CONSTANT DIFFERENTIAL GROWTH-RATIOS 13 



increase from one absolute size is regarded as equivalent to 

 an «-fold increase from another absolute size. 



The same total increase could be subdivided into fractions 

 of equal absolute size ; but this method of subdivision in 

 terms of additive growth has not the same biological value, 

 and will not be adopted. Thus for an increase from 1 g. to 

 256 g., equal fractions of the growth-period are best repre- 

 sented by the equal multiplicative increases from 1 to 4, 4 to 16, 

 16 to 64 and 64 to 256 g. ; and not by the equal additive 

 increases to 64, 128, 192 and 256 g. 



§ 3. Examples of Constant Differential Growth-ratios 



The expression y = bxu I shall refer to as the simple hetero- 

 gony formula. This formula, or an approximation to it, has 

 been found to hold for a number of other organs, e.g. the 



Fig. 4. — Increase of the logarithm of stem-weight against the logarithm of 



root-weight in various plants. 



