50 H. K. HAYES 



Several photographs from this bulletin are of some interest. A picture of 

 two inbred lines of maize and their Fi cross was one of the first published field 

 views of hybrid vigor from crossing inbred lines of maize. East told me that 

 such a demonstration of hybrid vigor would create a sensation if the material 

 had been grown in the corn belt. 



Some Fi crosses between species and sub-species in tobacco gave large in- 

 creases in vigor. Some species crosses were sterile. Some varietal crosses 

 within species showed little or no increase in vigor, other crosses gave an aver- 

 age increase of 25 per cent in height over the average of their parents. A few 

 wide species crosses were very low in vigor. One such cross beween Nicotiana 

 tabacum and Nicotiana alata graiidiflora was sterile and very weak in growth. 

 Photographs of the parents and hybrids bring out the fact that a lack of vigor 

 in a few cases was known to accompany the heterozygous condition. Natural- 

 ly such undesirable combinations had little importance either to the plant 

 breeder or as a basis for evolution. 



In 1910, G. H. ShuU summarized the effects of inbreeding and crossbreed- 

 ing in maize in a clear, concise, and definite manner. The student of heredity 

 in this early period had little conception of the complexity of inheritance. 

 Hybrid vigor was in many cases not clearly Mendelian. The term heterosis 

 was coined by Shull and first proposed in 1914. He used the term to avoid 

 the implication that hybrid vigor was entirely Mendelian in nature and to 

 furnish a convenient term to take the place of such phrases as "the stimulus 

 of heterozygosis." 



At this time it was usually stated that increased vigor in hybrids was due 

 to a more rapid cell division as stimulated by the heterozygous condition of 

 the genotype. A. F. Shull in 1912 attributed the vigor "to the effect of a 

 changed nucleus and a (relatively) unaltered cytoplasm upon each other." 



The purpose of this chapter is to discuss some phases of the development 

 of the heterosis concept since 1910. Three main topics will be presented cover- 

 ing utilization, breeding methods, and genetic concepts with particular ref- 

 erence to practical applications and to genetic explanations. 



UTILIZATION OF HETEROSIS BY THE PRODUCER 



The presentation of East and Hayes in 1912 emphasized the probable 

 practical value of heterozygosis. A review of experiments with maize was 

 made. In discussing Shull's (1909) plan for the use of single crosses between 

 inbred lines, it was stated that the procedure was desirable in theory but 

 difficult of application. At this early time the inbred lines of maize that were 

 available seemed so lacking in vigor that the use of Fi crosses between selfed 

 lines in maize for the commercial crop seemed impractical. Both Shull and 

 East believed that some method of direct utilization of hybrid vigor in maize 

 would be found. 



One is inclined to forget that the inbred lines of maize of today are marked- 



