CHAPTER VIII 



THE MECHANISM OF THE ACTION 



The development of the field has been so recent that 

 it is perhaps only to be expected that the inner mechanism 

 of the action of auxin on the growth process should be as 

 yet little understood. Nevertheless there are a number of 

 factors which make a successful attack more probable in 

 this than in any other comparable case. In the first place, 

 the active substances are relatively simple and their con- 

 stitutions are completely known. In the second place, the 

 process they affect, at least in so far as cell enlargement is 

 concerned, is directly open to physiological analysis, and the 

 action, compared to that of some other hormones (see 

 Chapter XIV), is very direct. Finally, the cells on which the 

 auxins act are of the least differentiated and simplest 

 type. 



There are two possible methods of approach to such a 

 problem, and both have been to some extent exploited. 

 One is to attempt to unravel the chain of physiological 

 processes which ultimately results in growth. This we may 

 call the physiological approach. The other, which has been 

 opened up by the fact that a number of different substances 

 have growth-promoting action, consists in an attempt to 

 identify those properties of the molecule which give it its 

 activity. This we shall term the chemical approach; in 

 its later stages it becomes interrelated with the physiological 

 analysis. To adopt Emil Fischer's simile of the key in the 

 lock — already used by Kogl — one may call the auxin the 

 key which opens the lock to allow growth, and it is clear 

 that the opening process may be studied either by analyzing 

 the lock (physiological approach) or by analyzing the key 

 (chemical approach) . Both methods will be discussed in this 

 chapter. 



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