No. 532] THE GENOTYPES OF MAIZE 242 



to have its mean row-number at 14 shows that the fall in 

 row-number from 12 to 8 in the other family has been 

 due to internal rather than to external causes. 



The change in variability in number of rows on the 

 ears has also been studied from year to year. Continued 

 self-fertilization has resulted in a gradual decrease of 

 variability in the number of rows per ear in each of the 

 self -fertilized lines. This is a fluctuating character, and 

 so far as present evidence goes, the number of rows 

 per ear in any strain can not be fixed at a definite num- 

 ber. While it is probable that none of my self -fertilized 

 families has yet reached an absolutely pure-bred con- 

 dition, several of them have become so nearly pure-bred 

 that their various relations can be used to demonstrate 

 that they are approaching purity as a limit. 



In 1909 two of these nearly pure-bred families were 

 compared with their reciprocal hybrids in the first and 

 second generations, with reference to the variability in 

 number of rows. 5 It was found that the average varia- 

 bility in these two self-fertilized families was 9.08 per 

 cent. The variation in number of rows in their Fj prog- 

 eny was 9.06 per cent., and in the F 2 12.63 per cent. A 

 comparison of these coefficients of variability shows at 

 once that the variation in number of rows in the F x is 

 essentially identical with that in the self-fertilized lines 

 used for the cross. Theoretically this should be so if 

 the strains used were pure genotypes, because in that 

 case all germ-cells in each pure strain were alike, and 

 therefore, when individuals belonging to these two lines 

 were crossed, equal sperms met equal eggs ; consequently 

 there should be no variability in their offspring due to 

 germinal differences, but only those due to environment 

 in the widest sense. As the pure-bred families and their 

 Fj and F 2 progenies were grown beside each other dur- 

 ing the same season, they were subjected to as nearly 

 identical environmental influences as can be attained. 



6 Shull, G. H., "Hybridization methods in corn breeding, " Am. Breeders' 

 Magazine, 1: 98-107, 1910. 



