CHAPTER 18 



Growth Substances 



The chemical substances discussed in the previous chapter are primarily 

 important in the nutrition of the plant, and most of them are required 

 in relatively large amounts since they contribute to the composition of 

 protoplasm. In relatively few cases, however, have they been shown to 

 be of any very great morphogenetic significance. 



There is another group of physiologically active substances which are 

 not concerned with nutrition and which, although present in most 

 cases in very low concentrations, are of great importance for the growth, 

 development, and differentiation of plants. They have been given various 

 names-hormones, inductors, Wuchsstoffe, activators, evocators, growth 

 regulators, and growth substances. They were first studied in animal 

 physiology but in recent decades they have assumed major importance 

 for an understanding of the development of plants as well. They are 

 diverse in character and effect and are the chemical means by which 

 many morphogenetic processes are controlled. Among these processes 

 are growth, tropisms, many correlations, and the determination and 

 differentiation of specific organs and structures in the plant body. Indeed, 

 as our knowledge of these substances increases, more and more of the 

 activities in development and differentiation are found to be affected by 

 them, and, in interaction with the genes, they seem to be the chief 

 agents in morphogenesis. It must not be forgotten, however, that they 

 are agents merely and that the ultimate control of development lies in 

 the factors that determine the concentration, distribution, and interaction 

 of these and other chemical and physical mechanisms. Here lies the ulti- 

 mate problem. 



Many of these substances, in great diversity, are produced and con- 

 trolled by the plant itself and are thus of particular interest in normal 

 morphogenesis. It has been shown, however, that a large number of 

 synthetic compounds have effects comparable to naturally occurring ones, 

 and this has greatly extended the means for experimental attack on de- 



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