sept. 1, 1920 Genetics of Rust Resistance 537 



These families were selected in the field as examples of resistant and 

 susceptible F3 lines. Since the inoculation results in the greenhouse 

 corroborate the field observations, there is good reason for believing that 

 the field observations are reliable. This greenhouse experiment shows 

 clearly that transgressive segregation occurred. Common, durum, and 

 emmer sorts were obtained with a higher degree of resistance than that 

 of either of the durum parents. 



These results show that, if the numbers are sufficiently large, it is pos- 

 sible to obtain resistant wheats with common head characters by cross- 

 ing resistant durum and susceptible common varieties. In the F2 

 generation, out of a total of 128 com^mon segregates only 2 were rust- 

 resistant, and both of these were of little commercial value. Hov\^ever, 

 several resistant plants with the head characters of common wheats 

 were obtained in the F3 generation. 



Emmer-common crosses. — Only a few plants were available for the 

 study of crosses between emmer and common varieties. Two different 

 white spring emmers (Minnesota 1165 and C I 1524) were used in this 

 study. Minnesota 11 65 is a very vigorous variety which was practically 

 immune from the form of stemrust experimented with, both in the field 

 and in the greenhouse. C I 1524 is quite similar to Minnesota 1165, 

 except that it occasionally produces small uredinia. 



The difference between emmer and common wheat is greater than that 

 between durum and common wheat. The shape of the head of the emmer 

 parent, which is proportionally very narrow in face view, together with 

 the strongly keeled glumes, dift'erentiates it from the Marquis parent. 

 The kernels of emmer are tightly inclosed by the glumes, and recognition 

 is easy. 



The Fj generation of the cross between emmer and common is inter- 

 mediate for the differential characters mentioned above as separating 

 the parent sorts (PI. 98). The keels of the F^ generation are inter- 

 mediate, but they more closely resemble those of the emmer parent. 

 The head shape of the F^ generation is likewise more nearly emmerlike. 

 Fifty-six per cent of the kernels were naked and 44 per cent had adher- 

 ent glumes after they were thrashed in an individual plant thrasher. 

 The emmer parent produced 19 per cent naked kernels and 81 per cent 

 hulled kernels. The F^ generation (Table V) is nearly as resistant as 

 the emmer parent. Thus, the Fj generation more nearly resembles 

 emmer. 



Fifty F2 plants of the cross between Marquis and emmer (Minnesota 

 1 165) were grown in the Fg generation. Of this number 5 plants were 

 emmers, and i was a very susceptible, lax, common type. The progeny 

 of 5 F2 plants thrashed like common wheats in the F3 generation, but 

 the heads were very compact (PI. 100). Two F, plants bred true to the 

 common keel condition and segregated, producing compact, intermediate, 



