CHAPTER XXXI 

 GROWTH, ASSIMILATION, AND ACCUMULATION 



That plants more or less continuously increase in size and produce new 

 organs at least intermittently throughout their life history is one of the most 

 self-evident of natural phenomena. The term "growth" is popularly employed 

 to designate this complex of processes, and in a loose sense, at least, is so em- 

 ployed by botanists. Growth is the one plant process with which few persons 

 are unfamiliar, even if they have never observed it on any larger scale than 

 a potted plant on a window sill. For farmers, horticulturists, foresters, and 

 all others who depend upon the productivity of plants for their livelihood the 

 phenomenon of plant growth holds the center of the stage of interest. 



In many discussions of plants as living organisms emphasis is laid upon 

 the "structure" and "function" of various organs and tissues. IMost such dis- 

 cussions overlook or at least fail to emphasize that structure is also a result 

 of "function" if this latter term is considered to refer to physiological activity. 

 The coordinated development or growth of plant organs and tissues is just as 

 clearly a form of physiological activity as such relatively simpler processes as 

 photosynthesis and respiration. However, because of the complexity of the 

 process, the physiology of growth has been studied much less intensively than 

 the end products — cells and tissues — of growth activity. 



Assimilation. — The dry matter which is incorporated into the structure 

 of both protoplasm and cell w^alls during growth comes almost entirely from 

 foods. The process whereby foods are utilized in the building of protoplasm 

 is often called assimilation. For convenience in discussion we will use this 

 term in a blanket sense to refer to the construction of both protoplasm and 

 cell walls from foods. In the synthesis of protoplasm the foods assimilated 

 are largely proteinaceous, while those assimilated in the fabrication of cell 

 walls are almost entirely carbohydrates. The chemical reactions involved in 

 assimilation are principally condensations in which simple, soluble foods 

 are converted into complex, insoluble constituents of cell systems. These 

 reactions are probably catalyzed by enzymes. As a result of assimilation a 

 growing region invariably increases in dry weight during growth. 



Apparent exceptions to the principle of increase in dry weight during 

 growth are sometimes cited. A seedling developing in the dark, as described 



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