Hybridization 111 



and published the results of a number of experiments that 

 have not been equaled by any other worker. 



What plants can be hybridized f It is a fact of prime 

 importance that plants so different as to be classed by 

 botanists in widely different families never yield offspring 

 when crossed ; for example, it is impossible successfully 

 to cross Indian corn and lilies or the apple and the wal- 

 nut. Usually plants diverse enough to be considered as 

 belonging to clearly distinct genera, even though of the 

 same natural family, are perfectly sterile when crossed ; 

 for example, Indian corn yields no offspring when cross- 

 pollinated with wheat, nor does wheat when crossed with 

 oats, although all belong to the great family of grasses. 

 Plants belonging to the different cultivated races or to 

 natural varieties of the same species are almost invariably 

 fertile when crossed. Indeed, as will be shown later, they 

 are sometimes more fertile when crossed with a related 

 species than when fertilized with their own pollen. Dif- 

 ferent species of plants closely enough related to be placed 

 in the same genus by naturalists are very often, though by 

 no means always, capable of being hybridized. 



Gartner found that "one of the tobaccoes, Nicotiana 

 acuminata, which is not a particularly distinct species, 

 obstinately failed to fertilize or to be fertilized by no less 

 than eight species of Nicotiana." Darwin states that "in 

 the same family there may be a genus, as Dianthus, in 

 which very many species can most readily be crossed; 

 and another genus, as Silene, in which the most persever- 

 ing efforts have failed to produce, between extremely close 

 species, a single hybrid." Again, there is considerable 

 diversity in results in certain reciprocal crosses between 



