300 LEROY POWERS 



increase over the VV plants, and with the exception of the Brbr genotype for 

 height of plant and length of awn, the differences of Vv-VV are greater than 

 the differences for VV-vv. 



These facts concerning the data reveal that Vv is associated with an in- 

 crease in all four quantitative characters. For spikes per plant, height of 

 plant, and length of awn this increase results in heterosis. 



Hypotheses for Difference in Vigor 



If the increase noted is due solely to an interaction between V and v such 

 as is depicted by East's physiological hypothesis, then it would not be pos- 

 sible to obtain homozygous lines possessing any of this increase. However, if 

 the heterosis noted is due to a combination of favorable and unfavorable 

 genes linked with V and v, it should be possible to obtain lines in which 

 some of the favorable genes are recombined. These lines should show some 

 increase in the four quantitative characters studied. In the event that linkage 

 of genes favorable and unfavorable to an increase in the quantitative charac- 

 ters was found to furnish the most logical explanation, an intraallelic interac- 

 tion such as depicted by East's physiological hypothesis still may be having 

 some influence as the two systems are not mutually exclusive. 



Tables 19.1 and 19.2 show that Vv results in an increase of all four charac- 

 ters: weight of seed per plant, number of spikes per plant, height of plant, 

 and length of awn. This fact is most simply explained by assuming the pro- 

 duction of a favorable growth-promoting substance which influences all of 

 them. Then such being the case, on the basis of East's (1936) physiological 

 hypothesis, V and v supplement each other, resulting in greater development. 

 Next consider the development of the lateral florets which determines the 

 number of rows of kernels per spike (two-row or six-row spikes). The Vv 

 segregates are two-row types, whereas the vv segregates are six-row types. 

 Hence, as regards the character number of rows of kernels per spike, the in- 

 teraction between V and v is such as to prohibit the development of the 

 lateral florets, resulting in a two-row barley spike rather than a six. Summing 

 up, on the basis of the physiological hypothesis, in the case of four quantita- 

 tive characters the interaction between V and v is such as to stimulate de- 

 velopment. In the case of number of rows of kernels per spike the interac- 

 tion is such as to prohibit development of the lateral florets. From physio- 

 logical genetic considerations such a pleiotropic effect seems rather im- 

 probable. 



Explaining the heterosis associated with Vv plants on the basis of linkage, 

 a simple interpretation would be that the favorable linked genes and their 

 alleles interact according to Jones's (1917) hypothesis to produce a substance 

 favorable to growth processes, resulting in the heterosis noted; and that V 

 is dominant to v resulting in Vv (Fi) plants having two-row barley spikes. 

 This explanation does not require the assumption that V and v stimulate 



