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THE AMERICAN NATURALIST [Vol. XL VIII 



the relative yields of the varieties tested, and that certain 

 ones were more sensitive than others to these influences. 



If we turn to the relation between stand and yield, we 

 shall again find a strong disturbance of the normal corre- 

 lation as shown in the following : 



TABLE III 



The exceptionally low coefficient of the fifth cutting was 

 due to the low yields on the part of plots which had good 

 stands but were relatively inactive during the hot 

 weather and partial water famine which occurred at this 

 period. On the other hand, certain plots through their 

 resistance to heat and consequent activity at this period, 

 overcame to a large extent their handicap of poor stands, 

 and nearly obliterated the usual plus correlation between 

 stand and yield. 



The data thus studied en masse indicate at least two 

 physiological groups which are unequally sensitive to the 

 climatic changes which occur in the course of a growing 

 season, and whose reactions were sufficiently strong to 

 change almost completely the order of the productivity of 

 the plots. In order to test this conclusion let us turn to 

 the individual plots and endeavor to discover and classify 

 the physiological varieties indicated above. 



If, now, we arrange the forty-four regional strains 

 according to their morphological characters and geo- 

 graphical origin, we shall have five more or less distinct 

 groups as follows: Mediterranean, Peruvian, European, 

 American and Turkestan. The behavior of these varietal 

 groups through the course of six cuttings during the sum- 

 mer of 1910 substantiates the conclusions already drawn 

 and illustrates the sharp differences in climatic reactions 

 which may be observed in the several varieties of a single 

 species. 



