IPOMOEA BATATUS 



501 



In the upper hypocotyl, the four arcs of scattered phloem groups 

 are extended tangentially so as to form a discontinuous ring of 

 phloem elements some of which are collateral with the metaxylem. 

 The first strands of inner phloem are differentiated in the pith and 

 lie between the two laterally divergent arms of the metaxylem of 

 each xylem pole. (Fig. i6i, B.) The inner phloem is not con- 

 tinuous with the primary phloem of the root, and arises later in 

 ontogeny than the outer phloem of the hypocotyl which is con- 

 tinuous with that of the root. It consists of strands that have 



-iph 



mx 



A 



B 



Fig. i6i. A, transection of lower portion of hypocotyl of three-weeks-old seedling show- 

 ing tangentially oriented metaxylem arms ; B, transection through upper portion of same 

 showing further divisions of metaxylem and appearance of inner phloem : i ph, inner phloem ; 

 WZA-, metaxylem ; <? ^/>, outer phloem ; /)x, protoxylem. 



differentiated downward from the cotyledonary plate as continu- 

 ations of the inner phloem of the bicollateral bundles of the cotyle- 

 donary traces. 



There is usually no connection between the outer and inner 

 phloem, and the strands of the latter end blindly in the pith of 

 the hypocotyl. In the case illustrated in Figure 161, no inter- 

 connection between the outer and inner phloem occurred above or 

 below the level figured; but, in a few instances, they may be 

 transversely connected by the differentiation of parenchymatous 

 cells lying in the gap between the divergent and partially separated 

 arms of the primary xylem. The relationship of the inner phloem 

 to the outer has been variously interpreted by investigators who 

 have worked with other members of this family. Scott (11), in 

 his investigation of Ipomoea versicolor Meissn., states that 



