Oct. 8,1921 Comparative Vigor of F^ Wheat Crosses and Their Parents 6i 



be included as the best 3nelders. In subsequent generations as the lines 

 become homozygous their productivity may decrease. In generations 

 beyond the F5 the heterozygous condition of the population rapidly dis- 

 appears. 



A method of breeding which, according to Babcock and Clausen (j), 

 has been used by the Svalof Station, seems worthy of wider application. 

 As self-fertilized crops approach homozygosis rapidly in generations follow- 

 ing a cross, it is suggested that a cross be made between varieties selected 

 because of the desirable characters which they possess. After 6 to 10 

 years have elapsed, during which time progeny of the cross has been grown 

 in bulk plots, selection of individual plants may be made with the assur- 

 ance that a high percentage of these plants will give homozygous progeny. 

 While this system requires some length of time before results are ob- 

 tained, it requires a minimum of labor. 



When making crosses with the hope of increasing yield through a re- 

 combination of the desirable factors of both parents, the parents will 

 naturally be selected on the basis of their yielding ability. The chances 

 of favorable recombinations of yield factors in generations following a 

 cross will presumably be greater when dealing with a cross which shows 

 maximum increased yield over the parents in the Fj generation. Genetic 

 linkage, however, may make certain combinations difficult or impossible. 



The sterility of the specific crosses, with the exception of crosses of 

 varieties of Triticum vulgare with Little Club, is partially or wholly 

 responsible for the low grain yield of the crosses as compared with the 

 parental averages. The fact that Little Club behaves in every way as a 

 variety of T. vulgare agrees with the view of Tschermak (75), who believes 

 that T. compactum and T. vulgare are closely related. Little Club 

 crosses readily with varieties of vulgare. When Little Club or varieties 

 of vulgare are crossed with Emmer or Mindum, the same high degree of 

 sterility is shown. 



SUMMARY OF RESULTS 



(i) An increase in seed weight was obtained in all varietal crosses as 

 an immediate effect of cross-pollination. The only significant differ- 

 ence shown by the immediate hybrids of specific crosses was a decrease 

 in seed weight obtained in Velvet Chaff X Mindum. 



(2) In the Fj generation some of the hybrids exceeded the parental 

 average in height of tallest culm, and in total culm length others showed 

 a decrease. In all varietal crosses the F^ hybrid exceeded the parental 

 average in yield of grain per plant, and six out of eight crosses exceeded 

 the yield of the better parent. 



(3) Crosses between Little Club and varieties of Triticum vulgare gave 

 results similar to those of crosses between vulgare varieties. 



(4) The F^ generation of Emmer or Mindum crossed with varieties of 

 Triticum vulgare or with Little Club showed a high degree of sterility. 



