THE METHOD OF MOVEMENT 231 



others that solutes move chiefly along walls in the absorbing root or to 

 the apical meristem are not well founded. 



33. DeVries seems to have been the first clearly to formulate the hypothe- 

 sis that protoplasmic streaming may be concerned in transport of solutes. 

 Largely because of failure to observe streaming in mature sieve tubes, the 

 hypothesis seems to have been discarded. Fairly satisfactory evidence has 

 been presented that explains failure to observe streaming in sieve tubes. 

 Several points seem to favor transport by streaming protoplasm. Such 

 movements would allow for transport at rates immensely more rapid than 

 by diffusion alone; it could account for selective transport to special tissues; 

 it could account for transport to great distances without requiring excessive 

 pressures or concentration differences; and it could account for transport out 

 of or into tissues independent of the osmotic concentration or turgor of 

 the tissues. 



34a. Although protoplasmic streaming would seem to account for trans- 

 port over considerable distances at rates immensely more rapid than would 

 diffusion alone, calculations indicate that the mechanism is not adequate. 

 These calculations, however, may be misleading because rates of streaming 

 in specialized elongated cells may greatly exceed those observed in paren- 

 chyma cells on which the calculations are based; and the measurements of 

 sieve pores and sieve tubes, which are based on observations of cut tissues, 

 may be much too low because the cutting has released high internal pres- 

 sures. It is also conceivable that strands of protoplasm may actually move 

 through sieve pores, and possibly even through plasmodesma, and thus 

 allow for more rapid transport. 



h. Though protoplasmic streaming has not been observed in mature sieve 

 tubes, its almost universal occurrence in other living cells and the fact that 

 injury attendant upon observation would easily account for failure to find 

 it in sieve tubes reduce the seriousness of this objection. 



c. Tests with aquatic plants have indicated that protoplasmic streaming 

 does not greatly hasten the transport of lithium and caffein, even though 

 they penetrate into the vacuoles. This, however, may be due to the type 

 of substance used which may not be taken up by the moving protoplasm. 

 Fluorescein has been demonstrated to be carried in protoplasm, though it 

 has also been denied that the moving protoplasm is concerned in its 

 transport. 



d. If materials are transported by moving protoplasm, it would seem that 

 there might be simultaneous transport in both directions. There is some 

 evidence for simultaneous movement and also for restricted unidirectional 

 movement. The best evidence for unidirectional movement is that on the 

 polar movement of the growth substance in Avena coleoptiles. There is 

 also evidence for unidirectional movement of certain viruses and that this is 

 not polar but in the direction of major movement of sugars. There is also 

 evidence of a simultaneous movement of virus in both directions. In 

 certain regions or under certain conditions fluorescein seems to move in 

 but one direction but under other conditions in both directions. 



e. It has not been demonstrated that the movement of materials is or 

 is not in the direction of a diffusion gradient. Mason and Maskell have 

 given strong evidence indicating that sucrose is the principal sugar trans- 



