SOLANUM TUBEROSUM 



52-5 



final stages of development, the epidermis and cortex become 

 fissured and fragmented down to the endodermis. At such points, 

 the endodermis develops thickenings on the inner tangential walls 

 and the underlying pericyclic cells may divide tangentially to form 

 a periderm. 



Vascular Transition. — The transition occurs in the hypocotyl 

 and cotyledonary petiole. It involves a reorientation of the stelar 

 elements, which is here described beginning at the root-like por- 



FiG. 2.79. A-I, diagrams representing vascular transition. Dotted areas indicate primary 

 phloem; solid areas, protoxylem ; lined areas, metaxylem. (Adapted after Theil.) 



tion of the lower hypocotyl and proceeding in an ascending direc- 

 tion to the cotyledons. 



The first variation from the diarch radial protostele is the 

 division of the protoxylem strand into two sectors owing to the 

 fact that the cells of the central region remain parenchymatous 

 instead of differentiating into metaxylem. (Fig. 179, B.) At the 

 same level, each of the primary phloem regions subdivides into two 

 or three distinct groups. Above this point, there is a change 

 in the direction of differentiation of the later formed proto- and 

 metaxylem; which, instead of being laid down centripetally, on 

 the same radius as the protoxylem, is differentiated to the right 



