Studies on the Growth of Tobacco Roots : I. The formation of branching roots in tobacco plant

In tobacco plants the lateral root originates in the pericyclic cells of the diarch root between one protoxylem and one protophloem, but in the root more than triarch opposite to a protophloem, whereas all adventitious root primordia arose from the outer part of cambium. On the lateral root primordia, some neighbouring pericyclic cells undergoes periclinal division to in two layer (fig. 2-3) followed by anticlinal division (fig. 4-5). In this process, endodermis cells undergo anticlinal division until calyptrogen differentiation (fig. 4-7). On the other hand, in the adventitious root formation, the phloem ray cells undergoes periclical division to in two layer, and developed to root primordia by random division (fig. 8-12). In the tobacco plants root dimorphism were recognized the same as woody plant, one was thicker in diameter than the other. The distinction between big and fine roots has been shown to be based upon a number of development characteristics including the size in the apex cell, the degree of primary growth and arrangement of primary vascular tissues in the stele (fig. 19-22). The primordia of the two types appear to be distinct in origin, fine root arise from pericycle in diarch root, while the origin of big root is pericycle in multiarch root or phloem ray outer cambium. A according as the mother root became thickening the number of the root primordia increased to high at from 1 to 2 mm in diameter (fig. 23). In this stage, the organization in the root vascular cambium, now a cylinder, has produced additional secondary tissues (fig. 29). Poor branching capacity of may be related to their formation of periderm. On the other hand, the formation of multiseriate rays makes possible the development of root primordium and is an important means of excentric growth of root.