566 GROWTH, ASSIMILATION, AND ACCUMULATION 



corresponding to this cyclic variation in growth is termed the grand period of 

 growth. That this is a universal pattern of growth behavior is shown by 

 the fact that it is exhibited by such diverse types of growth phenomena as the 

 rate of elongation of segments of the root axis, the rate of elongation of seg- 

 ments of the stem axis, the expansion in area of leaves, the increase in weight 



8 10 12 



DAYS AFTER PLANTING 



Fig. 122. Growth curves of muskmelon hypocotyls. {A) curve of daily increment 

 of elongation. (B) curve of total increment of elongation. Data of Edwards, et al. 



(1934)- 



of fruits, the growth of annual plants expressed in terms of dry weight incre- 

 ment, and even the growth {i.e. increase in population) of micro-organisms. 



Although environmental factors may influence the length of time required 

 for completion of the grand period of growth, or in extreme cases may cause 

 complete cessation of growth thus causing interruptions in the cycle, usually 

 the general trend of the grand curve of growth is immutable, indicating 

 that it is primarily controlled by internal factors. This does not mean that 

 the magnitude of the growth in a given plant may not vary greatly in ac- 



