CHAPTER v. — GROWTH AND MOVEMENT 



I. GROWTH 



Ideas involved. — Nothing about plants as a whole is more readily 

 seen than that they grow, and in due course unfold new organs. How- 

 ever small and simple, however large and complex, growth is almost 

 always obvious, and sometimes it becomes striking because of its ra- 

 pidity or its long duration. Two ideas are involved in the term growth 

 as ordinarily used, (a) an increase in size and {h) the formation of new 

 organs. The latter is sometimes distinguished under the term develop- 

 ment, and if one speaks of growth and development, the term growth must 

 be limited to the enlargement of already formed cells. But the terms 

 are nearly synonymous; though growth may be restricted for a time to 

 cells already formed, it normally leads to the formation of new organs; 

 and though development is possible without enlargement, it is usually 

 accompanied by an increase in size. The production of new organic 

 material is not essential; when the corn seedling, raised in the dark, 

 grows into a plant many times larger, the stored organic material has 

 been merely rearranged, with the addition of water, and when the surplus 

 food has been fully used for growth, there is actually a smaller total of 

 dry matter than when growth began. Additional organic matter can 

 be produced only when the conditions for photosynthesis are fulfilled. 

 Few plants have so definite a cycle of development as most animals. 

 • In some cases leaves produced in the juvenile period differ from those of 

 later stages.' Again, leaves developed at certain periods are so different 

 in form and texture as to be really different organs, as in the case of bud 

 scales, floral parts, etc. But these periods of flowering or seed formation 

 or other reproductive process are determined largely by external con- 

 ditions, and little or not at all by the fact that the plant has reached a 

 certain stage of maturity, though of course the formation of the special 

 organs, as of all others, is conditioned by the supply of constructive 



' These juvenile forms, however, may appear later under suitable conditions. See 

 Part III, p. S96. 



417 



