894 PLANT BREEDING 



in number, length, and strength. It is desirable for most 

 purposes (e.g., automobile tire casings) to have them as 

 long, strong, and flexible as possible. Cotton in nature 

 readily crosses, so that pure seed can be produced only by 

 guarding against cross-pollination. The method of ac- 

 complishing this is both unique and effective. In certain 

 isolated valleys of the arid southwestern United States 

 cotton can be grown by irrigation and can easily be kept 

 pure provided only one variety is grown in each valley. 

 Growers in a valley agree on a certain strain and then pre- 

 vent by law the planting of any other kind. Selection 

 for improved seed is then made in isolated breeding places 

 until a suitable variety has been secured, and then only 

 pure seed is sown in the valley. It remains pure merely 

 because it has no chance to cross. 



Seedless Oranges illustrate a sort of plant breeding 

 which can only be practiced with perennial plants which 

 can be multiplied in asexual ways by grafts, cuttings, etc. 

 All the Washington navel oranges in California are de- 

 scended from two trees originally brought from Brazil. 

 Just as these first seedless oranges originated from seeded 

 varieties by mutation, so they in their turn have pro- 

 duced many new strains from bud sports. By a bud 

 sport is meant that a mutation occurs in the cells of a 

 bud, and the bud in turn grows out into a branch show- 

 ing this mutation. Whenever a new and better mutation 

 is observed it is multiplied by taking buds from the 

 mutated branch to graft into seedlings. Aside from muta- 

 tions, plants of this sort do not change (except in response 

 to the environment) and no special care is necessary to 

 keep them pure. Some trees are more productive than 

 others, however, and are now used by the up-to-date 

 nurseryman as the source of his buds. 



Disease Resistance has been found to be a heritable 

 character in many species of plants. Two illustrations 

 will show how this can be made useful in combating loss 

 from disease. 



