Does GA Act Through Auxin-mediated Mechanism? 599 



as bulk or fresh weight increases, one might conclude simply that GA 

 and lAA both promote growth and that the two interact to produce 

 a maximal effect on the same growth system. It should be noted in 

 passing that perhaps one reason for attempts to show close linkage 

 between gibberellin and auxin actions was the belief that the former, 

 like the latter, promoted cell extension or expansion rather than cell 

 division, although this generalization is not even completely valid for 

 auxin. In any case, several papers, such as that cited above and that 

 of Sachs et al. (10), have shown in elegant fashion that GA can act 

 as a potent cell-division factor as well. 



To associate a single growth factor with one component of growth, 

 as if others were not involved at all, is almost certainly an error, and 

 one which we do not wish to commit. Probably all the known growth 

 factors, and more, are necessary for each plant cell; experimental 

 techniques are such as to identify as participating only those which 

 can be made limiting to a given process. It seems unlikely, however, 

 that all or even several act in a single biochemical process limiting 

 many stages of development, and the value of thinking in terms of 

 some such master growth reaction is questionable. It might be just 

 as well to discard the general term "growth" entirely in such dis- 

 cussions and look in more specific morphological, cytological, and 

 biochemical detail at the phenomena in question before proposing 

 simple mechanisms for their interactions. 



ACKNOWLEDGMENTS 



We are indebted to Professor A. W. Galston for his encourage- 

 ment and criticism, and to Dr. B. A. Bonner for reading the manu- 

 script. W. S. Hillman was supported by a National Science Founda- 

 tion grant (G-4433) to Professor Galston, and W. K. Purves by an 

 NSF Predoctoral Fellowship. Part of this material was included in a 

 doctoral dissertation (Purves, 1959) submitted to the Graduate School 

 of Yale University. 



LITERATURE CITED 



1. Bertsch, W. F. Effects of composition of the medium on the Hght sensitivity of 

 etiolated pea section growth. Plant Physiol. 33 (suppl.): xxxii, xxxiii. 1958. 



2. Brian, P. W. Effects of gibberellins on plant growth and development. Biol. 

 Rev. 34: 37-84. 1959. 



3. Burstrom, H. Auxin and the mechanism of root growth. Symp. Soc. Exper. 

 Biol. 11: 44-62. 1957. 



4. Galston, A. W., and Hillman, W. S. The degradation of auxin. In: W. Ruhland 

 (ed.). Encyclopedia of Plant Physiology. 14: Vic, b6. Springer-Verlag, Berlin. 

 (In press.) 



5. Hillman, W. S. Interaction of growth substances and photoperiodically active 

 radiations on the growth of pea internode sections. In: R. B. Withrow (ed.), 

 Photoperiodism and related phenomena in plants and animals, pp. 181-196. 

 American Association for the Advancement of Science, Washington, D. C. 1959. 



