GENERAL CONCLUSIONS 247 



treme base of the coleoptile cannot cause growth because 

 the cells can no longer respond to it. Root formation fur- 

 nishes a still more striking example. Here, as shown in 

 XI E, the formation of roots is dependent upon the co- 

 operation of a number of factors. The concentration of 

 auxin determines not only the number of roots formed, 

 but also the cells which will form them. In general, the 

 response in organ formation is localized. To explain this 

 localization of the action Went (1936) has suggested that in 

 addition to its master-reaction effect, auxin acts by affecting 

 the transport of the other factors, so that they become ac- 

 cumulated at the point of highest auxin concentration. 

 However this may be, it is clear that when the influence of 

 the other factors is taken into account, the apparently mys- 

 terious action of the auxins in bringing about so many 

 responses no longer seems so obscm'e. It is true that we do 

 not yet know the nature of the fundamental master-reaction, 

 but there is good reason to hope that it is within our reach. 



G. Abnormal Growth 



If internal or external factors affect growth, they must do 

 it through the growth-controlling system we have discussed. 

 In some cases this will be through their effects on auxin. 



Numerous examples of this have been given. The effects 

 of external factors, particularly radiation, are numerous, 

 and a number of them have been analyzed in terms of auxin. 

 In the case of visible light, these include phototropism 

 (KE), auxin production (IV ^), and sensitivity to auxin 

 (X B) ; in the case of x-rays, their general inhibiting effect 

 on growth has been explained (V F). Of the effects of inter- 

 nal factors, only one group has yet been considered to any 

 extent, namely the genes. The genes set up a chain of inter- 

 nal reactions which terminate in the observed effect ; but the 

 last link in this chain, where genes affecting growth are 

 concerned, is the effect on auxin. This has been partly 

 analyzed in V E. 



Many of the infections to which plants are susceptible 



