HYBRIDISATION AND WHEAT HYBRIDS 399 



The mean glume-length of the extracted F 2 polonicums is usually 

 25 per cent less than that of the grandparental polonicum, the glumes of 

 the other extracted homozygotes being somewhat longer than those of the 

 corresponding grandparents. 



The means of the curves of the glume-lengths of the homozygotes 

 are nearer to the mean of the intermediates than are the means of the 

 original parents. This condensation effect is permanent, the changes in 

 the average glume-length of the extracted homozygotes being hereditary 

 and transmitted without further apparent alteration to the subsequent 

 generation. 



It is possible that the observed deviation of the extracted homozygotes 

 from the pure parental forms in the crosses of polonicum with other 

 wheats may be due to irregular reduction division in the F l generation. 



In normal reduction division, a certain number of the gametes pro- 

 duced in the F x generation contain only polonicum chromosomes, and a 

 certain number only durum chromosomes. In irregular reduction 

 division, however, the gametes may never receive chromosomes derived 

 entirely from either parent, and the zygotes from the fusion of such 

 gametes would therefore always exhibit the characters of both parents in 

 greater or lesser degree. 



5. HYBRIDS OF T. turgidum WITH OTHER WHEATS. 



a. T. turgidum x T. monococcum (see p. 391). 



b. T. turgidum x T. polonicum (see p. 397). 



c. T. turgidum x T. Spelta, 



T. turgidum (Rivet), var. dinurum x T. Spelta, var. Duhamelianum (Red 

 Tyrol Winter Spelt. Beardless Red Chaff). This hybrid obtained by 

 H. Stoll gave an extraordinary number of forms in F 2 , among which were 

 several " Squareheads " with and without awns and a beardless compactum. 



The glumes were white, red, or bluish and pubescent, like the Rivet 

 parent. One of the beardless Spelta forms with red chaff and large, 

 plump spikelets and grain was selected and remained constant after 1904, 

 when it was placed on the market as Stoll 's Early Giant Spelt. 



The Fj in the Spelta-vulgare and Spelta-turgidum crosses is inter- 

 mediate in ear characters, with a more or less brittle rachis and tightly 

 fitting glumes, like the Spelta parent. So far as can be gathered from the 

 published accounts, segregation occurs in the F 2 generation in the expected 

 Mendelian ratios, and no forms appear of races other than those of the 

 two grandparents. 



d. T. turgidum x T. vulgare. 



i. T. turgidum (Rivet), var. iodurum $ x T. vulgare (Red Schlan- 

 stedt) S- Rimpau crossed these wheats in 1875. The F x resembled 

 the male parent. In the F 2 were many intermediate forms between 

 completely bearded and beardless. 



