94 Growth and Form 



Other cells cannot convert N2 to NH3, but if given NH3 and pyruvic acid 

 they can make alanine. Still other cells, which do not possess any of the 

 enzymes that catalyze these reactions, must be supplied with alanine itself 

 if they are to grow. The diversity of nutritional requirements results from 

 the inability of cells to synthesize one or another protoplasmic constitu- 

 ent. 



The S-Shaped Curve of Growth 



Figure 44 shows growth curves which, as you can see, are S-shaped. 

 The abscissa is time— minutes, days, hours, years, depending on the organ- 

 ism studied. The ordinate is growth and can describe the number of cells 

 in a bacterial culture, the number of human beings on earth, the size or 

 weight of a sunflower seedling or a rat, the size or weight of the heart or 

 the brain. In other words, the ordinate is a measure of the growth of a 

 population or a single organism or any of its parts and, when growing 

 under optimal conditions, all show precisely this sort of growth curve. 

 Many questions come to mind when we look at this curve. Why does 

 growth start? Why does it stop? Why is the curve S-shaped? 



In this discussion of the factors affecting growth, we shall deal with 



Fig. 44a. The growth curve of the human population of the world. The 

 circles indicate census totals, the solid line the mathematically fitted, 

 smooth, S-shaped curve (after Pearl and Gould, from Allee, et al.. Prin- 

 ciples of Animal Ecology). 



