338 Plant Life and Evolution 



been described among the willows, oaks, and 

 sedges, and probably many more remain to be re- 

 corded. Of the oak hybrids may be mentioned that 

 between the white oak and the bur oak, and a species 

 described as Qnercus brittoni which was shown to 

 be a cross between Q. Marylandica and Q. ilicifolia. 

 A fern, Asplenium ebenoidcs, is supposed to be 

 a hybrid between the walking fern (Camptosorus 

 rhizophyllus) and Asplenium ebeneum. 



Artificial Hybrids. The first artificial plant 

 hybrids of which there is a definite record was ob- 

 tained in 1760 by Kolreuter, whose further work in 

 hybridization was very important, although not 

 fully appreciated by his contemporaries. His first 

 hybrid was obtained by crossing two species of to- 

 bacco, Nicotiana rnstica and N. paniciilata. Later 

 he made experiments with many other genera, and 

 demonstrated most of the important phenomena con- 

 nected with the crossing of plants. Subsequently 

 a long controversy arose over the question of the 

 sterility of hybrids compared with species, which 

 lasted for a long time. It is now clear that while 

 as a rule sterility results from crossing widely dif- 

 ferent forms, e.g., different genera, this is not nec- 

 essarily the case, and fertility or sterility of the off- 

 spring cannot be taken as the test for the validity of 

 a species. The importance of hybridization in caus- 

 ing plants to vary has been taken advantage of by 

 the plant-breeder, some of whom, like Burbank, rely 

 frequently upon such crossing, not only to unite cer- 



