8 BACTERIA IN RELATION TO PLANT DISEASES. 



To test the pathogenicity of the organism Dr. Cobb inoculated a few sugar-canes. 

 He writes as follows : 



In order to show whether the disease is caused by the microbes healthy plants were inoculated 

 with gummy matter from diseased cane. I found it to be unnecessary to first make pure cultures 

 in the way usually adopted, for the reason that a pure growth of microbes could be obtained without 

 this precaution. Plenty of cane could be found containing no other organism in its interior tissues 

 and advantage was taken of this fact in making inoculations. 



This is good as far as it goes, but to complete the proof and satisfy the skeptic, pure- 

 culture inoculations are necessary. 



Concerning the pathogenic nature of his Bacillus vascularum, Dr. Cobb also has the 

 following : 



What we wish mainly to know is whether the microbes are the cause of the disease. As a result 

 of hundreds of careful examinations, I am able to say that the disease of gumming, as described, 

 never occurs without the yellow gummy matter in the sap-vessels. 



The result of even a much greater number of examinations made with equal care showed that 

 the yellow gummy matter never occurred without the microbes, indeed it is very evident that the 

 gum is a product of the growth of the microbes. This leads to the conclusion that the disease never 

 occurs without the microbes being present, and it may be added that many cases were examined in 

 which, in spite of long and careful search, none but the merest traces of other foreign organisms could 

 be found. This is very strong evidence that the microbes are the cause of the disease, but it does 

 not amount to proof. 



Dr. Cobb's methods of inoculation are quite carefully described. Pure cultures were 

 not used, but all inoculations were direct (see this monograph, vol. I, p. 9). They were 

 made in healthy canes by means of a needle which was thrust into vertical slits made with 

 a needle-chisel. The surface of the cane was first covered with hot wax, the chisel was 

 thrust through this a half inch or more into the cane, then the needle-thrust followed. 

 The bacteria on the tip of the needle were obtained directly from the fresh ooze out of the 

 ends of cut bundles. The following precautions were used : Needle and chisel were steril- 

 ized by heat and protected till ready for use. Cane was selected which was occupied 

 apparently only by a pure growth of this yellow organism; the rind was removed, and the 

 segment was placed in a large glass-stoppered jar the inside of which had been smeared 

 with glycerin over night; all the operations were performed in a room as free from drafts 

 and dust as possible. When all was ready the protected segment of diseased cane was 

 shortened with a sterile knife and the yellow ooze from the freshly exposed surface of the 

 bundles was then touched with the inoculating needle and some of the slime was transferred 

 to the interior of the healthy canes. The lips of the wounds were then covered with more 

 hot wax. This paper contains no hint as to the result of these inoculations. 



All that relates to the morphology of this organism is given in the three cuts here 

 reproduced from Dr. Cobb's paper. The magnification is said to be X 2,700, which would 

 make the organism about the same size as Bacterium campestrc. Judging from these 

 figures it is also variable in form like the latter. 



We are told by Dr. Cobb next to nothing about the cultural characters of his Bacillus 

 vascularum. All I can find is the following: 



Cultures on agar-agar containing about 5 per cent cane-sugar gave at first roundish colonies on 

 the surface, having to the unassisted eye no structure either radiated or concentric. In the course 

 of a few weeks the yellowish-white and somewhat opalescent growth had extended several inches 

 along the edge of the tube on the surface of the agar-agar and between the tube and the agar-agar, 

 without causing any liquefaction. On gelatin the growth was much slower and remained circular 

 and almost imperceptibly concentric. Its color, &c., were as on agar-agar. 



If the "&c."is entirely inclusive, then we must assume that this organism did not 

 liquefy gelatin. From its slower growth on this medium, it is not unlikely that litmus 

 neutral gelatin exerts on it a retarding influence the same as it does on Bad. campestrc. 

 Figure 3, which shows the dwarfing effect on cane, is borrowed from Cobb's paper. 



