134 PHYTOHORMONES 



at all, while those in solutions of I, II, or III elongated 

 vigorously. Application of the acids in the form of their 

 sodium salts was equally effective, so that this confirms 

 that the substances do not act by their effect on the pH. 

 We know in addition that the pea test is practically insensi- 

 tive to acid (see III D 2). 



Another observation led to the elucidation of the behavior 

 of these substances. It was found that the curvatures on 

 Avena produced by indene-3-acetic acid are restricted to a 

 very short region, about 3-4 mm. from the point of apph- 



15 10 7 14 13 24 32 24 32 



Mean 12° Mean 27° 



Indole-propionic acid Indole-acetic acid 



Fig. 44. Short curved zone caused by indole-3-propionic acid (93 mg. per 

 liter) contrasted with long curved zone caused by indoIe-3-acetic acid (0.32 

 mg. per liter). 



cation (see Figure 44, which shows a similar effect of indole- 

 propionic acid). This suggested that the rate of transport of 

 the substance limits its activity. Hence tests like those on 

 Avena sections and Pisum stems, which are carried out by im- 

 mersion of the test plant, would be expected to give higher 

 activities. The inability of these two substances to be trans- 

 ported was confirmed by experiments on root formation and 

 bud inhibition (see XI C and XII). 



Thus the physiological activity of a substance depends 

 not only on its direct effect, but also on its secondary prop- 

 erties which determine whether it will reach the place of 

 action or not. To use again the simile of the key in the lock, 

 the effectiveness of the key is determined not only by the 

 wards but also by the grooves in the side of the key, which 

 in themselves, of course, have nothing to do with the open- 

 ing process. Still, the incorrect arrangement of the grooves 

 may prevent an otherwise correctly shaped key from open- 



