A. J. HAAGEN-SMIT 13 



penoorth (42), who found that a coleoptile cylinder suppHed with auxin-a 

 showed phototropic behavior, whereas those with indoleacetic acid did 

 not. The destruction under the influence of Hght has been explained by 

 the conversion of auxin-a-lactone to inactive lumiauxone. These changes 

 take place rapidly by irradiation with ultraviolet light, and in the 

 presence of both a- and /3-carotene the inactivation occurs in the visible 

 spectrum. The proposed mechanism of phototropic action finds support 

 in the presence of carotenes in the coleoptile and in the agreement 

 between the spectral sensitivity of the coleoptile to light and the ab- 

 sorption spectra of the carotenes. Recently, however, Galston (15) has 

 shown that indoleacetic acid is easily destroyed by light when a suitable 

 activator is present. Lactoflavin was found to have such an action, and 

 rapid decarboxylation and oxidation is observed in vitro in sunlight, 

 and the action spectra also agree with that of the coleoptile. Van 

 Overbeek (44) found that the growth inhibition of Avena coleoptiles 

 after exposure to light does not seem to occur when the growth hormone 

 applied to the top of the coleoptile is indoleacetic acid instead of auxin-a. 



There is a possibility that an additional method of distinguishing 

 indoleacetic acid and auxin-a may be found in the experiments of 

 Guttenberg (17) on deseeded, derooted coleoptiles of Avena and hypo- 

 cotyls of Helianthus annuiis. After one-sided application of tip growth 

 substances, the curvatures appear during the first two hours, whereas 

 those from application of indoleacetic acid take from 3 to 10 hours to 

 develop. This slow reaction of indoleacetic acid is explained as being due 

 to the production or release of auxin-a under the influence of indoleacetic 

 acid. Further support for this theory is found in experiments whereby 

 auxin-free coleoptiles are treated on the outside with indoleacetic acid. 

 After extraction, the extracted growth substance behaves like auxin-a in 

 acid and alkali stabihty tests. 



Another method which holds promise in distinguishing between the 

 auxins could be based on enzymatic destruction. Thimann (58) observed 

 a considerable loss in activity when auxin was incubated with leaf 

 extracts of Vicia faba and Helianthus and attributed this effect to the 

 enzymatic destruction of the auxin. The importance of this phenomenon 

 for the regulation of plant growth was shown by van Overbeek (43), who 

 found that dwarf corn contained greater than normal amounts of this 

 destructive agent. Larsen (36) made some steps toward purification of 

 the auxin inactivating substance and found it to be of enzyme nature. 



