286 PLANT PHYSIOLOGY 



plasm in the sieve tubes by its movements of circulation and ro- 

 tation probably aids the diffusion processes, which would other- 

 wise go on very slowly. In this process, the nuclei would be of no 

 advantage by their immediate presence but can function equally 

 well from the adjacent companion cells. Downward movement 

 is thus seen to differ from the upward movement in the xylem 

 and to require the cooperation of living cells. 



The chief objection to the phloem as the sole carrier of mate- 

 rials downward in plants like the potato and yam (Dioscorea), 

 where much translocation must take place, is that it seems hardly 



Fig. 17. — A schematic representation of the arrangement of the phloem 

 cells in the corn. The small cells are the companion cells and the large ones are 

 the sieve tubes. In most plants the arrangement is not so regular as in the corn. 



big enough. To accomplish the observed results, the downward 

 diffusion must proceed at the rate of about 88 cm. per hour (Mason 

 and Lewin, 1926). Nevertheless, Mason and Maskell (1928) 

 in a series of very careful experiments on cotton found that the 

 mass movements of carbohydrates occurred only in the outer 

 part of the stem, i. e., in the phloem, and that here diffusion of 

 sugar was 40,000 times faster than would occur by physical 

 means only. This is to say that down through the protoplasmic 

 cells of the sieve tubes diffusion was 40,000 times faster than a 

 2% sugar solution would diffuse in water. This is almost as fast 

 as molecules the size of sucrose would diffuse in air! Evidently 

 the living protoplasm of the sieve elements has a way of " getting 

 speed out of" the sugar molecules that we are not yet able to 

 understand. Furthermore, these two workers found (1931) that 

 the inorganic elements (Ca, N, K, P) also travel downward in the 

 phloem. 



Since the phloem is especially rich in proteins (60% of the dry 

 weight in some cases) it has been proposed that a part of the pro- 

 tein synthesis takes place here. Such questions, however, still 

 await their ultimate solution. All that can be said is that the 

 downward translocation of materials in some way takes place 



