PISUM SATIVUM 349 



In the lower hypocotyl, at a level slightly above the triarch root 

 described above (Fig. 175 and Fig. 177, A), definite changes occur 

 in the arrangement of the stelar elements. The xylem is not triarch 

 and the radial primary xylem strand is organized into groups that 

 are tangentially mesarch in their orientation. A central pith is 

 differentiated just below the level of divergence of the cotyledonary 

 traces and the first leaf trace from the hypocotyledonary stele. 

 Tangential bands of metaxylem connect these three potential traces, 

 thus forming with them a ring of vascular tissue which surrounds 

 the medullary parenchyma. (Fig. 177, B.) 



Slightly above this point, the three phloem groups are each 

 separated into two distinct phloem strands. (Fig. 177, C) At 

 succeedingly higher levels, the groups of primary xylem, which are 

 directly continuous with the cotyledonary traces, increase their 

 angular divergence from one another and incline outward. (Fig. 

 177, D, c-i and c-x.) Associated with the xylem of each cotyledon- 

 ary strand are two groups of phloem which originate as branches 

 from the adjacent strands of the root phloems, and the remaining 

 halves of the original phloem groups of the root anastomose out- 

 side the xylem of the first leaf trace. (Fig. 177, C, ph I i.) Each 

 group of cotyledonary phloem is branched, and the resulting strands 

 that lie in the sector between the cotyledons are again divided into 

 several groups, the two median ones anastomosing to form the 

 phloem of the second leaf trace. (Fig. 177, D, I 2..) The two 

 outermost groups of phloem in the intercotyledonary plane, to- 

 gether with small strands of xylem which are diverged from the 

 adjacent metaxylem, continue into the epicotyl as cortical fibro- 

 vascular bundles. (Fig. i-yy, D, cortfv b.^ The remaining phloem 

 groups increase in size and form the four lateral phloem strands of 

 the epicotyl. 



At the cotyledonary node, the protoxylem of the second leaf trace 

 is differentiated and lies in the sector between the cotyledonary 

 traces, although Compton notes that the position of the xylem of 

 this trace may vary. (Fig. 177, D, / i.) In some instances, its 

 initial position is external to the intercotyledonary band of meta- 



that given by Gerard, Compton, Gourley and other earlier authors whose work is reviewed. 

 He concludes that the special structure of the central portion of the axis of the first four 

 internodes does not show any relation to the vascular transition of the root and stem, ascribing 

 the characteristic organization of the lower internodes to the peculiar orientation of the 

 vascular bundles that supply the first bract-like leaves and to the manner in which they become 

 confluent. 



