244 PHYTOHORMONES 



organizer, it appears to furnish both field and evocator, and 

 it must therefore be compared to the sum of auxin and 

 polarity. 



F. The Stimulus Concept and the Nature of 



Auxin Action 



As has been shown in the latter half of this book, auxins 

 bring about a number of different responses in plants. 

 These are, specifically: growth by cell-elongation; the forma- 

 tion of roots — both on stems and on roots themselves; the 

 inhibition of buds; the activation of the cambium; inhibition 

 of root growth, and certain growth phenomena which involve 

 cell enlargement and division together. We have seen al- 

 ready that a number of different substances can bring about 

 the same growth response. It is also true that any one 

 of the active substances can bring about all these different 

 responses. This raises the question of the mechanism of 

 these effects. There are two possibilities which may be 

 considered; (1) the auxins bring about some master-reaction 

 within the cell, the results of which will be determined by 

 the presence and amount of other factors (''condition of the 

 cell"); and by the conditions of the experiment; or (2) the 

 auxins are stimulating substances setting free the energies 

 stored up in the Uving protoplasm ("latente Reizbarkeiten"). 

 This latter view has recently been emphasized by Fitting 

 (1936); "I am convinced that we are right in including the 

 hormones with the 'stimulus-substances' (Reizstoffe). By 

 these are understood, following physiological usage, all those 

 compounds which exert their physiological action through 

 the intervention of the living substance, i.e. whose first 

 point of attack is the living plasma. The typical physiolog- 

 ical actions of stimulus substances show all the character- 

 istics of stimulations. . . ." By the latter are meant latent 

 time, presentation time, threshold concentration, excitation 

 of the protoplasm, anti-reaction and recovery, etc. 



To make this somewhat old-fashioned idea clear to the 

 modern reader it is necessary to picture the organism as con- 



