SIGNIFICANCE OF SEXUALITY 



247 



laled two strains of corn and prop)agated ihem by self-fertiliza- 

 tion and selection until they appeared almost if not quite homo- 

 zygous. In this condition they were weak and stunted, and 

 their ears had degenerated to nubbins (Fig. 137, A and D). 

 He then crossed these two strains reciprocally, that is, the strain 

 that was used as the mother in one cross was employed as the 



Fig. 137. — A and D, Ears from two nearly homozygous strains of Indian corn: B. 

 offspring of strain .4 fertilized by strain D; C, offspring of strain D fertilized by strain .4 , 

 (After Shull.) 



father in the next, and vice versa. Immediately the offspring 

 from these crosses mounted in vigor, even above that of the 

 cross-fertilized race from which the strains had been derived; 

 yielding ears as shown in Fig. 137, B and C. Here A pollinated 

 by D produced B, and D pollinated by A produced C. 



These results led to the notable suggestion by Shull that since 



