132 



PHYTOHORMONES 



auxin may be the free auxin acid discussed above, and the 

 bound form its salt. This, however, is improbable, because, 

 if so, the so-called acid curvatures should migrate downwards 

 below the point of appHcation of acid, which appears not 

 to be the case. It is also remarkable that the immersion of 

 coleoptiles in acid buffers, which sets free auxin acid and thus 

 accelerates the growth rate, does not set free any auxin which 

 can be redistributed under the influence of gravity (u). 

 Table XI, which is partially hypothetical, combines these 

 facts with those of Chapter VII into a composite scheme of 

 the various possible states of auxin in the plant. 



TABLE XI 

 States of Auxin in the Plant 



Auxin in plant 



inactive in Avena test (precursor, auxin-esters, storage form 



in seeds, easily transformed into 

 auxin) 



'free moving (transport form, can be collected by 

 diffusion method, redistributed by 

 light or gravity) ("free auxin")- 



active in 



Avena 



test 



inside cells 

 (can be ex- 

 tracted by 

 organic sol- 

 vents only) 

 ("bound 

 auxin"). 



auxin acid (active in producing 

 growth) 



auxin in salt form (not active 

 in growth, reserve auxin in 

 cells). 



II. THE CHEMICAL APPROACH 



G. Primary and Secondary Activity 



We have seen in VII E that a number of different sub- 

 stances have been found to possess acti\dty to a greater 

 or lesser extent on Avena. This fact has been subjected 

 to a physiological analysis by Thimann (1935). He com- 

 pared the activity of the three substances indole-3-acetic 

 acid, I; indene-3 -acetic acid, II; and cumaryl-1-acetic 

 acid, III: 



