GROWTH AND SENESCENCE 



49 



rat, gives us the impression that it bears some resemblance to a 

 letter S, with the difference that the bend or inflection in the 

 curve is not in the middle. The bend occurs, not in the center of 

 the curve, as in the case with the letter S, but about one-third 

 of the distance from the base line to the top of the curve. Differ- 

 ent species of animals differ with regard to the position of this 

 bend in the curve, as we shall presently show by means of 

 graphs, but the general shape of the curve is the same in all 

 animals. 



Up to the point of inflection in the growth curves, the time 

 rate of growth increases with the increase in size of the animal 

 and we shall for this reason 

 term this phase of growth, 

 the self-accelerating phase. 

 After the point of inflection, 

 the time rate of growth de- 

 creases with the increase in 

 size of the organisms, and 

 we shall, therefore, call 

 the phase of growth follow- 

 ing the point of inflection 

 the self-inhibiting phase of 

 growth. We may note inci- 

 dentally that we have ex- 

 amined a large number of growth curves of man, animals, and 

 plants and found that the junction between the self-accelerat- 

 ing and the self-inhibiting phases of growth occurs, apparently 

 without exception, at the time of puberty in animals, and at the 

 time of flowering in plants. 



2. The course of growth during the self -inhibiting phase of 

 growth. — (a) Linear growth. — Linear growth, that is, growth 

 as measured by means of a measuring rod as, for example, 

 growth in length of the body, presents a simpler type of growth 

 curve than growth as measured by increasing weight. We shall, 

 therefore, first consider curves of linear growth. 



Figure 3r. Skeletal measurements taken of 

 dairy cattle. The measurements are referred 

 to in the following charts by the numerals 

 indicated on the photograph. 



