THE METHOD OF MOVEMENT 163 



flow of solution through pores, such as plasmodesma or 

 sieve pores, must necessarily have a tendency to carry 

 materials that lie across these pores. In other words, the 

 outer layers of protoplasm and the protoplasm within the 

 pores themselves, unless it were highly porous and allowed 

 the solution to filter through it readily, would thus be 

 forced from cell to cell. This would favor the transport 

 of the outer layers of protoplasm, would not allow for its 

 return, and would not allow for direct transport of vacuolar 

 contents where much of the osmotically active material, 

 particularly in parenchyma cells, is probably located, and 

 yet it is the osmotically active material which, according 

 to the hypothesis, is responsible for the transport and is 

 itself transported. It is true that plastids manufacturing 

 or storing carbohydrates are not located in the vacuole 

 but the osmotic concentration of the vacuole must equal 

 or exceed that of the cytoplasm. Miinch, though he pic- 

 tures the protoplasmic layer as the moving layer, seems 

 not to consider this as a difficulty. In the sieve tubes, 

 however, if the sieve pores are so large that they are merely 

 lined with protoplasm, thus leaving the vacuole continuous, 

 this criticism would not apply, but it would seem to apply 

 to movement out of the parenchyma cells and into the 

 receiving cells of any and all kind of plants, and even in 

 sieve tubes it has not been definitely demonstrated that 

 the vacuolar contents are continuous. Although sieve 

 pores have been described as having a parietal layer of 

 protoplasm (Hill, 1908), that this is universally true or 

 even occasionally so has not been conclusively demon- 

 strated. In fact. Crafts (1932) has failed to find any indi- 

 cation of continuity of vacuolar contents. 



To allow for mass flow of the sort postulated it would 

 seem that those types of phloem that have few or only 

 occasional sieve tubes, as well as those in which the sieve 

 pores are filled with protoplasm (see Lecomte, 1889; 

 Schmidt, 1917; and Crafts, 1932) would not be very effec- 

 tive as channels for transport. Furthermore, sieve tubes 

 often are imperfectly developed close to the shoot and root 

 meristem to which food must move fairly rapidly. 



