376 M or pho genetic Factors 



Jensen, Cholodny, Dolk, Fitting, Kogl, Laibach, Paal, and Went. The 

 modern understanding of auxin was established by an important paper 

 by the last-named botanist in 1928. 



Growth substances have been shown to be present in many species 

 and throughout the plant kingdom, in algae, mosses, ferns, and a great 

 variety of seed plants. They have relatively less effect on the growth of 

 fungi. 



TYPES OF GROWTH SUBSTANCES 



There are many growth substances but the best known and those ap- 

 parently of the greatest general importance are the ones that occur 

 naturally in plants and are usually termed the auxins. These were identi- 

 fied first in the oat coleoptile but are now known to be present in almost 

 all plants and to be concerned in a great variety of developmental 

 processes. There has been a good deal of discussion as to how many 

 auxins there are, but one of them, identified by Kogl and his colleagues 

 (1933) as 3-indoleacetic acid, seems to be of primary importance, and 

 all known auxins are related to it chemically. In the present discussion the 

 term "auxin" will be generally limited to this and chemically similar 

 compounds. 



Our knowledge of indoleacetic acid (IAA) is now very substantial 

 but there are a considerable number of other naturally occurring growth 

 substances, or substances which have been thought by many to exist in 

 plants, which are unrelated to this chemically but are important in various 

 ways. About these we know much less. Here belong the wound hormones 

 ( traumatins ) ; the various flower-forming substances (florigens); the so- 

 called root-forming, stem-forming, and leaf -forming substances or calines; 

 the gibberellins; the hormones concerned with the determination of sex; 

 various substances that amplify or antagonize the effects of auxin; and 

 others. About the chemical nature of most of these little is known, and 

 the very existence of some of them is in doubt. The conception of "organ- 

 forming substances," in the narrow sense, is far from being established 

 and has been involved in the general discussion of determination. It is 

 clear, however, that there are a great many compounds, some of them 

 very specific in their influence, which in minute amounts have important 

 effects on one or another of almost all phases of plant development. An 

 understanding of their nature and mode of action is proving of much 

 importance for the solution of morphogenetic problems. 



It should again be emphasized that most growth substances in plants 

 are much less specific in their effects than are the hormones of animals. 

 Thus IAA is concerned in cell expansion, cell division, cambial activity, 



