IMPROVEMENT BY SELECTION. 25 



favors the general applicabilit}- of the idea. The production of ber- 

 ries, or seed balls, is held by some English breeders to be an indica- 

 tion of such vigor, and therefore presumabl}^ of disease resistance, 

 although no one claims that the absence of these is equally strong 

 evidence in the opposite direction. It is worthy of passing note that 

 berries are formed much more commonly in Europe, especially in 

 Germany and Holland, than in America, apparently because of cli- 

 matic diU'erences. 



THE RELATION OF HYBRIDITY TO DISEASE RESISTANCE. 



By a "new variet}'*' of potato, as the term is commonh^ used, is 

 meant one recently developed from the seed. Sports may appear and 

 are indeed frequent in some varieties. These will, however, be men- 

 tioned later. The seed in all cases presumabl}" represents a sexual 

 origin, i. e., comes from a fertilized flower, but this may have been 

 either self-fertilized or cross-fertilized. One would expect greater 

 vigor to result from the cross-fertilization, and potato breeders are of 

 the opinion that it is secured. On the other hand, while varieties 

 recently originated from seed ma}^ show a high degree of disease 

 resistance, this is not necessarily the case according to the verdict of 

 English and German breeeders, man}' new varieties proving as sus- 

 ceptible as old ones. 



Reference has already been made to the work of Cathcart and Sut- 

 ton in England and of Meckel and Labergerie in France based on the 

 hope of advantage from using one or another of the wild Solanums 

 for such hybridizing. There are interesting possibilities along this 

 line, since there are many wild forms of «S'. tuherosum in addition to 

 other species of tuberiferous Solanums. Thorough trials of these are 

 now being made by Stuart ** at the Vermont Experiment Station which 

 will be reviewed later in this paper. It should be emphasized at once, 

 however, that while the use of the wild Solanums does ofter interest- 

 ing possibilities, there is no record of practical success from their use, 

 if we except the doubtful one of Labergerie's variety of S. eoinmer- 

 smiii previously referred to. On the other hand, great practical 

 improvements have been secured by various breeders from crossing- 

 varieties of cultivated potatoes. 



IMPROVEMENT BY SELECTION. 



All plants tend to vary. One of the commonest ways to improve 

 plants is by selection from among the var3^ing individuals. For the 

 first two years after their origin, variants, or "rogues," are not uncom- 

 mon with the seedling potato, but the variety is "fixed" by the weed- 

 ing of such rogues before it is distributed for general culture, and 



« Stuart, W., Disease- Resistant Potatoes, Vermont Exp. Sta, Bui. 115 (May, 1905). 



