296 HOW CROPS GROW. 



and vegetable kingdoms the limits within which hybridiza- 

 tion is possible appear to be very narrow. It is only be- 

 tween closely allied species that fecundation can take place. 

 Wheat, oats, and barley, show no tendency to " mix " ; the 

 pollen of one of these similar plants being incapable of 

 fertilizing the ovules of the others. 



In flower and fruit-culture, hybridization is practised or 

 attempted, as a means of producing new kinds. Thus the 

 celebrated Rogers' Seedling Grapes are believed to be hy- 

 brids between the European grape, Vitis vinifera, and 

 the allied but distinct Vitis labrmca, of North America. 



Hybridization between plants is effected, if at all, by 

 removing from the flower of one kind, the stamens before 

 they shed their ;pollen, and dusting the summit of the pistil 

 with pollen from another kind. 



The mixing of different varieties, as commonly happens 

 among maize, melons, etc., is not properly hybridization, 

 this word being used in the long-established sense. We 

 are thus led to brief notice of the meaning of the terms 

 species and variety, and of the distinctions employed in 

 botanical classification. 



Species. The idea of species as distinct from variety 

 which has been held by most scientific authorities hither- 

 to, is based primarily on the faculty of continued repro- 

 duction. The horse is a species comprising many vari- 

 eties. Any two of these varieties by sexual union may 

 propagate the species. The same is true of the ass. The 

 horse and the ass by sexual union produce a hybrid the 

 mule, but the sexual union of mules is without result. 

 They cannot continue the mule as a distinct kind of ani- 

 mal as a species. Among animals a species therefore com- 

 prises all those individuals which are related by common 

 origin or fraternity, and which are capable of sexual fer- 

 tility. This conception involves original and permanent 

 differences between different species. 



