UTILIZATION OF HYBRIDS IN 1'LANT BREEDING 



355 



until 1908 and in that year "the general environmental conditions were 

 much above normal. For opposite reasons, poor soil and badly distributed 

 rainfall, the yields of 1909 are somewhat too low and the yields of 1911 

 are very much too low." With these facts in mind an examination of 

 the table shows that the strains became more and more differentiated as 

 to yield as inbreeding progressed. "The first strain, No. 6, is a re- 

 markably good variety of corn even after five generations of inbreeding. 

 It yielded 80 bushels per acre in 1910. ... In the field, even in 1911, 

 the plants were uniformly vigorous and healthy and were especially 

 remarkable for their low variability. The poorest strain, No. 12, is 

 partially sterile, never fills out at the tip of the ear and can hardly 



FIG. 148. Inbred strains of Learning dent corn compared with F\ and Ft hybrid gener- 

 ations. The yields per acre were as follows: No. 9 (at the left) 47.7 bu.; No. 12, 16.6 bu. ; 

 (12 X 9)Fi, 117.5 bu.; (12 X 9) F t , 91.5 bu. (After East and Hayes.) 



exist alone. . . . When two of these inbred strains are again crossed, 

 the FI generation shows an immediate return to normal vigor. The 

 plants are earlier and taller, and there is a greater total amount of dry 

 matter per plant. For example, in 1911 the average height of all the 

 strains of inbred Learning dent was 84 inches while the average height of 

 the 16 hybrid combinations was 111 inches and the height of the shortest 

 hybrid combination was 1 foot greater than that of the tallest inbred 

 strain." In general it seems that the combinations into which strain 

 No. 7 was introduced were the best while those in which the poorest 

 strain, No. 12, was used were the poorest. However, a cross between 

 these two strains in 1911 yielded 60.2 bushels per acre. The F 2 genera- 

 tion from a number of the crosses was grown and in every case there was 

 a decided falling off in production. This would be expected as a matter 

 of course under conditions of random mating in FI inasmuch as some 

 homozygous combinations would be formed among the F z zygotes. 

 Fig. 148 shows types of ears and comparative yields in strain No. 

 9 after 4 generations of inbreeding, strain No. 12 in the fifth inbred 

 generation, and the F\ and F 2 hybrids, all grown in 1910. 



