WOUND INFECTIONS. 



53 



the writer would seem to indicate that it does not enter through unbroken surfaces. Three 

 of the four plants which were atomized thoroughly with a virulent culture did not contract 

 the disease, and the fourth showed signs of it only after 20 days. One of the three which 

 remained free bore aphides. More experiments should be made. 



Injuries due to insects and other small animals, e. g., nematodes, undoubtedly chiefly 

 favor the entrance of bacteria, but I think we must also regard injuries due to frost and to 



F.g. 7.* 



hail as among the predisposing causes in certain cases. Pure frost injuries when slight 

 may also resemble bacterial leaf spots as in case of the tender magnolia shown in fig. 8. 



Many bacterial plant-parasites are able, however, to enter the plant in the absence 

 of visible wounds, and this brings us to the question of infection through natural openings. 

 These bacteria we will now consider. They are a more interesting group than the strict 

 wound-parasites for this reason, if no other, that they are able to begin business on a 

 smaller capital. 



*Fig. 7. Cucumber leaf wilted by Bacillus tracheiphilus and then gnawed by striped beetles Diabrolica vittata; 

 introduced to show partiality of this beetle for diseased leaves. One other wilted leaf on this plant was gnawed in 

 same way, while a dozen turgid leaves were scarcely touched. These two leaves were primary infections transmitted 

 by Diabrolica from squash leaves which were wilting as a result of my pure culture needle-puncture inoculations. The 

 presence of the bacteria in great numbers in several different lobes of this leaf was demonstrated microscopically 

 after photographing. 



