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Hormonal Regulation of Plant Cell Groivth 



Peter M. Ray ^ 



Regulation of the activities of plants by hormones was first dis- 

 covered in the growth responses, or tropisms, of plant shoots to 

 gravity and illumination. The growth-promoting hormone which is 

 involved in these responses is called auxin. The natural auxin which 

 has been detected most widely is 3-indoleacetic acid; many synthetic 

 (not naturally occurring) compounds also show auxin activity. 

 Auxins have been found to influence a wide variety of physiological 

 processes in plants, and they are now believed to play a role in regu- 

 lating many aspects of plant development and function, other than 

 cell enlargement in \'Oung stems or stemlike organs, although pro- 

 motion of this latter kind of growth is still considered one of the 

 most diagnostic actions of an auxin. More recently other types of 

 growth substances, which probably exercise a hormonal influence, 

 have been found in plants, notably the gibberellins and kinins. 



Comprehensive treatments of the various aspects of plant growth 

 and its hormonal regulation have recently been given by Thimann 

 (1960) and Audus (1959). Audus has brought together the many 

 theories about tlie "primary action" of auxin, which attempt to ex- 

 plain how it promotes cell enlargement. This subject has been 

 reviewed recently also by Galston and Purves (1960). They point 

 out that the "primary" action, by which is meant the way in which 

 the hormone molecule itself interacts directly with a process in the 

 plant, is not necessarily exerted on cell growth, something which; 



1 Department of Botany, University of Michigan, Ann Arbor, Michigan. The ex- 

 perimental work described here was supported by National Science Foundation Grant 

 NSF G-8705. 



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