ECONOMIC OR USEFUL PLANTS 



411 



579. Citrous fruits. The citrous fruits belong to the family 

 Rutacece, which includes also our native prickly ash (Xantho- 

 xylum) . They belong to the genus Citrus, shrubs or small trees 

 native in tropical and subtropical Asia. Three types of the 

 citrous fruits are cultivated in the United States. The culture 

 of the citrous fruits is often attended with considerable risk be- 

 cause of hard frosts which sometimes nearly or quite destroy 

 the crop, or even kill the trees. Experiments are now in prog- 



Fig. 394- 



Naval orange tree, ten years old. From Bureau Plant Industry. 



ress by the United States Department of Agriculture which 

 have for their object the development of more hardy varieties. 

 The sweet orange has been crossed with Citrus trifoliata (which 

 has an unedible fruit) , and the result is a hardier tree which can 

 be cultivated about two hundred miles farther north than the 

 present limit of orange culture. A number of varieties have been 

 obtained and a few are promising. The fruit is called citrange. 

 Two varieties give promise of being used as substitutes for lemons. 



