INHERITANCE IN THE BARLEY SPIKE. 21 



density. More extensive study would be needed to determine whether 

 these apparent centers are of any significance. 



DISCUSSION OF RESULTS. 



From the fact that segregates homozygous for density are apparent 

 in the measurements of the F 3 and F 4 generations, it seems safe to 

 conclude that internode length in the barley rachis may be explained 

 on the factor hypothesis. The number or value of the factors involved 

 is not readily estimated. In a general way the results of the Man- 

 churia X Svanhals and the Pyramidatum x Jet crosses seem to 

 indicate a single main, factor difference. The proportion of homo- 

 zygotes is roughly satisfactory, and the absence of homozygotes differ- 

 ing greatly from the mean of their parents is also in favor of this 

 belief. The dominance of density in the F x generation in the first 

 cross and its intermediate expression in the second is of interest. 



The results in the Hanna X Reid Triumph cross in the same way 

 indicate a broad difference of two factors. In this cross forms were 

 isolated' that were homozygous for intermediate densities, as well as 

 forms having densities near those of the parents. These results can 

 be interpreted very satisfactorily on the basis of two main factors 

 for internode length. These factors are cumulative in effect, both 

 being necessary to produce the extreme type. The results show that 

 a sort may be homozygous for one of the factors and heterozygous 

 for the other. At least, heterozygous forms whose progeny range is 

 from the intermediate group to one or the other parent are so 

 interpreted. 



The Hanna X Zeocriton cross gave homozygous intermediates of 

 unlike value, as well as homozygous sorts which were like the parents. 

 If the presence and absence hypothesis is here used, three main 

 factors may be postulated to explain the genetic facts. These factors 

 may be supposed to be of like value, each inherited independently, 

 each allelomorphic to its absence, the number showing a hetero- 

 zygous condition being half the homozygous sorts. This hypothesis 

 explains the genetic fact fairly well. Other minor factor differences 

 are doubtless necessary to explain all of the results. One known 

 minor character of some density significance separates the parental 

 forms. This is a difference in the progressive density from the base 

 to the tip of the rachis, the Zeocriton parent being the only sort 

 which shows a constant increase in length of internode from the base 

 to the tip of the spike. 



A comparison of the Pyramidatum X Jet cross with the Hanna X 

 Zeocriton cross illustrates some facts regarding the mode of inherit- 

 ance of density. These are the two widest crosses made hi the study. 

 The first produced no homozygous intermediates. The second pro- 

 duced many. An F x generation was grown of the Pyramidatum X 



