174 TRANSLOCATION IN PLANTS 



meristem. Some additional mechanism for bringing solutes 

 into the meristem must be necessary because each crop of 

 new cells can hardly depend upon the foods released by the 

 maturing ones for their nutrition, even if the mechanism 

 were so perfect as to allow no loss whatever by leakage. 

 In the third place evidence demonstrating that solutions 

 can or do flow within the walls of such meristematic cells 

 is lacking. Steward (1930) has given evidence that points 

 very clearly to the complete inadequacy of diffusion along 

 walls for supplying growing tissues with necessary solutes. 



33. Hypothesis Accounting for Transport by Moving 

 Protoplasm. — The hypotheses thus far discussed assume 

 that there is a mass flow of solution through the phloem 

 cells, Miinch suggesting that the flow is through the lumen 

 and Crafts suggesting that the entire phloem cross section, 

 lumen, protoplasm, and cell wall form the channel. Among 

 the older proposals we find the suggestion that moving 

 protoplasm itself may carry the solutes. Hartig (1858) 

 was among the first to observe protoplasm moving within 

 cells and suggested that it might be moving nutritive sap. 

 DeVries (1885) pointed out that diffusion alone is altogether 

 too slow to account for translocation and suggested that 

 protoplasmic streaming would greatly hasten the move- 

 ment. He observed and measured the rates of streaming 

 in companion cells and phloem parenchyma. The hypothe- 

 sis that this protoplasmic streaming would account for 

 translocation seems to have been rather generally dropped 

 shortly after its proposal, however, for Lecomte (1889), 

 Strasburger (1891), and others found that streaming, 

 though active in the individual sieve-tube elements when 

 they are young, is no longer visible after the cytoplasm 

 forms a thin layer next to the wall and the pores appear 

 to open between sieve-tube segments. This is just the 

 stage at which one would expect the tubes to become 

 functional, but since movement was not observed after 

 this stage, it was assumed that there must be some other 

 mechanism concerned in transport. 



As was pointed out in an earlier paper, however (Curtis, 

 1929), it seems highly possible that the failure to observe 



