6o Journal of Agricultural Research vd. xxii. No. a 



The parental varieties showed an average of 19 per cent barren florets. 

 Intercrosses of vulgare varieties and .crosses between Little Club and 

 vulgare varieties showed an average of 15 per cent barren florets. The 

 Fi crosses of Marquis, Velvet Chaff, and Little Club with Emmer gave 

 an average of 75 per cent barrenness. These same common varieties 

 and Little Club crossed with Mindum showed a barrenness of 70 per 

 cent. These data confirm the results of Hayes, Parker, and Kurtzweil (10) 

 and show conclusively that in some cases Fj crosses between varieties 

 of T. vtdgare and T. durtim or T. dicoccum are highly self-sterile. 



DISCUSSION OF RESULTS 



It has been pointed out by East and Jones (7) that the increase in 

 productivity of a cross is due to an increase in the number of growth 

 factors of which the maximum number can be obtained only in a hetero- 

 zygous condition. In a crop such as com, this heterozygous condition 

 is kept up by cross-fertilization. Selfing corn varieties reduces the hetero- 

 zygosity and consequently the vigor. In wheat the continued selfing 

 natural to the crop has brought about a condition of homozygosity. 



In the present experiment all varietal crosses gave an increase in seed 

 weight as an immediate effect of cross-pollination. An increase is also 

 shown in the F^ crosses for average yield of grain per plant as compared 

 with the parental average. The increase ranged from 4 per cent in 

 Penny X Bobs to 32 per cent in Marquis X Bobs. Before attempting to 

 utilize the vigor of the heterozygous condition by growing Fg and F4 

 generation crosses as the commercial crop, it seems logical to combine 

 in one variety the maximum number of growth factors possible. When 

 the possibilities of combination have been exhausted and a variety, or 

 a series of varieties, has been secured which contains this maximum 

 number of growth factors, it may be desirable to follow out the sugges- 

 tion of Anderson (j). This method probably could not be used to 

 advantage except under intensive farming conditions. In case one 

 desired to use such a method it is logical to assume that the more desirable 

 crosses to make are those which show the greatest increase in yield of 

 grain in the F^ generation. 



There is an indication that the increased productivity of the heterozy- 

 gous condition is a factor which must be considered in comparing F3 and 

 F4 lines for yielding ability. In the cross Marquis X Bobs the Fj genera- 

 tion showed on an average a 32 per cent increased yield of grain per plant 

 as compared with the average of the parents and a 10 per cent increase as 

 compared with the higher-yielding parent. In the Fj generation of such 

 a cross it is highly probable that some of the most vigorous plants will be 

 those with the greatest degree of heterozygosity. These heterozygous 

 individuals will produce F3 progeny the vigor of which likewise will be 

 partially due to the heterozygous condition. If the F3 lines are classified 

 on a basis of their yielding ability, some of these heterozygous lines will 



