4 PHYTOHORMONES 



such substances in plants. However, since in this book we 

 shall deal only with the plant kingdom, the prefix can suit- 

 ably be dropped. Thus our conception of hormone is essen- 

 tially that of a correlation carrier, where correlation (as 

 used in regard to plants) is defined as the influence exerted 

 by one part of the plant upon another, — not in the sense of 

 statistical correlation, but in the sense of causal relationship. 

 In the beginning of the work in this field, the non- 

 committal terms growth substance, Wuchsstoff, growth 

 regulator, and growth hormone were used, but as our knowl- 

 edge developed, it became clear that the substances causing 

 cell elongation must be regarded as a separate group. Since 

 recent work indicates that this group is heterogeneous, the 

 term auxins, first suggested by Kogl and Haagen Smit 

 (1931), will be arbitrarily restricted to those substances 

 which bring about the specific growth reaction which is con- 

 veniently measurable by the curvature of Avena coleoptiles. 

 Whenever used in the physiological sense, the terms growth 

 substance (g.s.) and growth hormone will be used through- 

 out this book in the sense of auxins. The term Wuchsstoff 

 in particular has been used for some of the growth substances 

 of lower plants, such as Bios, but it cannot be too strongly 

 emphasized that only those substances whose activity is 

 determined on higher plants, preferably by the standard 

 methods which are described in Chapter III of this book, can 

 be termed auxins. 



C. Previous Reviews of the Field 



The rapid development of the field has resulted, as would 

 be expected, in the publication of a number of reviews and 

 summaries. Such reviews have rarely more than temporary 

 interest, and many of these are already only of historical value 

 (Babicka, 1934; Cholodny, 1935a; Kogl, 1932, 1932a, 1933, 

 1933a, 19336, 1933c; Laibach, 1934; Loewe, 1933; Malowan, 

 1934; Pisek, 1929; Snow, 1932; Soding, 1927, 1932; 

 F. A. F. C. Went, 1927, 1930, 1931, 1932, 1932a, 19326, 1933, 

 1933a). 



