UTILIZATION OF HYBRIDS IN PLANT 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 
: Fic. 148.—Inbred strains of Leaming dent corn compared with F, and F2 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 & 9) F2, 91.5 bu. (After East and Hayes.) 
exist alone. . . . When two of these inbred strains are again crossed, 
the F; 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 Leaming 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 M2 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 F; inasmuch as some 
homozygous combinations would be formed among the F, 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 /; and F hybrids, all grown in 1910, 
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