560 THE NATURE OF THE GROWTH RATE 



we shall use /+9 as the time instead of /, the observed time from birth. 

 By a series of approximations, 128 was taken as the value of a^ 

 and 0.08 as the value of k. The formula used was then x = 

 128 (l-g-oo8(' + 9)) (Fig. 6). The agreement for the most part is 

 strikingly close. The close correspondence between the observed 

 heights and those calculated by the formula bears out the obser- 

 vation of Eckles and Swett that the growth of the skeleton of the 

 cow is less influenced by conditions of gestation and lactation than 

 the other criteria of growth. 



Summing up the data on the growth rate of Jersey heifers as given 

 above, it appears that they grow by a definite rate which can be repre- 

 sented as a reaction in which the increase in any given interval is 

 proportional to the amount of growth yet to be accompHshed. It 

 does not greatly matter whether one uses height at withers or weighty 

 though the former is slightly more accurate. 



In the case of maize and of heifers it appears that growth proceeds 

 by a definite quantitative rate, no matter whether height or weight 

 is used as a criterion. 



SUMMARY. 



1. The growth rate of organisms may be considered as a chemical 

 reaction which gives the mature organism as its end-product. The 

 organism grows at a definite rate which is, at any moment, propor- 

 tional to the amount of growth yet to be made. 



2. Shoots of young pear trees measured at weekly intervals during 

 the growing season showed a rate similar to that of an autocatalytic 

 reaction. 



3. Young walnut trees showed distinct cycles of growth in a single 

 season, but the growth in each cycle proceeded at a rate corresponding 

 to an autocatalytic reaction, 



4. The growth rate follows a definite, quantitative course though 

 judged by different criteria. Data are presented for maize in which 

 green weight, dry weight, and height of the plant are used. Data 

 for cattle show that either weight or height of the animal may be used 

 as a criterion. 



