104 TRANSLOCATION IN PLANTS 



would explain continued transfer when the xylem is 

 completely removed (Curtis, 1925). Furthermore, as is 

 explained in Sec. 25, ringing is not necessarily followed by 

 plugging and yet solute translocation is seriously inter- 

 fered with. Dixon and some of those supporting his 

 hypothesis claim the ringing interferes with upward 

 translocation because it results in plugging the outer 

 layers of the xylem, but according to MacDougal's sug- 

 gestions the upward translocation, in the pine at least, is 

 chiefly in the inner layers and little or none occurs in the 

 outer layers which are supposed to be concerned largely 

 in downward movement. 



Arndt (1929), on observing the movement of eosin 

 solutions introduced into cut stems or roots of the coffee 

 tree, concluded that solutions move both downwards and 

 upwards through the xylem vessels and that the downward 

 movement would be adequate for transporting foods. He 

 found that the presence of leaves was not necessary for 

 such movements, for they occurred even after he had 

 supposedly eliminated influences of negative gas pressure, 

 capillarity and saturation deficits of the xylem. He 

 found greatest movement — and this occurred simulta- 

 neously in both directions — in the outer layer of wood and 

 suggests that this layer may be ''composed of closely 

 associated conduits which are hydrostatically isolated 

 from each other" and that "a more complicated vascular 

 mechanism exists in plants than has usually been postu- 

 lated." In common with many other investigators Arndt 

 seems to fail to recognize that movement in conduits 

 after they are cut open may have no bearing on normal 

 water or solute movements. 



19. Comments on Evidence Supposed to Prove Backward 

 Flow through the Xylem. — The hypothesis that the xylem is 

 the principal channel for the backward transport of solutes 

 seems to have been proposed because injected solutions 

 failed to be carried through the phloem in appreciable 

 quantities, and because the narrowness of the phloem 

 tubes together with their high content of seemingly 



