DOWNWARD TRANSPORT THROUGH THE XYLEM 105 



viscous, colloidal material, and the frequency of their 

 cross walls seemed to preclude the possibility of an ade- 

 quate transport through the phloem. The xylem tubes, on 

 the other hand, are large, have relatively infrequent cross 

 walls, a content of low viscosity, and injected solutions 

 are easily and rapidly carried in either direction or can be 

 readily forced through the tissue in either direction. The 

 seemingly frequent presence of sugars occurring naturally 

 in these xylem tubes also tends to favor this hypothesis. 



The evidence discussed in Chap. II, however, especially 

 that which demonstrates that translocation takes place 

 when the xylem is completely removed, seems to show 

 conclusively that the phloem can and does carry solutes 

 at a rate at which they seem normally to be carried. In 

 the Ught of this evidence, therefore, negative evidence, such 

 as the seeming inability to force solutions through the 

 phloem, or that it is difficult to conceive of a highly efficient 

 mechanism, is not very convincing. 



Though it is possible that occasionally there may be a 

 small amount of backward flow of water through the xylem, 

 it seems probable that the frequency and extent of such 

 movement are normally much less than those indicated by 

 injection experiments. As is discussed in more detail in 

 Sec. 20, the backward flow of an injected solution could be 

 expected to occur whenever a conducting system under 

 tension or reduced pressure is opened and solutions at 

 higher pressure are introduced. That backward flow is 

 frequent or normal has not been demonstrated. Even 

 if backward flow were as frequent and extensive as might 

 be assumed to be indicated by injection experiments, the 

 commonly low sugar content, or almost complete lack of 

 sugar found in the water-conducting tubes of the xylem 

 at the season when extensive backward translocation 

 must be taking place, reduces the likelihood of appreciable 

 amounts of sugar being carried through this channel. 



Even if backward flow were frequent and extensive, and 

 even though there is considerable resistance to transverse 

 flow of water through the xylem, there is no indication of a 



