomes circular or ring like. Now the whole cambial ring becomes active and produces secondary xylem on the inner side than on the outer, and consequently, secondary wood increases more rapidly to form a continuous cylinder with primary xylem on the inner side and primary phloem get destroyed. As the secondary growth continues, the addition of more secondary xylem results in primary phloem pushed out and get crushed. But the primary xylem bundles still remain intact and can be recognized by their exarch nature and central medullary rays in the general mass of secondary xylem and secondary phloem. The part of phloem is called phloem rays. Other smaller and thinner medullary rays are also formed later by the cambium. Medullary rays are larger and more prominent in the roots than in the stem. They help in the radial conduction of water, minerals and food materials.
Origin and Activity of Cork Cambium
Secondary growth in dicot roots results in the addition of more secondary tissues in the stelar region. It exerts a pressure on the peripheral tissues and that may be ruptured. To give protection to the roots, primary peripheral tissues including the epidermis are replaced by a more efficient protective covering known as periderm, formed from the origin and activity of another cambium in the stele itself called cork cambium or phellogen.
The pericycle is already partly meristematic, forming a part of vascular cambium that forms the secondary vascular tissues. In most of the plants, the rest of the pericycle also become meristematic, all round the root. It is the cork cambium or phellogen. The phellogen cuts off cells to the inner side and also outer side of it, more to the outer side and less to the inner side. The cells produced towards the inner side differentiate into the secondary cortex or phelloderm. The secondary cortex of the root does not contain chloroplast. The cells produced towards the outer side become gradually suberized in their walls, lose their protoplasmic contents and become a dead tissue, the cork or phellem. The three tissues, the phellem, phellogen and phelloderm together constitute the periderm. The cork has many lenticels to facilitate gaseous exchange. The annual rings in dicot roots are not so well distinct, as the case of woody dicot stem because the temperature usually remains the same in the soil throughout the year. The bark in roots include all the tissues which lie outside the cork tissues. As the pericycle becomes phellogen, the endodermis, the cortex and the epidermis which lie out of the cork become dead and form the bark of the root.