474 THE STRUCTURE OF ECONOMIC PLANTS 



centrifugally from the xylem differentiate into new elements of this 

 tissue, while new phloem cells arise on the opposite side of the bundle. 

 Since, however, the differentiation of elements progresses faster than 

 the division of procambial cells the xylem and phloem gradually 

 approach each other." (Fig. 142..) 



Fig. 2.4X. A-C, transections through portions of petioles showing differentiation of pro- 

 cambial strand ; E, G, I, transections showing early differentiation of vascular tissue ; D, 

 F, H, diagrams of entire bundles from which drawings £, G, I were made : obi, obliterated 

 phloem ; d, oil duct ; ^r, procambium. (Afcer Esau, Hilgardia.') 



