INHERITANCE IN THE BARLEY SPIKE. 



23 



These results show that, although density is a very stable size 

 character, in some crosses numerous factors are involved which, by 

 recombination, produce homozygous forms showing an almost con- 

 tinuous range of density from the very lax to the dense types. It is 

 only reasonable to conclude that if a greater number of varieties had 

 been studied, together with crosses between them, a continuous range 

 for the average length of internode of homozygous forms could be 

 obtained which would show only small differences in average density 

 between types. These results are of considerable interest in barley 

 classification. While dependable in the isolation and description of 





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Fig. 2.— Diagrams showing the densities of parental forms and of the F 2 generation in a cross between 

 the Zeocriton and Hanna barleys (upper), of four pure lines (middle), and of several heterozygous 

 lines (lower). 



strains, groups founded on this character are likely to overlap and 

 hence to be of limited value for taxonomic purposes. 



While the general genetic results of these crosses are explained on 

 a broad factor basis of differences of one to three factors, the fact 

 remains that the homozygous segregates corresponding to the parents 

 do not always have the exact density of the parents. Likewise, the 

 forms homozygous for intermediate densities do not all fall together 

 but in groups, which, in the Hanna X Zeocriton cross become almost 

 continuous, even where limited numbers are concerned, and might 

 become wholly continuous if it were possible to carry the full number 

 to the fourth generation. Obviously, there are modifying factors, 

 and so far as they affect density they may be considered as minor 

 density factors. Several explanations are possible. These varia- 



