PLANT BREEDING 363 



that may profitably be raised in a region where the cultiva- 

 tion of that kind of crop has hitherto been impossible. This 

 is especially true if the new variety is suited to dry regions, 

 such as the arid and semi-arid parts of the western United 

 States. Soil conditions, too, differ greatly; some varieties 

 of plants grow best in sandy soil, others in clay ; some in soils 

 that are rich in organic matter, and others in soils that are 

 comparatively poor. 



Sometimes varieties must be bred to meet local tastes and 

 fashions. In some localities there is almost no market for 

 apples unless they are large and highly colored, while in other 

 places at the same time dull-colored apples of ordinary size 

 but of good flavor sell readily. It is often true, too, that dif- 

 ferent varieties of the same or of related species must be bred 

 for different purposes. Thus there are table grapes, raisin 

 grapes, and wine grapes ; apples that must be used soon after 

 they are picked, and apples that can be kept in good con- 

 dition for winter use. 



374. Selection. — No two plants of the same variety are 

 exactly alike. In an ordinary field of wheat, some plants 

 have larger heads than others. The grains from the plants 

 with larger heads are likely on the average to produce larger- 

 headed plants than will the grains from plants with smaller 

 heads. Therefore, if we wish to obtain a race of wheat with 

 large heads, we may select our seed grain from those plants 

 which come nearest to being what we desire, and on the whole 

 .we shall improve the variety of wheat in this particular re- 

 spect. In the same way we may select seed grain with ref- 

 erence to the number of heads per plant, the size of individ- 

 ual grains, the strength of stem, the abiHty of the plant to 

 endure drought, its resistance to rust, or with reference to a 

 combination of any two or more of these qualities. Such 

 a selection of the seeds of the most desirable plants has played 

 a very large part in the breeding of cultivated varieties. 



Until quite recent years seed selection has usually consisted 



