THE METHOD OF MOVEMENT 195 



sucrose about forty thousand times as great as the diffusion 

 constant in a 10 per cent solution of sucrose in water. 

 This, they point out, '4s almost identical with the diffusion 

 constant for molecules of the size of the sucrose molecule 

 diffusing in air." 



In their analyses of sugar concentrations in the bark and 

 the boll of cotton, Mason and Maskell found in the bark 

 a concentration of 2.441 per cent reducing sugars, and 

 2.647 per cent of sucrose (estimating the amounts in the 

 "sieve tubes" or phloem proper these would be 1.019 

 reducing sugar, and 4.992 sucrose). The concentrations 

 in the boll were 4.870 per cent reducing sugar and 0.626 

 per cent sucrose. It is obvious, therefore, that the con- 

 centration gradient for sucrose would tend to cause its 

 diffusion toward the boll. That for hexoses, however, would 

 tend to cause them to be carried back toward the leaf 

 tissues unless one postulates, as do the authors, that there 

 is the equivalent of a membrane between the ''sieve tube 

 and other tissues of the boll, permeable to sucrose but 

 impermeable to reducing sugars." (See Sec. 42 for addi- 

 tional data on gradients.) If the sugars pass readily from 

 cell to cell through plasmodesma, however, unless these 

 have also the properties of membrane semipermeability, it 

 would seem that the hexoses would pass back into the 

 sieve tubes, and be rapidly carried back to the supplying 

 cells by the same protoplasmic streaming mechanism that 

 brought sucrose from the supplying cells. One can be 

 certain, however, that protoplasmic streaming is rarely if 

 ever a simple mass stirring of cell contents, but that there 

 is considerable structural organization (see especially 

 Scarth, 1927) which may be highly effective in controlUng 

 the movement of special constituents. This structural 

 streaming in narrow strands can be readily seen in many 

 living cells and especially in phloem cells. The different 

 solutes, whether merely different carbohydrates or different 

 in other respects, may occupy different parts of the living 

 cell. Thus certain solutes, such as sucrose, may be present 

 in the moving parts while others, such as hexoses, may be 



