CHAPTER XVI 

 HYBRID VIGOR 



The phenomenon of hybrid vigor has been mentioned 

 in a general way, but its relation to genetics remains to be 

 considered. Geneticists have proposed various explana- 

 tions of hybrid vigor, and some of these will be presented. 



The first record of observations on hybrid vigor is 

 that of KOLREUTER in 1776, who states that crossing 

 results in an increase in general vegetative luxuriance 

 and an increase in the facility of vegetative propagation 

 and viability. Later Gartner discussed the same 

 phenomenon but gave no important new ideas. Finally, 

 hybrid vigor attracted the attention of Darwin, who 

 states that crossing hastens the time of flowering and 

 maturing and increases the size of the individual. He 

 adds the very important fact that it is not mere crossing 

 that gives the stimulus, but crossing forms that differ 

 in the constitution of their sex elements; in other words, 

 crossing between individual flowers on the same plant 

 gives no advantage. It is Darwin who is responsible 

 for bringing hybrid vigor to the attention of botanists, 

 although the modern popular impression would be that 

 Burbank deserves this credit because of his experience 

 in producing some remarkably fast growing, large, and 

 vigorous hybrids. Among the modern investigators of 

 hybrid vigor, the work of Shull and East will be con- 

 sidered, both of whom have worked upon the subject 

 during the last ten years. 



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