ALLIUM CEPA 191 



slightly enlarged, the metaxylem vessels are somewhat smaller, 

 and the alternate, radial position of the xylem and phloem still 

 persists. (Figs. 83, C; 86, B.) 



The development of the cotyledonary xylem is exarch through- 

 out. In most cases, the protoxylem consists of spiral elements, 

 although some annular elements may occur; and annular thick- 

 enings are frequently present in the tapering ends of the tracheids. 

 The cotyledonary bundle has one protoxylem point, and a band of 

 metaxylem is later differentiated on each side of the youngest or 

 most recently formed protoxylem. This results in the formation 

 of a Y-shaped xylem region with an accompanying zone of primary 

 phloem at the end of each arm of the Y. The orientation of the 

 xylem and phloem is shown diagrammaticallyat successively higher 

 levels in the cotyledon in Figure 83, C-F. In some cases, the xylem 

 is separated into three or four groups by parenchymatous cells; 

 while, in others, the Y-shaped configuration may be flattened so 

 that it appears like a short-stemmed T. 



The maturation of the phloem occurs first at points farthest 

 removed from the protoxylem. Chauveaud (i), in describing the 

 development of the cotyledonary bundle in seedlings eight to ten 

 days old, reports a disintegration of the protoxylem as the meta- 

 xylem differentiates, attributing this to a complete resorption of 

 the protoxylem elements and their replacement in situ by meta- 

 xylem. Studies of this region by Hoffman, especially in longi- 

 section, indicate that there is not a complete resorption of these 

 elements; but that "the protoxylem vessels become so stretched 

 that the walls between thickenings collapse; and, in transection, 

 the vessels are seen as only a very small patch of cellulose, easily 

 overlooked." This stretched protoxylem is more difficult to find 

 as the cotyledon becomes older; but can be seen in longisections 

 of material less than four weeks old, at which time the second and 

 third leaves have appeared and the cotyledon has begun to wither 

 from its tip downward toward its base. 



Adventitious Roots. — All the roots, except the primary one, 

 are adventitious in origin. During the development of the seed- 

 ling, the first of these arises from the meristematic region of the 

 inner cortex. (Fig. 86, C.) Subsequent roots originate in a similar 

 manner, being produced in great abundance so that a plant five 

 months old may have up to a hundred root primordia and roots. 

 The later-formed roots originate in the pericycle, rather than in the 



