GENERAL CONCLUSIONS 245 



tinuously in touch with its surroundings: every quality of 

 the surroundings, such as hght, gra\dty, or chemical change, 

 acts as a stimulus on the organism, and the perception of 

 this stimulus is followed by a response. These stimuh do 

 not supply the organism with the energy which it uses in 

 its response, any more than a Ughthouse supplies a ship 

 with the energy to move out of its way. The stimulus is 

 perceived by the helmsman and the response of the ship is 

 achieved by the release of much larger amounts of energy. 

 According to Fitting's definition the perception of the light 

 stimulus must necessarily be through the hving protoplasm 

 of the helmsman, although this is not entirely true because 

 the helmsman can very well be replaced by a photoelectric 

 cell. (A similar example has been given by Loeb [1918] 

 in his "heliotropic machine.") 



However, it must be made clear that the responses of 

 plants to auxin are by no means typical stimulus responses. 

 Let us take as example the effect of light in causing photo- 

 tropism. It was at first believed that the light acted by 

 releasing the stored-up "bending tendency" in the plant. 

 Blaauw and Froschel's ''Reizmengengesetz" dealt a serious 

 blow to this concept by establishing a quantitative relation 

 between the amount of energy applied (intensity X time) 

 and the amount of response. Not only this, but the curva- 

 ture of an Avena coleoptile is, as we now know, principally 

 due to the redistribution of auxin between the light and 

 dark sides. This redistribution requires far less energy than 

 is suppUed by the light which causes it (Went, 1936). The 

 ship is, in fact, being steered by the photoelectric cell itself. 

 Thus one of the most typical properties of a stimulus, i.e. 

 its action through the release of stored-up energy of the 

 protoplasm, is here absent, and for phototropism, therefore, 

 the stimulus concept is an unnecessary compUcation. 



The relation between auxin and growth also has nothing 

 in common with stimulation. We have seen in the fore- 

 going chapters, especially in III C and III D, that a given 

 amount of auxin produces, if the conditions are constant, a 



