86 PHYTOHORMONES 



curvatures down the Avena coleoptile, and the autonomic 

 straightening of the curved zones ("autotropism"). If an 

 unequal distribution of auxin between the two sides of the 

 plant has been brought about by gravity or other means, 

 then the side with the more auxin grows rapidly (convex 

 side) and thus temporarily reduces the supply of food factor. 

 When the auxin distribution becomes equal again, the side 

 which has grown less (concave side) is better supplied with 

 food factor and hence reacts more to the auxin. It thus 

 becomes slightly convex, and this results in a straightening. 



Weber (1931) has raised a number of objections to the 

 two-factor theory, based on experiments on geotropic 

 curvature of Hordeum. The growth measurements which 

 he presents, however, are scarcely accurate enough to allow 

 detailed conclusions to be drawn. '\ 



It may be concluded that this scheme of the 'limitation 

 of growth either by auxin or by the food factor provides 

 an acceptable theory for the growth of a great many objects. 



E. Auxin Inactivation and Dwarf Growth 



One of the most marked chemical properties of the auxins 

 is their extreme sensitivity to oxidative destruction. They 

 are readily destroyed also by plant enzymes, probably by 

 the peroxidase or oxidase system, as we have seen (Chapter 

 IV ^). That auxin disappears within the plant tissue was 

 shown by the analyses of Bonner and Thimann (1935) on 

 coleoptiles previously treated with auxin. If low concen- 

 trations were applied the amount of auxin disappearing was 

 proportional to the resulting growth, but when high con- 

 centrations were used there was a rapid disappearance of 

 the excess auxin without any accompanying growth. 



A phenomenon similar to this was found by van Overbeek 

 (1935) in dwarf forms of Zea Mays. These differ by only 

 one gene from the normal type, and their growth rates, 

 especially of the mesocotyl and stem, are greatly reduced. 

 In nana, one of the dwarf races, the seedlings were found to 

 produce less auxin than the normals and also to respond 



