ALLIUM CEPA 193 



through the outer cortical cells, finally penetrating the bases of 

 two or three fleshy leaf sheaths. In some instances, the root pene- 

 trates the inner epidermis of a leaf sheath but fails to pierce the 

 outer, and the root grows upward through the mesophyll, eventu- 

 ally dying when the sheath becomes desiccated. The emergent 

 adventitious roots may branch after attaining a length of 10 to 15 

 cm.; but, during the first year, the older roots, as well as the lower 

 part of the crown stem and outer leaves, shrivel and die so that the 

 base of the bulb appears somewhat flattened. 



The adventitious roots are tetrarch, pentarch, or hexarch, the 

 pentarch being most common; and all three types may occur in the 

 same plant. (Fig. 87, A, B.) Their development resembles that 

 of the primary root, the chief difference being the larger histogenic 

 region and the correspondingly greater number of cells formed. 



The Vascular Anatomy of the Crown Stem. — The structure 

 of the entire hypocotyl is root-like; and, near its top, the two 

 protoxylem strands separate, one continuing into the cotyledon, 

 while the other ends blindly beneath the point of divergence of 

 the primordium of the second leaf. (Fig. 86, B.) Above the 

 point at which the hypocotyledonary xylem terminates, the xylem 

 of the cotyledonary bundle and of the stem develops in one direc- 

 tion — away from the protoxylem of the cotyledon and toward the 

 center of the stem axis. About ten days after germination, the very 

 short stem contains no mature xylem, except that of the coty- 

 ledonary bundle; and approximately ten more days are required 

 before the xylem of the trace of the second leaf is completely dif- 

 ferentiated in the first internode. The elements of this trace are 

 reticulate cells about three times as long as wide. 



All the bundles of the stem are common, representing the down- 

 ward divergence of leaf traces; and the vascular arrangement of the 

 stele is determined largely by the manner in which the leaves are 

 differentiated. As each leaf matures, the bundles in its cylindrical 

 sheath extend into the stem and anastomose with the ring of vas- 

 cular tissue. The bundles of each successively higher leaf trace 

 are vertically connected with those below, so that the central pith 

 is surrounded by a cylindrical stele consisting of a complex network 

 of vascular strands. The larger bundles of each leaf trace are con- 

 nected directly with the inner perimedullary portion of the vascular 

 cylinder, while the smaller ones are anastomosed with the periph- 

 eral bundles of the stele. 



