Interrelation of Plant and Animal Pathology 61 



discovered in the crown gall, and that therefore we may confidently 

 expect the eventual discovery of a microorganism as the cause of 

 human cancer. Smith reported in Science a few weeks ago that he 

 has actually succeeded in producing tumors on plants by injecting 

 extracts of the bacteria. 



I need only in this connection remind you of the numerous bac- 

 terial diseases of the human body, such as typhoid fever, diphtheria, 

 tuberculosis, etc., to convince you of the similarity of behavior of 

 plants and animals toward this class of parasites. 



A pathological condition manifesting itself as galls or tumors, 

 caused by insects and other animals, is a matter of every day obser- 

 vation. Phylloxera of the vine, a destructive European trouble, and 

 woolly aphis of the apple in this country furnish familiar examples. 

 The question, however, has been raised here whether bacteria sim- 

 ilar to the species causing crown gall may not be the real agent in 

 producing these galls. 



If I had the time to take up the behavior of plants and animals 

 toward poisons, it could easily be shown that as a general fact the 

 same substances are poisonous to organisms in both kingdoms. 



As regards the general question of pathological anatomy, Kuster 

 has shown in his pathologische Pflanzenanatomie, published in 1903, 

 that the terminology introduced by Virchow many years before ap- 

 plies with scientific precision to abnormalities of plant structure. 



We have now seen that plant and animal diseaseo show most 

 striking resemblances in both their causes and effects. Let us now 

 consider for awhile the mechanism by which representative organ- 

 isms from the two kingdoms protect themselves against the attacks 

 of various diseases. 



In the first place, we find that individual plants of the same 

 species show striking differences in disease resistance, and that in 

 many cases at least this power of resistance is transmitted to the 

 offspring, so that it often becomes a comparatively simple matter 

 to produce by selection or breeding a variety resistant or immune 

 to any particular disease. A number of important diseases of 

 economic plants has already been controlled in this way. The 

 anthracnose of clover just referred to is a striking example of this 

 kind. A number of other similar instances might be mentioned, 

 as the cow-pea wilt and cotton wilt by Orton, the tomato blight by 

 Essary, and the flax wilt by Bolley. It has been shown that disease 

 resistance of plants may be transmitted to the offspring in strict con- 



