370 TEXTBOOK OF BOTANY 



pure. Accidental hybridization may be prevented by plant- 

 ing each variety at a distance from any other that might be 

 crossed with it ; or by covering the flowers with sacs of gauze 

 or of oiled paper during the time that their stigmas are able 

 to receive pollen. But occasionally the new combination of 

 qualities that appears in a hybrid makes it a more useful plant 

 than either of its parents. Plant breeders produce hybrids, 

 when they TOsh them, by artificial cross-pollination — that is, 

 they deposit the pollen from a flower of one variety by means 

 of suitable instruments upon the stigma of a flower of another 

 variety. In such a case, the stigma must be protected from 

 pollen of its own kind. This is done by bagging the flower, 

 and, if it contains both stamens and pistil, by removing the 

 anthers before their pollen is ripe. 



The exact combination of qualities that appears in a hy- 

 brid is not as a rule permanent, because the seeds of a 

 hybrid commonly grow into plants that differ from one an- 

 other. It is sometimes said that the offspring of a hybrid 

 " split up " into different sorts of plants. These offspring 

 combine in themselves in various ways different qualities 

 of the original parents of the hybrid, including qualities 

 that did not appear in the hybrid as well as qualities that 

 did. Suppose now that a hybrid combines the qualities of 

 its parents in such a way as to make it a useful new sort of 

 plant. We cannot as a rule hope to obtain a crop of plants 

 like the hybrid by planting its seeds, because it is likely that 

 many of the plants that grow from these seeds will be differ- 

 ent from the hybrid. But we can make the hybrid the start- 

 ing-point of a new variety of plants like itself if it can be 

 multiplied in vegetative ways — for example, by buds, cuttings, 

 or grafts — because a plant produced by vegetative multi- 

 plication is, with very rare exceptions, like its parent. Since 

 apple trees are multiplied by grafts and cuttings, a new 

 apple variety may be propagated from a hybrid. Many of 

 our most valuable varieties of apples have in fact been 



