120 



PLANT BIOLOGY 



131. Artificial crossing of related species. Not only can man 

 secure new varieties of plants by watching for favorable variations 

 and perpetuating them from year to year, but he can actually be 

 instrumental in producing new kinds of plants. This process is 

 known as plant breeding. It depends fundamentally on the prin- 

 ciples we learned in treating of cross-pollination in flowers. Let 

 us illustrate plant breeding by the following account of the work 

 which has been done for the U. S. Department of Agriculture by 

 Dr. H. J. Webber of Cornell University. 1 



In the winter of 1894-1895 a heavy frost destroyed practically 

 every orange tree in the northern and central part of the State 



of Florida. The loss was over 

 $75,000,000. The problem that 

 confronted the orange growers 

 of the State was that of start- 

 ing their groves anew and if 

 possible of preventing a repeti- 

 tion of such an experience by 

 planting a more hardy kind of 

 orange tree. Dr. Webber, in 

 casting about for such orange 

 trees, finally chose a type called 

 the trifoliate orange (Fig. 56) 

 often used for an ornamental 

 shrub, and one that would not 

 be killed by winters as far north 

 as Philadelphia. The fruit of 

 this tree, however, is small, bitter, 



and worthless for eating pur- 

 FIG. 56. Spray from trifoliate TT . , ,, - 



orange, showing leaves and fruit. PS6S. His task, therefore, was 



to combine the characteristics of 



a juicy, sweet-flavored fruit of the ordinary Florida orange tree 

 with the hardy, cold-resistant character of the trifoliate type. He 

 proceeded in this fashion: 



* See Year-books of U. S. Department of Agriculture, 1904, 1905, 

 1906. 



