12 PLANT GROWTH SUBSTANCES 



occurs in the Avena coleoptile tip and in other plants? For the identifica- 

 tion of the auxins in plant tissues it would be preferable to isolate the 

 active components in a pure condition. This is, of course, generally not 

 feasible. In the case of the Avena tip which contains 1/50,000,000 of a 

 milligram, it would take years to collect enough material. An additional 

 difficulty in the case of auxins-a and -b is that both substances are con- 

 verted into inactive products within a few months. 



Indirect methods of determination are therefore usually the only 

 means of answering the question of which growth substances are present 

 in plant tissues. Went used the molecular weight determination by 

 diffusion as a means of determining the size of the active material 

 diffusing from the Avena tip. Since that time this method has served in 

 a number of cases to decide between the occurrence of auxins-a and -b 

 or indoleacetic acid. Went found for the tip auxin a molecular weight of 

 378, which was close to the molecular weight of auxin-a; but recent work 

 has shown that when a diffusate purified by ether extraction is used, a 

 much lower figure is found (32,71). We must accept this revision of 

 the molecular weight with caution, since acid and alkali destruction 

 tests were in agreement with the presence of auxin-a. It has been found 

 in the case of the pure auxins that heating with hydrochloric acid 

 destroyed indoleacetic acid, whereas auxin-a was resistant. On the other 

 hand, treatment with sodium hydroxide showed the opposite effect. 

 Auxin-b and indoleacetaldehyde were destroyed by both treatments. 

 In the inactivation of indoleacetic acid by acid, oxygen plays an im- 

 portant role, and it has been shown that in a nitrogen atmosphere no 

 destruction takes place. Since the destruction method is used extensively 

 it should be more carefully standardized. 



A third possibility of detecting the difference between the action of 

 indoleacetic acid and the tip growth hormone is based on the phototropic 

 behavior of plants. In the Avena coleoptile Went showed that with low 

 light intensity of 20 to 100 meter candle seconds a lateral movement of 

 the auxins takes place, causing an increased concentration at the dark 

 side. At the same time there is a lowering of the total auxin content, 

 this destruction being of the order of 25 per cent. If one or the other 

 auxin is responsible for the phototropic curvature, artificial application 

 of one of these substances to growth-hormone-free coleoptiles should 

 give similar response to that of the intact coleoptile. This experiment has 

 been carried out by Koningsberger and Verkaaik (31) and by Op- 



