176 Plant Genetics 



future such a phenomenon as hybrid vigor may be 

 explained in terms of the stabilities and reactivities of 

 the constituents of specific protoplasts. 



In conclusion, attention should be called to the danger 

 of confusing phenomena of hybrid vigor with those of 

 cumulative factors. The two situations are somewhat 

 similar, but careful consideration will discover distinct 

 differences. It is perhaps unsatisfactory to assume 

 absolute lack of dominance in the case of cumulative 

 factors and the essential presence of dominance (partial 

 at least) in explaining hybrid vigor. Such assumptions, 

 however, are not necessarily contradictory. The two 

 things should be recognized as distinct phenomena, 

 although they are at work simultaneously and probably 

 interact to give complex results. 



i. COLLINS, G. N., and KEMPTON, J. H., Effects of cross-pollination 

 on the size of seed in maize. U.S. Dept. Agric. Circular 124. 



2. EAST, E. M., and HAYES, H. K., Heterozygosis in evolution and 

 in plant breeding. U.S. Dept. Agric., Bur. PI. Ind. Bull. 243. 

 pp. 68. pis. 8. 1912. 



3. JONES, D. F. Bearing of heterosis upon double fertilization. 

 Bot. Gaz. 65:324-333. figs. 3. 1918. 



4. - , Dominance of linked factors as a means of accounting 

 for heterosis. Genetics 2:466-479. 1917. 



5. KEEBLE, F., and PELLEW, C., The mode of inheritance of 

 stature and of time of flowering in peas (Pisum sativum). Ge- 

 netics 1:47-56. 1910. 



6. SHULL, G. H., Hybridization methods in corn breeding. Amer. 

 Breeders Mag. 1:98-107. 1910. 



