This line of work lately has been found to give results of enormous crop value. One has only to save the seed from the types that best serve the purposes, and persist in doing so to gain greatly in this respect. This is the newest field of work along the line of controlling plant diseases, but it is sufficiently past the experimental stage to allow one to assert with confidence that any farmer who will may thus greatly benefit himself and aid all mankind toward the elimination of plant diseases. For example, if we gain a type of wheat that does not produce on its leaves one-third as much rust as has been produced previously in that region on the common types of wheat, it is a self-evident fact that there will not be so much rust to be distributed to other fields. If, by care ful and consistent selection of varieties and indi vidual strains from the varieties, the farmer finally attains a crop of potatoes that is no longer open to the attack of potato-rot and potato-blight, it is a self-evident fact that his fields will not be distributers of the disease to other fields. It is too much to expect, perhaps, that this process will eliminate entirely some of the most destructive diseases, such as rust of wheat, rot of potatoes, blight of pear, root-rot of cotton, and wilt of flax, yet the results gained in this direction in the past ten years are such as to convince the most skep tical that herein lies a most effective means of reducing the destructive action of plant diseases.
The process is so simple that any one may engage in it with success. Diseases weaken, mar, shrivel and lessen the produce from plants that are non resistant. Mother plants that are resistant produce the more perfect products. It is from such that one should propagate the succeeding crops. It is but to put the " survival of the fittest" principle into direct action in crop production.
Literature.
The literature on plant diseases is voluminous. It is impossible here to cite monographs. Refer ences to these may be found in writings specially devoted to this subject. Many of the diseases that have to do with special crops are discussed or referred to under these crops. Most of the experi ment stations and the United States Department of Agriculture have issued general and specific bulletins on plant diseases. The card catalogue of experiment station literature, issued by the United States Department of Agriculture, is especially helpful in this connection.