64 



THE STRUCTURE OF ECONOMIC PLANTS 



of the vascular bundles of the leaves and branches which in the 

 aggregate constitute the stele. 



The meristematic regions of the root and shoot are similar with 

 respect to the characteristics of the cells which comprise them, but 

 they differ markedly in other ways. One obvious difference is that 

 the root meristem is surrounded and protected by cells differentiated 

 from it which constitute the root cap, while the stem meristem is 

 exposed except for the protection afforded by the leaves diverged 



Fig. i6. Median longisection of the stem tip of Linum showing several stages in the ontog- 

 eny of the leaf: a, section of a very young leaf primordium; h and c, older primordia showing 

 early development of provascular strands; /»/', pith. (After Crooks.) 



