Hybrid Vigor 169 



cent. This is really the clearest demonslralion of iiybrid 

 vigor that has ever been pro\'ide(l, for the conditions of 

 the experiment were ideally constant. 



It is interesting to note in this connection that there is no 

 selective action favoring foreign pollen when these pollen mix- 

 tures are applied. In fact, the results indicate that own pollen 

 is successful in bringing about fertilization in a slightly greater 

 number of cases than is foreign pollen. 



It has been stated that the amount of hybrid vigor varies 

 directly with the width of the cross. Of course this statement 

 applies only within certain limits. The situation is somewhat 

 clarified by considering the following series of cases which is 

 arranged with respect to width of cross. 



1 . Parents so diverse that cross cannot be made. 



2. Cross possible but seed obtained fails to germinate. E.\- 

 ample, certain Nicotiana crosses. 



3. Hybrid seed germinates, but resulting hybrid plants are so 

 weak that they fail to reach maturity. E.xample, other Nicotiana 

 crosses. 



4. Hybrid plants mature and are extremely vigorous, but are 

 sterile except possibly in back crosses. Example, cabbage-radish 

 hybrid, an enormous but completely sterile plant. Example from 

 animal kingdom, the mule. (On this matter see also chapter on 

 "Sterility.") 



5. Hybrid plants more vigorous than parents, and completely 

 fertile. Example, corn crosses and many others. 



6. Parents to cross so closely related that no protluction of 

 hybrid vigor is noticeable. 



(An interesting phenomenon appears in certain wheat crosses, 

 where it is found that the Fi endosperms are well developed in the 

 fertile crosses, but shriveled in those crosses which are to produce 

 sterile or partially sterile F, plants. Even in these latter cases, 

 however, hybrid vigor appears in the vegetative parts of the F, 

 plants. Sax ii.) 



Obviously, it is only within the limits of classes 4 and 5 that 

 it can be said that hybrid vigor varies directly with the width of 

 the cross. It is impossible to say where the species boundary tits 



