1086 BIOLOGICAL EFFECTS OF RADIATION 



plant is only one two-hundredth of that which is present in 1 cc, and 

 this quantity is termed a plant unit. The units defined above have only 

 an arbitrary value, since their application is limited to measurements in 

 which the procedure described above is rigidly adhered to. The same 

 dependence upon the conditions of the test applies to the biological assay 

 of all animal hormones. The variety of oat used, the culture conditions, 

 the size of the agar blocks, and the time relations between the various 

 manipulations will all exert a considerable influence upon the final result." 



Several methods for purifying this growth-promoting substance were 

 examined and one obtained which gives extracts of fairly constant 

 purity. Traces of peroxide, present in freshly distilled ether, are sufficient 

 to destroy some 20 per cent of the activity. From their investigations 

 it is shown that the growth substance is an acid with a dissociation 

 constant of 1.8 X 10"^. Some of the chemical properties are given as 

 well as its stability to acids and alkalies. 



A few months later Kogl (25) reported the results of his chemical 

 studies of the growth substance ("auxin") which Went found in the tips 

 of oat seedlings and which is similar in its reaction on decapitated seed- 

 lings to the material found by Nielsen (31) in mushrooms. A similarly 

 acting product was also extracted from "mash" obtained in the process of 

 making alcohol by fermentation of molasses. Kogl attempted to extract 

 the pure growth substance from 100,000 tips of Avena but found human 

 urine a much better source of supply. He realizes that it is too soon to 

 state that the active substances from these different sources are identical, 

 although they bring about similar responses in the test plants. During 

 his extraction and purification processes he tested the material on sprouts, 

 making use of the Avena unit (AE), i.e., the amount of substance which at 

 a temperature of 22° to 23°C. and a humidity of 92 per cent will bring 

 about a curvature of about 10 deg. 



The auxin behaves as a monobasic acid. After recrystallizing from 

 alcoholligroin, it was found to have a melting point of 196°C. and an 

 activity of about 50,000,000 AE/gm. The auxin lacton melts at 170°C. 

 and has an activity of about 30,000,000 AE/gm. The molecular weight 

 by the Rast method is 338 and by titration 340. Microanalysis gives 

 good agreement with the formula C18H32O5. Urine obtained about 3 hr. 

 after the main meal gives a substance with maximum activity. Other- 

 wise no difference was found as to sex or age. 



Cubes of skin and muscle tissue were placed on the sides of decapitated 

 coleoptiles instead of the small blocks of agar in the AE test, but only 

 negative results were obtained. Cubes cut from cancer tumors on mice 

 were also negative. 



GENERALIZATION 



Priestley (36) in a review of the subject of phototropic growth curva- 

 tures has attempted to show that the suggestion of de Candolle still 



