311 THE STRUCTURE OF ECONOMIC PLANTS 



vessels; and, in most cases, there is a large, centrally located, 

 reticulate metaxylem vessel which may or may not be in direct 

 contact with the protoxylem at all points. Three primary phloem 

 groups develop between the xylem arms, and the peripheral phloem 

 cells differentiate as fibers adjacent to the pericycle. (Fig. 163.) 

 The pericycle is uniseriate at first; but early in ontogeny, there 

 may be tangential divisions of the cells outside the protoxylem 

 points, and later the entire pericyclic region becomes multiseriate. 



Fig. 163. Transection of young primary root showing triarch xylem: co, cortex; en, 

 endodermis ; ep, epidermis ; fib, phloem fibers ; tnx, metaxylem ; pel, pericycle ; ph, phloem ; 

 px, protoxylem ; r h, root hair. 



The cortex consists of an endodermis and four or five layers of large 

 parenchymatous cells which are limited outwardly by the epider- 

 mis. Lateral roots originate in the pericycle on the same radii as 

 the protoxylem points so that they are arranged in three longitudi- 

 nal rows, except in cases where the root is tetrarch. 



Vascular Transition. — The vascular system of the seedling 

 axis has been investigated by van Tieghem (31), Compton (8), 

 Winter (36), and others. Unlike the pea, the transition is entirely 

 hypocotyledonary, and the reorientation of the vascular tissues 



