572- 



THE STRUCTURE OF ECONOMIC PLANTS 



ferentiated tangentially, approaching the periphery of the stele 

 more closely; and the protoxylem develops in a more centrad 

 position. The relation of the proto- and metaxylem is not com- 

 pletely endarch at the level of the cotyledonary plate. Coincident 



px 



~o ph 

 en 



Fig. 199. A-F, successive stages in vascular transition from diarch, root-like, lower 

 hypocotyl to cotyledonary node. Completely endarch bicollateral bundles occur in cotyle- 

 donary petiole : en, endodermis ; / ph, inner phloem ; mx, metaxylem ; ph, outer phloem ; 

 /»<*r, parenchyma ; pc/, pericycle ; ph, phloem; /ix, protoxylem. (After King.) 



with the reorientation of the xylem, the strands of inner phloem 

 form two groups that are located on the inner faces of the primary 

 xylem groups, and the outer phloem is oriented outside them. 

 As a result of adaxial differentiation of the protoxylem and abaxial 

 orientation of the metaxylem in the cotyledon, the endarch condi- 



