Dicotyledonous roots have limited number of radially arranged vascular bundles. With the initiation of secondary growth some parenchymatous conjunctive tissue cells become meristematic beneath the phloem groups, thus forming strips of meristems equal to the number of phloem groups present.
The cells of one-layered pericycle now divide. The meristems by further extension reach the pericycle abutting the protoxylem and form a complete wavy band of cambium passing internal to phloem and external to xylem. The first formed meristem is much more active than the lately formed ones and produces more xylem on the inner side. As a result the wavy cambium ring is rendered circular.
Now the cambial cells divide to produce secondary tissues in the same manner as in dicotyledonous stems. Cambium against every protoxylem group gives rise to parenchyma cells which run as longitudinal strands and are called main medullary rays.
As a result of addition of new tissues in the intra-stelar region a pressure is exerted on the epidermis. The pericycle as a whole becomes meristematic and constitutes the phellogen or cork-cambium. Cork and bark are produced in the same manner as in stem. Lenticels may also be formed. (Fig. 160)
Fall of Leaves:
Deciduous trees usually shed leaves with the approach of winter, when the leaves usually turn yellow due to breaking down of chlorophyll into colourless substances and increase in the concentration of carotinoid pigments. The immediate structural reason for the fall of a leaf is the formation of a separation-layer, called absciss layer, at the base of the petiole surrounding the bundles (Fig. 161).
The parenchymatous cells there undergo chemical changes often dissolving the cementing materials and thus form the absciss layer. Now the leaf remains attached only by the bundle. By its own weight or due to wind it breaks and the leaf falls. The wound is healed up by the formation’ of cork, leaving a mark there called leaf- scar.