54 Journal of Agricultural Research voi.xxn.No. a 



acters but which may be heterozygous for growth factors. There is the 

 possibility that this heterozygous condition may cause the Fg or F. 

 hybrid to give a high yield. After several further generations this hete- 

 rozygous condition may be lost, with a consequent loss in growth stimu- 

 lus. A knowledge of the amount of added vigor in the F, generation is 

 of value in determining whether heterozygosis in Fg and F^ lines would 

 modify their yields sufficiently to interfere seriously with a determina- 

 tion of their value as improved varieties. 



With these points in view a study has been made in wheat of the imme- 

 diate effect of cross-pollination on seed weight and the increased vigor 

 of Fi crosses. Pure lines were used of seven varieties of Triticum vulgare 

 Vill. and one variety of each T. compactum Host. (Little Club) , T. dicoccum 

 Schr. (Spring Emmer) , and T. durum Desf. (Mindum). Varieties of T. 

 vulgare were crossed with each other and with Little Club, Spring Emmer, 

 and Mindum. Little Club was crossed also with Spring Emmer and 

 Mindum. 



IMMEDIATE EFFECT OF CROSS-POLLINATION 



Because of the phenomenon of double fertilization it is possible in 

 some cases to obtain an increase in weight of seed as an immediate eflfect 

 of cross-pollination. The increase is due principally to an increase in 

 weight of endosperm in such crops as com, where the proportion of endo- 

 sperm to embryo is large. 



Collins (5) observed open-pollinated ears of Chinese maize in which the 

 size of seed was increased by cross-pollination. Seeds which showed by 

 their color the effect of foreign pollen averaged 0.178 gm., while white 

 seeds from the same portion of the ear averaged 0.153 g^. Roberts (14) 

 mentions a similar instance with Chinese maize. Collins and Kempton (6) 

 compared the average seed weight of com from intravarietal and inter- 

 varietal pollinations. The intervarietal crosses exceeded the intra- 

 varietal in seed weight by 8.8 per cent. In a similar experiment, Wolfe 

 {16) found that 23 of 31 com varietal crosses yielded more grain than in- 

 travarietal pollinations. Carrier (4) obtained an increase in yield of 

 grain in strains of com when grown in a mixture as compared with any 

 one of the strains grown alone. 



That an increase is also obtained in the size of the embryo is clearly 

 shown by Lewis and Vincent {12) in a comparison of seeds of Newtown 

 apple from self- and cross-pollinations. The crossed seeds showed a 

 striking increase in weight over that of the selfed seeds. As there is 

 little or no endosperm in apple seeds, an increase in seed weight is due 

 largely to an increased size of the cotyledons. 



Since artij5cially pollinated seeds of wheat are usually smaller than nor- 

 mally pollinated seeds, spikes of each variety were emasculated in the 

 same manner as for cross-pollination and then pollinated with pollen 

 from plants of the same pure line. Seed from this intrapollination is 



