CHAPTER 2 



Growth in General 



The process of organic development, in which are posed the chief prob- 

 lems for the science of morphogenesis, occurs in the great majority of 

 cases as an accompaniment of the process of growth. The association be- 

 tween these two activities is not an invariable one, for there are a few 

 organisms in which growth is completed before development and dif- 

 ferentiation are finished, but far more commonly the form and structure 

 of a living thing change while it grows. Knowledge about growth is 

 therefore necessary for an understanding of development, and any dis- 

 cussion of morphogenetic problems in plants should be preceded by a 

 discussion of plant growth in general. This is the purpose of the first 

 few chapters of the present volume. 



Definition of Growth. The term "growth" has been variously defined 

 by biologists. For some (Hammett, 1936) 1 it includes not only increase 

 but also the accompanying phenomena of progressive differentiation. 

 Most regard this definition as too inclusive and would limit it in one re- 

 spect or another. Since much of the increase in volume is brought about 

 merely by gain in amount of water, increase in dry weight might seem to 

 be the best measure of growth, but sprouting seeds kept in the dark will 

 "grow" into large, etiolated seedlings through intake of water though 

 their dry weight actually decreases. In one sense these sprouts have 

 grown, but in another they have not. The fundamental fact in all growth, 

 of course, is the self-multiplication of living material, a process of much 

 biological significance. For this reason, growth might best be defined 

 as increase in amount of protoplasm. Even if we could agree, however, 

 as to what constitutes protoplasm (whether vacuoles, for example, are 

 parts of it), it would be impossible, as a practical matter, to measure 

 this. Furthermore, in every organism, and particularly in every plant, 

 there is much material ( such as cell walls and starch grains ) which is an 

 integral part of the organism but which presumably is not living, and it 

 seems illogical not to regard increase in such material as part of growth. 



1 For bibliographic information concerning books and papers referred to in the text, 

 see Bibliography, pp. 461 ff. 



11 



