432 PRACTICAL BOTANY 



Hybridizing sometimes (but not nearly always) aids the 

 plant breeder by giving him a large number of marked 

 variations from which to select. 



High cultivation, together with plant breeding, has brought 

 about many astonishing results. Plums three inches long have 

 recently been produced. A hybrid beach plum bears so abun- 

 dantly that the twigs are entirely hidden by the fruit. The 

 largest cultivated apples are many hundred times the bulk 

 of their remote wild ancestors. A new variety of blackberry 

 plant covers one hundred and fifty square feet of soil and 

 bears a bushel or more of fruit. 1 Most cultivated roots and 

 tubers have been greatly changed from their wild condition, 

 losing in the proportion of woody fiber which they contain, 

 and gaining immensely in size. 



398. Securing varieties immune to disease. One of the most 

 important problems for the plant breeder is how to secure 

 varieties immune to diseases. Two of the most notable 

 achievements of our Department of Agriculture in this direc- 

 tion have been the production of a disease-resisting variety 

 of sea-island cotton and of watermelon. The soil of valuable 

 cotton plantations had come to harbor a fungus (Fusarium) 

 which attacked the roots of the plants, plugged the vessels 

 with its hyphse, and destroyed almost the entire crop. In 

 consequence of this many planters gave up cotton growing. 

 Observation showed that often in a field where nearly all 

 the plants were killed, here and there an individual survived, 

 blossomed, and ripened its capsules. For four years plants 

 were bred from the seeds of these resistant individuals until 

 a variety was secured which withstood the attacks of the 

 fungus and made it possible to resume cotton growing on the 

 abandoned plantations. 



Extensive areas in the South, once devoted to the culture 

 of watermelons, became so infected with a fungus that melon 

 growing was no longer possible. The destruction was so 



1 See the article by D. S. Jordan, rt Some Experiments of Luther Bur- 

 bank," Popular Science Monthly, January, 1905. 



