216 



The Journal of Heredity 



formly effective in all three strains, or 

 is the low stem content due to the sea- 

 son? Evidence that the summer of 

 1922 favored low stem content was 

 obtained accidentally, and is presented 

 herewith. 



In 1922, because of severe damage 

 to the selected seed-heads by birds and 

 insects, it was deemed wise to plant 

 some seed from the 1920 crop to insure 

 continuation of the strains. This seed 

 came from remnants saved from the 

 previous year's planting and from some 

 second-choice heads that had been saved 

 over, all of which were free from cen- 

 tral stem. The data secured from this 

 planting show that the two-year-old 

 seed of the three selected strains when 

 planted in 1922 produced a crop con- 

 taining I I.I per cent of central stem, 

 whereas essentially the same seed 

 planted the previous year had produced 

 22.4 per cent of stem. This shows that 

 the season of 1922 favored low stem 

 content. On the other hand, the sharp 

 rise of stem percentage in the two com- 

 mercial varieties in 1922 indicates a 

 "stemmy season." No attempt is made 

 to reconcile the two sets of facts, unless 

 the method of securing seed of the 

 two commercial varieties for planting 

 the 1922 crop was unwittingly a selec- 

 tion for stem. 



At any rate there can he no doubt 

 that the changed environment in differ- 

 ent seasons has an important influence 

 on both percentage of stem and length 

 of brush, and it is not surprising that 

 many growers should conclude that 

 quality is entirely controlled by the 

 season. Yet one only needs to study 

 the graphs, or better still to see the 

 quality of the crops produced year after 

 year by some of the selections in com- 

 parison with the commercial varieties, 

 to be convinced that heredity also is an 

 important factor in determining the 

 character of the crop (see Figure 13). 



Genetic Factors 



From the graphs it is evident that the 

 three strains, 15, 26 and 56, isolated in 

 1917 and subjected to continuous selec- 



tion for low percentage of stem, have 

 maintained their individuality through 

 five generations. No convincing evi- 

 dence appears that these strains, after 

 five years of intensive selection for low- 

 stem, have been changed genetically in 

 any respect. During this time not a 

 single head of these strains containing 

 a central stem has been used for seed. 

 And yet, so far as we can see through 

 the fog of seasonal influence, they 

 seem to have kept, practically un- 

 changed, their original tendencies to 

 produce similar relative amounts of 

 central stem and lengths of fiber. 



The only opportunity for making a 

 direct comparison between two genera- 

 tions with environmental influence 

 largely eliminated was the accidental 

 one discussed above, when seed from 

 the crop of 1920 was planted in 1922 

 to eke out the seed raised in 1921. The 

 crop produced by seed of the older 

 generation contained 11. i per cent of 

 stem and that of the younger contained 

 7.1 per cent. The significance of this 

 difference is doubtful, although it may 

 indicate some slight effect of selection. 



The behavior of the strains as a 

 whole strongly suggests that they are 

 nearly pure lines. It is true there 

 have been apparently good opportuni- 

 ties for selection to operate, as the dif- 

 ferent head-rows representing each 

 strain have varied rather widely at 

 times and only the strains which 

 showed a low stem content were con- 

 tinued. Would these strains have re- 

 mained at a relatively low level of 

 stem production if continued selection 

 had not been practiced? Our most di- 

 rect answer comes from the behavior of 

 the two commercial varieties which rep- 

 resent conditions of nonselection. They 

 apparently have remained fairly con- 

 stant genetically, although they are 

 evidently made up of an admixture of 

 strains. 



In 1 91 8 strain 4 had a relatively high 

 stem content, but as it was desirable in 

 other respects, stemless heads were 

 selected for planting. In 191 9 two 

 head-rows produced respectively forty- 



