OTHER BACTERIAL DISEASES. 387 



in the leaves and stem being filled with water, or many of them 

 simply with air. It has been claimed that there is no likelihood 

 that bacteria can live under such conditions and that bacterial 

 plant diseases are, therefore, on a priori grounds, improbable 

 or impossible. Even in very recent years this claim has been 

 very vigorously supported, and disputes are still going on, in 

 the pages of bacteriological journals, in regard to the question 

 of the existence of bacterial disease in plants. Almost to the 

 very present day it has been insisted that there is no demonstra- 

 tion that bacteria can produce disease in plants. 



Although this claim was legitimately urged a few years ago 

 by conservative scientists, it can no longer be held in the light 

 of recent experiments. In the last few years the evidence for 

 such diseases has accumulated rapidly, and to-day the proof of 

 the existence of bacterial plant diseases stands on identically 

 the same basis as the proof of the bacterial diseases among 

 animals. In quite a number of well-known plant diseases it has 

 been possible, with the greatest of ease, to obtain the necessary 

 steps of proof. First, it has been shown that a certain definite 

 bacterium is present in the plant suffering from the disease in 

 question. Second, it has been found possible to isolate the 

 bacterium and to cultivate it in the laboratory by normal cul- 

 ture methods. Third, it has been possible to inoculate healthy 

 plants with the laboratory cultures of the bacterium in ques- 

 tion, with the result of producing inevitably a recurrence of 

 the original disease with all of its typical symptoms ; and in the 

 tissues of the plants thus affected the bacillus is found in great 

 numbers. When these steps of proof have been obtained, as 

 they have in several plant diseases, there is no longer any pos- 

 sible question that these diseases are attributable to the bac- 

 teria in question. We must, therefore, look upon bacterial 

 diseases of plants as certainly occurring. 



How abundant bacterial plant diseases are can hardly yet 

 be stated, for the subject is one of very recent study. It is 



