CHAPTER XII 

 HYBRID VIGOR 



The phenomenon of hybrid vigor has already been 

 referred to. It is a matter so intimately related to 

 genetics, particularly plant genetics, both on the theo- 

 retical side and in connection with practical breeding, 

 that it will be worth while to consider it in some detail here. 



The first record of observations on hybrid vigor is that 

 of KoLREUTER in 1776, who states that crossing results 

 in an increase of general vegetative luxuriance and in 

 an increase in the facility of vegetative propagation and 

 viability. Later Gartner discussed the same phenome- 

 non but gave no important new ideas. Finally, hybrid 

 vigor attracted the attention of Darwin (4), 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, cross- 

 ing between different flowers on the same plant gives 

 no advantage, nor does crossing two individuals which 

 are gemiinally identical. He assumed (incorrectly, see 

 p. 161) that any effective germinal dift'erence was to be 

 accounted for by the fact that the parents had been 

 growing under different environmental conditions.^ 



^ It is probably Darwin who is responsible for bringing hybrid 

 vigor to the attention of botanists, although the modern popular impres- 

 sion might be that Burbank deserves the credit because of his experience 

 in producing some remarkably fast-growing, large, and vigorous hybrids. 



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