HORMONAL REGULATION OF PLANT CELL GROWTH 187 



sure (P) against the elastically stretched, largely polysaccharide cell 

 wall. The following effects on water uptake have been suggested 

 as possible explanations for the promotive action of auxin on growth: 

 (a) causing the 11 of the cells to increase; (b) increasing the osmotic 

 permeability; and (c) inducing active transport of water, resulting 

 in more water uptake than would occur osmotically. The first of 

 these ( a ) has been excluded pretty conclusively in the case of grow- 

 ing potato tissue and oat coleoptile sections, for 11 may fall during 

 growth. 



A positive effect of auxin on osmotic permeability has been re- 

 ported, but this could cause a significant effect on growth rate only 

 if the growing cells were far from being in osmotic equilibrium ( i.e., 

 in a dilute medium P < < R ) , so that rate of osmotic water uptake 

 is "limiting" the rate of growth. On the other hand, if active trans- 

 port of water were involved, it would most likely act by increasing 

 P above the osmotic equilibrium value (i.e., in a dilute medium 

 P > n ) and thus "stretching" the cell. 



In the last few years it has become widely accepted that growing 

 plant tissues are so permeable to water that they must remain prac- 

 tically at osmotic equilibrium regardless of whether the continu- 

 ously occurring water uptake is entirely osmotic or involves some 

 tendency toward active transport, so that neither significant excess 

 nor deficiency of P could prevail. Unfortunately there is no satis- 

 factory method for determining P directly. Brauner and Hasman 

 ( 1952 ) and Ordin, Applewhite, and Bonner ( 1956 ) , using the con- 

 ventional indirect method of determining (11 — P ) for plant tissues, 

 reported, respectively, that growing potato tissue and oat coleoptile 

 sections were practically in osmotic equilibrium. The conventional 

 method would not, however, detect an "actively maintained" P with 

 any certainty. And there is considerable doubt as to its even ap- 

 proximate validity for a rapidly growing tissue such as the oat cole- 

 optile, in which possible growth during the interval of measurement 

 is large compared with elastic changes in size, due to changes in P, 

 which the method depends upon detecting. Convincing demonstra- 

 tion that active transport of water occurs in plants has not yet been 

 obtained, and it does appear likely that slow-growing tissues such 

 as potato are nearly at osmotic equilibrium. Levitt (1953) calcu- 

 lated that this should be so, but he considered only water uptake 

 into a single cell from the medium rather than tluough the tissue 

 and employed an osmotic permeability value determined for tissue 



