132 THE AMERICAN NATURALIST [Vol. LV 



plotted in the customary way. It has come to be accepted 

 that the effect produced by a given growth factor is de- 

 pendent on the size of the organism. For example, if a 

 growth factor increases the length of the internode by a 

 given amount, it is clear that the height of a plant with 

 30 internodes will be increased more than that of a plant 

 with only 15 internodes. In other words, the effects are 

 factorial instead of additive. A convenient method of 

 classifying a population on a factorial basis has been 

 proposed by Zeleny (1920), who takes the range of each 

 class as a constant percentage of the value of the mid 



