404 THE STRUCTURE OF ECONOMIC PLANTS 



The development of the first leaves of the adventitious bud is 

 accompanied by a differentiation of their vascular elements so that 

 protoxylem is present in them before cell division has begun in the 

 inner layers of the cortex of the hypocotyl. (Fig. xo6.) Follow- 

 ing the proliferation of the parenchymatous cells of the inner 

 portion of the cortex, the vascular tissue of the new bud is differen- 

 tiated centripetally to the endodermis from some of the derivative 

 cells thus formed. After several tracheids have been differentiated 



Fig. 105. Longisection of hypocotyl showing development of adventitious bud after 

 plant has been severed below cotyledons. (After Crooks.) 



in this manner in the cortical parenchyma, a number of tangential 

 and radial divisions occur in the endodermis. (Fig. io6.) Some 

 of these endodermal derivatives differentiate as tracheids and estab- 

 lish continuity between the vascular system of the bud and tra- 

 cheids which have differentiated from pericyclic cells and phloem 

 parenchyma. The phloem associated with these newly differenti- 

 ated tracheids consists chiefly of elongated parenchymatous cells, 

 and by the time the vascular connection described above is com- 

 pleted, a layer of cells surrounding the tracheids begins to function 

 as an active cambium. Within a few weeks, the secondary tissues 



