Sept. 1. 1920 Genetics of Rust Resistance 525 



segregating generations. These types were easily fixed after several years 

 of selection. The four types were believed to have a common ancestry 

 and to belong to one botanical species. Unsuccessful attempts were 

 made to cross these species with T. monococcum ly. This led Vilmorin to 

 conclude that T. monococcutn belonged to a separate species. 



Tschermak (25) confirmed Vilmorin's conclusions regarding the pro- 

 duction of common, durum, spelt, and poulard forms from crosses when 

 the groups differed in the solid and hollow stem character. He stated 

 that the polonicum type was obtained only when it was used as one of the 

 parents. Relationships were illustrated on a factor hypothesis by assum- 

 ing (i) that Triticum polonicum contained two dominant factors, (2) that 

 common wheat lacked these factors, and (3) that durum contained one 

 of these factors but not the other. Tschermak also thinks that T. mono- 

 coccum is different from other wheat species. He obtained hybrids 

 between T. monococcum and T. vulgare and one Fj plant, which soon died. 



Blaringhem (6), however, succeeded in crossing Triticum durum and 

 T. polonicum with T. monococcum. Sterility was at first of frequent 

 occurrence in these crosses, but in later generations it was much less 

 common. 



Explorations by Aaronsohn in 1906 (i) confirm the experimental 

 evidence cited above. A wild emmer, Triticum dicoccum dicoccoides,^ 

 found in 1906, is interesting because it tends to confirm the report that 

 such an emmer was collected as early as 1885. 



Crosses between Black Winter emmer and Fultzo-Mediterranean (i^) 

 had strongly keeled glumes with hard, adherent chaff in the F^ generation. 

 The F2 plants varied widely in type and exhibited transgressive segrega- 

 tion for some characters. There was considerable sterility in this cross. 

 The hairy chaff and black color of the emmer parent were linked in 

 inheritance. 



Freeman (12) found evidence of linkage between high ratio of width 

 to thickness of head and hardness of grain in a cross between durum 

 wheat and a variety of bread wheat. The bread wheat parent had a 

 square head with soft, opaque grains, while the durum parent had a 

 much flattened head and produced hard, translucent grains. 



In a recent article on heredity of quantitative characters in wheat {13) 

 a cross between durum and common wheat is reported. It gave 

 normally vigorous plants in the F^ generation. In the Fg generation, how- 

 ever, many seeds failed to germinate; and among those with a normal 

 vegetative development were found plants exhibiting every degree of 

 sterility, from perfectly sterile to fertile plants. 



Linkage has been reported by Engledovv and Biffen in crosses between 

 Rivet, Triticum turgidum, and common Fife wheat. Gray glume color 



1 From a cross between durum and common varieties. Love and Craig have produced a form which closely 

 resembles the wild emmer. LovE, H. H., and Craig, W. T. the synthetic production of wild wheat 

 FOR.MS. In Jour. Hered., v. lo, no. 2, p. 51-64, illus. 



