CHAPTER XII 



I 



CLASSIFICATION AND NOMENCLATURE OF 

 PLANT GROWTH-SUBSTANCES 



Becher and Stahl, pursuing further and further the combina- 

 tions of the primitive principles, and of those which they 

 term secondary, establish diiferent orders of compound 

 bodies, to which they give improper denominations. The 

 signification of many of these terms is even contrary to the 

 ideas usually affixed to them, and is liable to occasion 

 obscurity. — Beaum6. 



From at least a historical point of view, the three following 

 growth-substances seem to form a class : auxin-a (auxentriolic 

 acid); auxin-b (auxenolonic acid); and "heteroauxin" (3- 

 indole-acetic acid). To these may be added the synthetic 

 substances, homologous with or akin to indole-acetic acid, 

 and principally interesting the readers of this book. 



In addition to these, however, there is a bewildering variety 

 of more or less well-characterized substances derived from 

 animal glands, for at least some of which claims have been 

 made that they are capable of affecting plant growth (see, for 

 example, the paper by Havas and Caldwell, 1935). Amongst 

 these may be mentioned the male and female hormones, the 

 chemistry of which has reached a fairly advanced stage, 

 owing largely to the work of Marrian and of Butenandt. For 

 the chemistry and names of most of these substances, the 

 books by Harrow and Sherwin (1934) and Bredereck (1936) 

 should be consulted. 



Another, but ill-defined, class of substances is that known 

 chiefly for its effects on microbial growth. Amongst these 

 may be mentioned Williams's (1933) "pantothenic" acid, so 

 named on account of its wide occurrence in living material, 



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