THE METHOD OF MOVEMENT 159 



directive flow of solutes. If the direction of flow is deter- 

 mined chiefly by pressure or turgor gradients, and by the 

 resistance to flow in the connecting channels, it would seem 

 that those cells with the lowest turgor pressures and nearest 

 to the supply, or those having the lowest resistance in the 

 channels leading to them, should always receive the 

 greater part of the food coming from the leaves or storage 

 organs. This, however, seems not to be the case (see 

 Sec. 42). Very commonly those tissues having the lowest 

 concentration or those that are nearest to the supply fail 

 to receive the foods which pass close by them and move to 

 tissues much more distant and with higher turgor pres- 

 sures. Maskell and Mason (19306) report high flaccidity 

 in unfertilized cotton bolls and much greater transport to 

 fertilized bolls that were more turgid. To explain this, one 

 might have to postulate that the receiving cells in some 

 way control the flow to them by keeping the plasmodesma 

 and sieve pores open, while in the others they are closed or 

 the resistance to flow is in some way increased. While not 

 an impossible supposition, the necessity of assuming special 

 controls of this sort weakens the hypothesis. Further- 

 more, the actual emptying of materials from flaccid tissues, 

 as mentioned under h and c, demonstrates that the conduct- 

 ing system has not become plugged. 



4. The mechanism proposed by Miinch seems not to 

 offer a simple explanation for the movement of specific 

 types of materials to special tissues or organs. A receiving 

 tissue could steepen the osmotic gradient by removing 

 sugars for example, but the resulting unidirectional mass 

 flow would carry all the movable phloem contents toward 

 that same tissue. Through selective permeability the 

 receiving cells might, for example, prevent the entrance 

 of the nitrogenous materials, but there must be some 

 mechanism to remove these, for otherwise they would 

 accumulate in the conducting channels outside of the 

 receiving cells and thus lessen the osmotic gradient in the 

 conducting channel or tend to clog it mechanically. 

 The receiving tissue, according to this hypothesis, would 



