Transpiration and Water Movement 103 



movement of pure water. In osmotic transfer water does 

 move independently of substances in solution, but it is 

 always associated with substances in solution. 



The movement of water in the plant has been a line of 

 experimental inquiry since the dawn of plant physiology. 

 Many important facts have been clearly enunciated and 

 numerous interesting data accumulated, yet some of the 

 phenomena of movement observed find as yet no entirely 

 satisfactory explanation. A chief source of difficulty lies 

 in the complexity of the factors involved. 



In general, however, diffusion is important, but the rise 

 and maintenance of water are complicated by such factors 

 as capillarity, the cohesive strength of water columns, 

 the lifting power of evaporation, the peculiar structure 

 of the conducting vessels, and, under certain conditions, 

 the existence of high root pressures. 



It has been stated that in the root the region of hair 

 production is commonly characterized by a radial bundle 

 arrangement apparently permitting more readily the move- 

 ment of water from the pare.nchymal cells directly into the 

 woody portion of the bundle. There is, of course, no such 

 thing in plants as a true circulation, analogous to the circu- 

 lation of the blood in animals ; yet it is possible to dis- 

 tinguish in a very general way two types of movement in 

 vascular plants, as indicated below. 



(1) There is a " transpiration stream," from the absorb- 

 ing organs (of water containing some salts and com- 

 monly traces of organic substances) to the leaves. This 

 stream is directed mainly through vessels, the xylem part 

 of the woody bundles (Fig. 32). During this transfer 

 there is, moreover, general diffusion to all parts requiring 



