VI 



PREFACE 



made these advances possible. Even more we should determine what is going 

 on within the breeding and physiological systems through which heterosis 

 finds expression, if further increases in yields are to be obtained or better 

 systems of breeding are to be developed. 



Toward this end the conference topics were arranged under four major 



j^'u'X ''■*Je.Ai 





mi 



'.»*.. 



Controlled heterosis in the making through pollinations and fertilizations of selectively 

 purified genetic strains of corn (maize). (From G. F. Sprague.) 



lA 



z a: 



E z 

 o 





O V> -J 



Z -10- 



1 1, 



4 O <t 



K - Z 



u >- e 



> * . 



1930 



1935 



1940 

 YEARS 



1945 



1950 



Trends in acre production of maize before and after heterosis was in use and tame hay 

 over which there has been no such breeding control 1930-1950. (From G. F. Sprague.) 



