14 PHYTOHORMONES 



in phototropism he obtained evidence that an asymmetric 

 distribution of the growth substance occurs, this being the 

 cause of the curvature. 



In recent years a large literature on this subject has 

 developed. The later work, which will constitute the body 

 of this book, need only be briefly referred to here. First of 

 all must be mentioned the chemical investigation of the 

 active substances, auxins, which, mainly in the skillful 

 hands of Kogl, Haagen Smit, and Erxleben, led rapidly to 

 the isolation in pure form of three highly active substances 

 (this work will be discussed in Chapter VII). 



The occurrence and distribution of the auxins has been 

 investigated by Soding, Thimann, Laibach, and others. In 

 this connection the fact that the auxins have no specificity 

 of action was first demonstrated by Cholodny, and confirmed 

 abundantly by Nielsen, Soding, Uyldert, and others. The 

 investigations of van der Weij have brought light to many 

 remarkable facts concerning the transport of auxins in the 

 coleoptile, particularly the strict polarity of its movement. 



Since the approach to knowledge of the auxins was 

 through tropisms, it is natural that one of the main applica- 

 tions of the acquired knowledge of these substances should 

 be in the explanation of tropisms. The investigations of 

 Dolk, Cholodny, and Dijkman have shown that asymmetri- 

 cal distribution of auxin under the influence of gravity 

 quantitatively accounts for geotropic curvature. This rules 

 out any need for assuming the action of "tropohormones." 

 In phototropism the situation is somewhat more compli- 

 cated, but the main Hues have been elucidated by Cholodny, 

 Boysen Jensen, du Buy, and van Overbeek. In roots, the 

 studies of Cholodny, Snow, Boysen Jensen, and others have 

 made it clear that the effect of auxin is to inhibit, not to 

 promote, elongation, and this makes it possible to explain 

 geotropic behavior of roots along the same hues. 



Many attempts have been made to elucidate the mecha- 

 nism of the growth-promoting action. Heyn and Soding 

 succeeded in showing that one of the ultimate effects of 



