i6o 



Plant Genetics 



fertilization again begins. After that self-fertilization 

 brings additional loss in vigor, but this loss is less with 

 each succeeding generation. It is as though a very 

 definite limit were being approached and each genera- 

 tion goes down one-half of the remaining distance 

 toward that limit. Just why and in what way this limit 

 is approached will be considered later in connection with 

 the work of EAST. 



FIG. 36. Illustrating status of hybrid vigor in Fi and later genera- 

 tions. Vigor represented by height of rectangles. 



3. "A cross between sibs (sister and brother) within 

 a self-fertilized family shows little or no improvement 

 over self-fertilization in the same family." This, it will 

 be noticed, is simply carrying a little farther the point 

 that DARWIN originally discovered. DARWIN found 

 that crosses between flowers on the same plant did not 

 result in hybrid vigor. SHULL now finds that crosses 

 between different individual plants in the same race is 

 of no effect. We realize that an inbred race should be 

 homozygous; therefore all the individuals involved 

 would have the same germinal constitution. A cross 

 between any two such individuals certainly could not 



