CHAPTER 17 



Chemical Factors in General 



Chemical factors are of paramount importance for metabolism and for 

 physiology generally, but they also have an important part in the de- 

 termination of form and structure. Physical factors-light, water, tem- 

 perature, gravity-produce their effects on development chiefly through 

 the external environment, but chemical ones operate morphogenetically 

 both from inside and outside the organism, and in studying them it is 

 necessary to recognize this fact. Nitrogen, for example, by its presence in 

 the soil, markedly affects the growth and development of a plant rooted 

 there, but it does so because of what happens after it has entered the 

 living system of the plant. Here it may be moved from place to place and, 

 as a constituent of protoplasm, it affects the course of development in 

 many ways. The student of morphogenesis concerns himself, therefore, 

 not only with the visible effects of changing amounts of nitrogen in the 

 soil but with the history of this element as a part of the organic mecha- 

 nism. This is true for other chemical substances, whether taken in from 

 the outside or synthesized within the plant. Much of differentiation re- 

 sults from differential distribution of substances throughout the plant 



body. 



Many effects of these substances are local. In studying them experi- 

 mentally it is therefore not enough to examine their effects upon the 

 plant as a whole but to discover what happens in particular parts of the 

 plant as their concentration in these parts is altered. The most important 

 discoveries in the field of growth substances, for example, have been 

 made by studying their local effects on a developing root or leaf or ovary. 



The role of chemical substances in development also changes with time, 

 for the life of the plant is a life history and this history consists of specific 

 progressive changes. These are reflected not only in alterations in struc- 

 ture, as between juvenile and adult foliage, but in physiological changes 

 that go on within the plant. Some of these are gradual but others are 

 more sharply marked. Such is the transition from the vegetative to the 

 flowering state (p. 184), in which meristematic activity shifts from one 

 region and one type to others that are quite different and where there is 



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