May 2S, 1915] 



SCIENCE 



777 



problem of immunity can not be solved in 

 this way. 



The third way to obtain immune forms 

 is to select resistant individuals and from 

 them breed pure strains. In the case of 

 many diseases, although certainly not in 

 all, healthy individual plants are found 

 in the diseased plots, and the breeding of 

 immune strains from these individuals 

 would seem to be very simple, but experi- 

 ence has taught the contrary. All the fac- 

 tors pointed out in connection with the se- 

 lection of immune forms must be reckoned 

 with, but in a still greater degree. So long 

 as the appearance of the disease is the only 

 criterion by which to determine the sus- 

 ceptibility of the plants to disease the ex- 

 perimenter is exposed to all kinds of un- 

 known influences. 



Several attempts to breed kinds of wheat 

 immune to stinking smut have been made 

 without any real results. The question of 

 producing such kinds is of great impor- 

 tance, especially for the United States. In 

 the large wheat areas of Idaho and eastern 

 "Washington, for instance, stinking smut 

 is very serious, not infrequently causing 

 a loss of twenty-five per cent, of the crop. 

 Inspection of seed in that state discloses 

 the fact that a large part of it is covered 

 with the smut spores, and treatment of the 

 seed with copper sulphate is said to be 

 useless because the soil is so badly in- 

 fected. In many European countries, 

 however, smut has been completely con- 

 trolled. 



In the case of smut the possibility of in- 

 fection, as far as the fungus is concerned, 

 is very great. As infected plants are in 

 general not very productive on account of 

 the seed being destroyed by the fungus, it 

 might be supposed that smut-resistant 

 plants would propagate well and that the 

 strains would become immune. This, how- 

 ever, is not the case, and it shows that the 



breeding of smutless wheat by selection of 

 healthy individuals has little chance of 

 success, a fact which has been proved by 

 experiments already made. That this is 

 an impossibility, however, can hardly be 

 stated definitely, but success could be ob- 

 tained, if at all, only after tremendous 

 amount of labor in breeding and trying 

 hundreds of forms or by fortunate acci- 

 dent. 



It will be remembered that Orton by this 

 method of breeding succeeded in obtaining 

 varieties of cotton and watermelon resist- 

 ant to Fusarium wilt. As the original re- 

 sistant individuals found in the field gave 

 too small yields, he crossed them with pro- 

 lific varieties and in this way combined 

 the disease resistance of the one parent 

 with the productivity of the other. A 

 similar thing was done by Bolley with flax 

 and by L. R. Jones with cabbage, both of 

 whom bred wilt-resistant varieties by selec- 

 tion. In the case of wheat, it is the opinion 

 of the writer that there would be better 

 chance of breeding smut-resistant varieties 

 if strains rather than individual plants were 

 selected and crossed with productive vari- 

 eties. Orton very successfully selected a 

 certain variety of cowpea resistant to wilt 

 disease and root knot, that is, the iron cow- 

 pea grown in South Carolina, and crossed it 

 with a more desirable variety. By this 

 means also, that is by selecting certain va- 

 rieties, some of the Phytophthora-vesist- 

 ant varieties of potatoes were obtained, 

 and probably also the square head wheat 

 which shows immunity to Puccinia triti- 

 cina. 



Next to field experiments, those in the 

 laboratory might aid in the discovery of 

 resistant varieties of cultivated plants. 

 Such experiments have advantages over 

 those in the field and are practical in case 

 of diseases caused by parasites that may 

 be grown artificially in pure cultures. 



