TREATMENT OF DISEASE 213 



from the normal, and so far as the habits of the plant are 

 concerned, it is a disease. 



In this instance, however, the formation of the tuber- 

 cles resulting from bacterial infection in these plants is 

 of great benefit, for the bacteria in these tubercles fix the 

 free nitrogen of the air, thus adding to the soil nitrog- 

 enous compounds of the utmost value to the host plant 

 and to other species, wild or cultivated. While the 

 tubercles on the roots of leguminous plants are a disease, 

 they benefit a great variety of other organisms, and pre- 

 sumably — directly or indirectly — the leguminous plants 

 themselves. In this respect we have a very unusual con- 

 dition, for disease is naturally considered to be quite the 

 reverse of advantageous both to the individual and to the 

 community. 



Treatment of Disease in Plants. — The methods of 

 dealing with disease in plants fall into the same general 

 classes as the methods employed for human beings 

 namely, prevention, medical and surgical treatment. Thus 

 the instruction of the public in the means of maintaining 

 its own health, and the health of the living organisms 

 employed by man for his purposes, are both receiving 

 the most skillful attention. Just as the concern with 

 public health has resulted in preventing or greatly re- 

 ducing the occurrence of typhoid fever, so the intelligent 

 study of the conditions of plant diseases enables us to 

 control them through preventive measures much more 

 certainly than by treatment when they break out. 



The Prevention of Disease. — The means of preven- 

 tion, in addition to those which may be spoken of as gen- 

 eral hygiene, consist specifically in the application of 

 sprays and dusts to individual plants, thus attempting 

 to prevent infection. Such treatment is obviously 

 limited to those parts of the plant above ground 

 and so protected by water-proofed epidermis that the 

 poisonous sprays or dusts will not injure the tissue 

 of the host, while they poison the bacteria, fungi, or in- 



