Application of Biological Principles to Plant Breeding 129 



cross of orange upon trifoliata, is a rough, but thin-skinned 

 fruit, resembling an orange in appearance but a lemon in 

 flavor. It is used as a condiment or for citrangeade. The 

 Morton, coming from the same kind of cross as the Willits, 

 is a large, juicy, almost seedless fruit, only slightly more 

 bitter than the sweet orange. 



Young trees of these three varieties have endured a 

 temperature of eight degrees above zero, and it is thought 

 that by the use of them and of similarly obtained varieties, 

 citrus fruit culture can be extended fully 400 miles north 

 of the present region. 



In connection with this description of the production of 

 new citrus fruits it may be well to mention that they are 

 sometimes seedless. In fact, seedless fruits are often 

 obtained by crossing. In true annuals reproducing by seed 

 only, such productions would be of no value, for they would 

 perish at the end of the first season. Seedless perennials, 

 however, are among the most valuable horticultural varie- 

 ties simply because they can be propagated asexually. 

 In floral novelties, moreover, not only seedlessness but 

 entire sterility is not a drawback to commercial worth, 

 because sterile plants are often famous for their profuse 

 flower clusters. 



It was stated earlier that one phenomenon of hybridi- 

 zation was the production of fixed or constant first genera- 

 tion hybrids. This statement was made from hearsay 

 evidence. There are several cases in which either new 

 characters or blended characters that breed true appear 

 to have been formed, but they have not been studied with 

 sufficient care for their mode of inheritance to have been 

 accurately and finally decided. In crosses between cer- 

 tain true species, hybrids have been produced that are 



