104 



Plant Physiology 



water, or possessing a sufficient tension therefor. From 

 what has been said of water-loss and of the principles of 

 diffusion it will be apparent that the type of movement 



FIG. 32. Cross-section of primary fibrovascular bundle of Ricinus : 

 phloem (P), showing sieve tubes OS), companion cells (AC), parenchyma 

 and sclerenchyma ; cambium (C) ; and xylem (X), showing especially 

 vessels (V) and tracheids. [After Curtis.] 



here discussed is more than diffusion. Moreover, rapid 

 movement is essential in order to supply the demands of 

 transpiration, and it is this transpiration stream which 



