THE FACTOR HYPOTHESIS AS APPLIED TO PL.ANTS 



385 



Continuing these investigations, Nilsson-Ehle next discovered a 

 new strain of red-grained wheat, which, when crossed with the pure 

 white strain, yielded Fi hybrids of intermediate intensity of red as 

 before. The F2 generation, however, showed a different situation. 

 Reds and whites were obtained in the proportion of 63 : i ; the 63 reds 

 as before falUng naturally into different groups on the basis of degree 

 of redness. Applying the same conception as before Nilsson-Ehle 



• • • 



Pure Red 



Grades of Pink 



White 



Fig. 75. — Another method of \asualizing Nilsson-Ehle's 15:1 ratio (see 

 Fig. 74). {From Coulter and Coulter.) 



discovered that in this case he was dealing with a trihybrid situation. 

 Without constructing the usual Mendelian diagram, which would have 

 to be extensive enough for 64 individuals, the situation as it appeared 

 in the F2 generation may be represented by Fig. 76. If the graph is 

 surmounted by a curve we recognize the regular "probability curve," 

 exactly the kind of curve biometricians use to represent the fluctuating 

 individuals about a specific type. 



This conception of cumulative factors, therefore, has far-reaching 

 significance. For a long time biologists have recognized individual 



