Sept. 1, 1920 Genetics of Rust Resistance 533 



The crosses between resistant durum wheats and Marquis, Preston, 

 and Pioneer gave similar results, the F^ generation being rusted as badly 

 as the susceptible common wheat parent. 



Different results were obtained in the F^ crosses of emmer (Minnesota, 

 1 165) with Marquis and Preston. The emmer parent is practically 

 immune from this form of Pticcinia graminis triiici as determined both 

 by field data and experiments in the greenhouse. Two crosses, Marquis 

 X emmer and Preston X emmer, and reciprocals were studied. While a 

 few small uredinia developed on the F^ generation, all plants observ^ed 

 were as free from rust as the resistant durum varieties. (See PI. 97.) 



That susceptibility is a dominant character in crosses between resistant 

 durums and susceptible common wheats and a recessive character in 

 crosses between resistant emmer and susceptible common v/heats is 

 obvious from these results. These facts bear out the observation made 

 in the agronomy nursery in 191 6 — nam.ely, that an F^ cross between 

 emmer (Minnesota 1165) and Marquis v/as resistant, while in an adjacent 

 row an F^ generation of a cross between a resistant durum and Marquis 

 was severely infected. 



CORRELATION BETWEEN RUST RESISTANCE AND BOTANICAL HEAD 



CHARACTERS 



For the purpose of determining possible interrelation between inherit- 

 ance of rust resistance and other differential characters, correlated 

 individual plant data were taken on each Fj plant. 



Durum-common crosses. — Notes were taken on length of head, 

 number of spikelets. breadth of head in face and side viev/s, average 

 length of internode, condition of awns, and rust infection on each indi- 

 vidual F2 plant. These plants were then classified into six groups — 

 namely, emmerlike with seed inclosed by the glumes, durum, near- 

 durum, intermediate, near-common, and common. 



After the elimination of the emmerlike types the principal basis for 

 classification was the appearance of the keel of the outer glume. In 

 durum varieties the glumes are prominently and sharply keeled, while 

 in the common wheats the outer glumes of the spikelets are only slightly 

 keeled. The Fj generation of a cross between durum and common wheats 

 was intermediate for this character (PI. 98). This one character deter- 

 mined whether the plant in question approached more closely the durum 

 or the common type. The breadth of head in face and side views, the 

 presence or absence of the depression in the center of the outer glume, 

 and the condition of the collar were used in making the final classification. 

 The depression is characteristic of common wheats. In most durum 

 wheats the collar at the base of the lower spikelet extends all around the 

 neck of the culm, while in common wheats it extends only part ^vay 

 around. Since progeny of the 1% plants were grown in the F3 generation, 

 mistakes in classification were rectified. The individuals finally placed 



