FROST RESISTANCE 395 



A few years ago the cabbages grown extensively in Wis- 

 consin were nearly wiped out by a soil fungus. Professor 

 Jones observed in a field that one plant seemed not to 

 have been injured. He saved seed from it and planted 

 them in the infected soil and again saved seed from the 

 most resistant. In this way he bred a race sufficiently 

 resistant to make it profitable to grow cabbages again in 

 that region. 



When the southern watermelons were attacked by a 

 wilt fungus, Dr. W. A. Orton of the U. S. Department 

 of Agriculture, crossed them with the citron, which is 

 unpalatable but is resistant to this wilt. The hybrid 

 offspring were self-pollinated and from among this prog- 

 eny he eventually selected new combinations which are 

 resistant to wilt, and taste like watermelons, and in 

 addition stand shipping better than the original melons. 



Frost Resistance. — Some years ago an unusually 

 cold winter killed large numbers of orange trees in the 

 Florida orchards. The loss was so severe that growers 

 hesitated to invest again. The problem was eventually 

 solved by Dr. W. T. Swingle of the U. S. Department of 

 Agriculture who crossed the cultivated oranges with the 

 hardy Chinese wild species, Citrus trifoliata. Selections 

 from the numerous and varied recombinations of Fa and 

 later hybrid generations have yielded varieties not only 

 well adapted to stand the cold in the original citrus belt 

 but even to extend it nearly four hundred miles to the 

 north. 



Summary. — Plant breeding consists in: 



1. Isolating already existing pure lines in self-pollinat- 

 ing species. 



2. Selecting suitable hybrids from populations of 

 naturally crossed species and interbreeding them until 

 they form pure or approximately pure lines. 



3. Crossing species, varieties, or races to recombine the 

 desired characters from each in the second hybrid genera- 



