PLANT BREEDING 263 



the resulting seeds thus having the characteristics of two dif- 

 ferent strains. The plants from such seeds are called crosses, 

 or hybrid*. A few hybrids between different genera are known, 

 but usually only closely related species, or varieties, are likely 

 to cross, and the closer the relationship the more successful 

 the operation is likely to be. The apple will not hybridize 

 with the pine, nor the strawberry with the milkweed. The 

 reason species do not cross more readily is because the tendency 

 in nature is away from such crossing. If it were otherwise, we 

 would have an endless confusion of plant forms in which no 

 type would be recognizable. Among the more interesting forms 

 of commercial value that have been produced by hybridizing 

 are the plumcot, a hybrid between the plum and apricot; the 

 citrange, a hybrid between the trifoliate orange, or citron, and 

 the sweet orange ; and the tangelo, produced by crossing the 

 tangerine orange and the grapefruit (pomelo). Among plants 

 cultivated for their flowers, the canna, gladiolus, and orchid 

 have been extensively hybridized. 



Producing the cross. In crossing plants the essential thing 

 is to protect from all foreign pollen the stigmas of the flowers 

 to be pollinated. This is accomplished by slipping a small 

 paper bag over the flowers just before they open, and tying 

 the open end of the bag about the twig which bears them. If 

 one is to be absolutely sure of his cross, the flowers that are 

 to supply the pollen should be similarly treated. If the flowers 

 contain both carpels and stamens, as is usually the case, there 

 is a chance that the flowers to be crossed may be pollinated 

 by their own stamens unless these are removed. It is custom- 

 ary, therefore, to cut away the corolla with a sharp pair of 

 scissors before the flower expands, and remove the stamens 

 with small forceps or the scissors. In plants like the pump- 

 kin, cucumber, and corn, which bear their stamens and carpels 

 in separate flowers, this treatment is not required, though the 

 flowers should be protected from the wind, insects, and other 



