60 TOBACCO BKEEDING. 



fic gravity. One apparatus is sufficient to separate seed for an en- 

 tire community, a plan which is being followed in some cases. A 

 pound of seed may be separated in less than half an hour. Thus it 

 is seen that the apparatus and cost of operating are very small and 

 not sufficient to prevent any tobacco grower from eliminating all 

 light and poorly developed seed, in this way not only increasing 

 the yield, but also improving the uniformity and quality of his crop. 



DISEASE RESISTANCE. 



In practically all fields producing diseased tobacco plants where the 

 writers have made observations some degree of immunity has been 

 noticed in individual plants which have been found growing among 

 badly diseased plants on infected soil. These cases of immunity could 

 not be explained on the ground of any differences in treatment, but 

 their resistance to disease was evidently inherent in the individual 

 plants. The same conditions have been found by other investigators 

 and workers in other farm crops, and from these resistant individuals 

 many immune strains have been developed. Among the most notable 

 are the variety of wilt-resistant cotton, improved by Mr. W. A. 

 Orton and Mr. Eivers, and the Iron cowpea, which is resistant to 

 root-knot caused by nematodes, improved by Dr. H. J. Webber and 

 Mr. W. A. Orton. The transmission of this immunity found in indi- 

 vidual plants has made it possible to develop immune strains, and in 

 that way to produce thoroly healthy crops on disease-infected soils. 



In most cases where immune plants occur, if seed is saved from a 

 large number of such plants some of them will be found to transmit 

 their resistance to the progeny uniformly and thus give rise to the 

 easiest known method for the control of certain plant diseases. 



In the case of tobacco, the seed of the immune plants must be saved> 

 with precautions to avoid cross-pollination, to insure the best results. 

 In the season of 1903 the writers made selections of plants in several 

 tobacco fields in the Connecticut Valley which showed immunity to 

 the tobacco wilt. These plants stood out very plainly and strikingly 

 in the diseased sections of the field, making a normal growth, and 

 were apparently not affected by the wilt in any way, while plants 

 growing all around them were so badly diseased that they produced 

 no tobacco, and many of them died before maturing seed. Seed was 

 also saved from some of the diseased plants that reached maturity. 

 Two rows were planted the following ^year on the infected soil, one 

 from seed of a resistant plant and the other from seed of an immune 

 plant, with the results shown in Plate IX, figure 2. In this instance, 

 by reason of the foregoing and other observations, it was found that 

 complete resistance to the wilt was obtained by one year's selection. 



96 



