80 PLANT PHYSIOLOGY 



America through England in 1896, and the potato blight was im- 

 ported from Chile to Colorado. All of these destructive dis- 

 eases could have been prevented by the proper quarantine 

 measures. 



4. Breeding resistant varieties. This type of preventive measure 

 is obviously more easy to effect in plants and the lower animals 

 than in man. The "human" species, in fact, by its sentimental 

 methods of " humane" treatment is the only one which keeps in 

 the race all the weak and inefficient members of the species instead 

 of permitting the laws of natural selection to take their course. 

 In the plant world, this line of attack, i. e., breeding resistant 

 varieties, has met with very promising results. Hardly a month 

 passes without the announcement of the discovery and develop- 

 ment of a new variety especially resistant to some particular 

 disease. Among those developed to date are the anthracnose and 

 mosaic-resistant beans, the black-rot (Fusarium) resistant cabbage, 

 wilt-resistant cotton, and rust-resistant wheat. 



5. Vaccination. In this treatment the individual is given a mild 

 form of the disease which protects against the more virulent form 

 by placing at the disposal of the organism the means of combat 

 which the mild form of the disease has developed in it. Thus there 

 is vaccination for typhoid, smallpox, cholera, and many other 

 diseases. Since plants have no circulatory system which permits 

 of general treatment in this way, vaccines have been used on 

 them to only a slight extent and then with no marked success. 



6. Serum therapy. In the case of serum therapy an extract or 

 serum is given which counteracts the poison developed by the 

 disease. Serums have been developed for the toxins of diphtheria, 

 tetanus, and hog cholera, but for the same reason given above, this 

 method has not been applicable to plants. Although the serum 

 treatment is commonly used as a cure it may also be given as a 

 preventive, and can hence be listed here among the prophylactic 

 measures. 



7. Antiseptic surgery. In this category are included all the meas- 

 ures of antisepsis which bring about asepsis. The sterilization 

 of the instruments and of all bandages, the employment of lotions 

 which prohibit the growth of microorganisms, and all other such 

 operations are here included. In the plant world such prophylactic 

 measures are of importance chiefly in tree surgery. After pruning 

 and cutting, the wound should be well washed out and then covered 



