SOLUTE DISTRIBUTION AND BEHAVIOR 245 



in the fertilized group. This might well be explained on 

 the basis of increased streaming activity, as tentatively 

 suggested above. The backward gradient for reducing 

 sugars was greatly steepened in the fertilized bolls and less 

 so in the unfertilized bolls. During the 7-day period 

 the unfertilized bolls gained steadily, but to a smaller 

 extent, in both nitrogen and carbohydrates, so the period 

 of removal observed by Howie tt (1926) for apples had not 

 set in. If the unfertilized bolls later lose nitrogen, this 

 would point to a real reversal of movement, and concen- 

 tration gradient studies on this material might be more 

 valuable than those in which actual reversal has not been 

 demonstrated (see Sec. 35). 



It may be that the ability of the receiving cells to remove 

 a given solute from the moving stream, thus determining 

 the direction of the gradient, may be as important as 

 maintaining an actively moving stream between the 

 receiving and supplying cells. Phillis and Mason (1933) 

 give clear evidence that phloem cells can remove sugars 

 from living cells against a concentration gradient. If they 

 can do this, it is obvious that the relative abilities of the 

 receiving and supplying cells may be of greater significance 

 in controlling direction of movement than actual con- 

 centration gradients. The seeming demand of the proto- 

 plasmic streaming hypothesis for a positive concentration 

 gradient may therefore be eliminated. If sugars are 

 carried in an interfacial film and the movement of this film 

 is unidirectional in sieve tubes (see Sec. 36), the direction 

 would be controlled by the direction of film spread and 

 not by a concentration gradient. Van den Honert (1932), 

 however, assumed that the direction of film spread is 

 controlled by the sugar gradient itself. The demand of 

 the Miinch hypothesis for an osmotic gradient is not 

 fulfilled in the case of transport from fruit or leaves that 

 are about to abscise. 



On the assumption that rapid translocation is brought 

 about by streaming movements in living cells and that the 

 activity of cells in a given tissue may induce increased 



