234 PHYTOHORMONES 



hormones with that of auxin. There is Uttle or no species 

 specificity in either case, that is, one hormone performs the 

 same function in large classes of organisms. On the other 

 hand there is specificity of function, and this is far more 

 marked in animals than in plants. Thus the auxins bring 

 about a number of different effects, while the animal hor- 

 mones in general have one special function. The latter are 

 not as a rule merely growth-promoting, and their assay — 

 except in the case of the pituitary hormone — is not based on 

 growth measurements. An exception might be made for 

 the sex hormones, which certainly act to produce a rapid 

 growth of special tissues and which have actually been 

 referred to as " Wuchsstoffe " (Butenandt, 1935, 1936). The 

 specificity of function in animal hormones is strictly de- 

 pendent on their molecular structure, so that in the sterols, 

 for instance, small changes in the molecule completely alter 

 certain functions of the substance: hydrogenation of one 

 aromatic ring converts a female to a male sex hormone. 

 In the auxins small changes merely alter the ciuantitative 

 activity of the substance, but do not change the functions 

 it performs. However, quantitative changes in activity 

 with small changes in the molecule also occur in the sex 

 hormones. Whether these changes are due to differences in 

 secondary properties, such as penetration, inactivation, etc., 

 as with the auxins (see VIII G), has not yet been ascertained. 

 It is, however, very suggestive that the relative activity of 

 two substances in two different tests may be quite different. 

 Thus if the activity of androstandiol, a male sex hormone, 

 is taken as 1 in both the capon and the rat test, then iso- 

 androstandiol has an activity of 0.04 in the capon and 0.23 

 in the rat test (Butenandt, 1936). It seems justifiable, 

 therefore, to suggest that the difference in relative activity 

 between two such substances in the two tests is due to 

 differences in their secondary properties. 



The effect of animal hormones on plant growth scarcely 

 falls within the scope of this review, but may be mentioned 

 briefly. There is evidence that oestrone in particular has 



