CHAPTER IV 



Developmental Effects of Auxins 



The mechanisms in the plant which are controlled or at least influ- 

 enced by auxins are sometimes difficult to distinguish one from 

 another and any classification ot such effects will necessarily be rather 

 arbitrary. For convenience, the effects of endogenous auxins may be 

 grouped into six general classes: (1) the effects of auxins on growth 

 itself, (2) the effects on tropisms and movement, (3) the effects on the 

 inhibitions of development of various plant parts, (4) the participa- 

 tion in morphological differentiation, (5) the effects on flower and 

 fruit development, and (6) the control of abscission. 



GROWTH 



It is obvious that all of the six general effects of auxins given 

 involve the effects of auxin on growth itself. Certain generalizations 

 about growth phenomena can be recognized as applying in common to 

 all the auxin effects. 



The concept established by Went (1928), that without auxin 

 there is no growth, has been consistently borne out in the great body 

 of more recent studies. Strong differences in the specific auxin levels 

 required to stimulate growth in different plant organs were observed 

 by many early workers. In many cases a mistaken point of view was 

 adopted that, whereas auxins promote growth of stems, they inhibit 

 the growth of roots. The situation was corrected by Boysen-Jensen 

 (1936) who showed that roots simply have a higher sensitivity to 

 auxin, and thus are stimulated to grow by much lower concentrations 

 than those required to stimulate growth of stems. A generalized 

 scheme for the effects of auxins upon growth of roots, buds, and stems 

 was made by Thimann (1937) in which he graphically represented the 

 relative ranges of auxin responses in various organs as shown in 

 figure 46. From this schema it can be seen that concentrations of auxin 

 which promote growth of stems inhibit the growth of buds and roots. 

 Lower concentrations can promote the growth of buds and still lower 

 concentrations can promote the growth of roots. Other growth phe- 

 nomena could similarly be entered in this graph of relative sensitivity. 

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