176 LUTHER BURBANK 



cloth about the united stems for a few days until 

 union has taken place. It is not necessary to 

 use grafting wax, if protected from winds and 

 too hot sun. The operation is preferably per- 

 formed in the greenhouse. 



By this method, I grafted the tops of young 

 tomato plants on the main stalks of potato 

 plants, at a time when the stems were about one- 

 quarter of an inch in diameter. The reverse 

 operation, grafting amputated potato tops on 

 tomato roots, was performed at the same time. 



Of course the tomato and potato belong to the 

 same family, and it seemed reasonable to suppose 

 that such grafting might be successful. But, on 

 the other hand, numerous attempts have been 

 made to hybridize the two plants by cross- 

 pollination, and these have always resulted in 

 failure. I have tried it many times, and have 

 never been able to fertilize one plant with pollen 

 of the other. We know that, as a general rule, 

 plants that cannot be cross-pollenized cannot be 

 mutually grafted. The same barriers usually 

 exist in one case as in the other. 



The potato and tomato grafts, however, 

 proved very notable exceptions to this rule. In 

 both combinations the union between the foreign 

 stems took place quickly, and resulted in a stem 

 as strong as the ordinary stem of either plant. 



