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Prof. M. C. Potter. On the Parasitism of [Apr. 7, 



frangibility of the cell-wall and the bacteria, this difference enabling 

 the course of the bacteria to be distinctly followed.* 



Important evidence that P. destructans has the power of perforating 

 the cell-wall was also afforded by the well-known method of paraffin 

 sections. Small sterile pieces of turnip were inoculated with a pure 

 culture of P. destructans, and after 12 hours were found to be partially 

 rotten. These were then fixed in Muller's and also in Flemming's 

 fluid, washed, dehydrated, and embedded in paraffin. The sections 

 were then cut, floated in water, and fixed to the slide by means of the 

 white of egg. The paraffin was dissolved in turpentine. The slides 

 were next placed in absolute alcohol, and then in gradually decreasing 

 strengths of alcohol, for the purpose of staining. When fixed in 

 Flemming's solution, it was found that the bacteria did not readily 

 stain, and for this reason Muller's was preferred. 



In staining the sections a further problem presented itself, namely, 

 how to differentiate between the cell-wall and the bacterium. After 

 numerous trials it was found effective to employ a weak aqueous solu- 

 tion of ruthenium-red, which was first allowed to act, the cell-walls 

 being stained by this means but not the bacteria : the sections were 

 then washed in water and stained with Ziehl's carbol-fuchsin or other 

 aniline dyes. Subsequently ruthenium-red followed by Lowit's method 

 for staining flagella was found to give the best results. 



This method of fixing and double staining distinctly differentiated 

 the cell-wall and bacteria, and showed the latter fixed in the actual 

 process of perforating the wall, and various stages of penetration could 

 be distinguished (fig. 2). These results confirm my observation of the 

 penetration of the wall by P. destructans. 



An organism which is thus capable of secreting a powerful cytase 

 and toxin, producing such a remarkable effect in destroying living 

 plant cells, and which subsequently has the power of perforating the 

 cell-wall and entering the cell-cavity, must certainly be regarded as 

 producing a true plant disease. Indeed the parasitic action of this 

 bacterium upon living tissues is exactly comparable with that of 

 certain of the parasitic fungi, though differences in detail are naturally 

 presented from the different character of the organisms. 



According to my previous observations, an attack of P. destructans 

 could always be traced to a wound. I have found that this bacterium 

 has no power to penetrate the cuticle of the mature epidermis. A 

 number of blocks of turnip cut with the usual precautions, and to 

 include a portion of the uninjured epidermis, were inoculated on the 

 internal parenchyma, placed in sterile plugged test-tubes, and incubated 

 at 30° C. for five days. At the conclusion of this period the pieces of 



* It might be objected that the cells in the hanging drop were not under normal 

 conditions, and thus their vitality might be impaired, but no such objection can be 

 raised in the case of the cells of the blocks employed for the paraffin sections. 



