534 Journal of Agricultural Research voi. xix, no. n 



in the durum, common, and emmer groups were those which bred true. 

 Some of the near-durums may actually belong in the durum group, but 

 there was no attempt to rectify unimportant mistakes in these classes. 



The plants were placed in four rust infection groups: group i, practi- 

 cally immune, o to 5 per cent infection; group 2, resistant, 10 to 20 per 

 cent infection, uredinia smaller than in the normally susceptible varieties; 

 group 3, susceptible, 25 to 40 per cent infection; and group 4, heavily 

 infected. These groups were based on the behavior in the F3 generation 

 of the progeny of each Fg plant. 



In the F2 generation some spikes with glumes resembling common 

 wheat were as compact as those of the durum parents or more so. These 

 were very similar to the club wheats and were placed in the common group. 

 Likewise, lax-headed, sharply keeled wheats v/ere obtained which were 

 placed in the durum group. No differentiation was made between emmer 

 and spelt, all plants with adherent glumes being placed together. Forms 

 were obtained, however, which closely resembled true spelt. 



The results from Marquis X lumillo and Marquis X Kubanka were sim- 

 ilar, and the data were combined. In the final summary in Table VI 

 each plant is classified as resistant or susceptible. The writers believe 

 that the plants classified as resistant would not be seriously injured in a 

 severe epidemic of this rust form. However, there is considerable 

 variation within each group. The resistant plants belong to either 

 class I or 2 for rust infection. 



There were two classes of susceptible Fg plants, those which produced 

 all susceptible progeny in the Fg generation and those which produced 

 both resistant and susceptible progeny. All resistant plants bred true 

 in the Fg generation ; at least this was the criterion used to determine 

 whether a particular Fj plant was really resistant. 



Several conclusions can readily be drawn from Table VI. This is a 

 tabulation of 404 Fj plants, each of which was tested in the Fg genera- 

 tion. Of these 404 plants 4, 81, and 33 bred true to the botanical head 

 type respectively of emmer, common, and durum. All 4 emmers and 8 

 out of 33 durums were resistant, while 81 Fg common segregates were 

 all susceptible. This strongly indicates that in these crosses resistance 

 and susceptibility are linked in transmission with the botanical head 

 characters which differentiate durum and common wheats. The results 

 again show that it is easy to obtain resistant durums, while it is much 

 more difficult, if not impossible, to obtain resistant common wheats. 



As has already been mentioned, both lax and dense durumlike 

 segregates (Pi. 99) as well as compact keelless common wheats were 

 obtained. There seems, however, to be a relation between average 

 length of internode and the botanical classes. Thus, the average mean 

 head densities^ of the five groups, durum, near-durum, intermediate, 



> Density was calculated by dividing the length of the head in millimeters by the number of spikelets 

 less one. 



