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



" On the Parasitism of Pseuclomonas destrurtans (Potter)." By 

 M. C. Potter, M.A., F.L.S., Professor of Botany in the 

 University of Durham College of Science, .Newcastle-upon- 

 Tyne. Communicated by Sir Michael Foster, K.C.B., Sec. 

 E.S. Received April 7 —Read June 12, 1902. 



Since the publication of my paper upon a " Bacterial Disease of the 

 Turnip,"* in which the existence of both a cytase and a toxin secreted 

 by the bacterium was proved, I have pursued my investigations 

 further, studying the action of the cytase and toxin upon the living 

 cells, and have succeeded in tracing the passage of the bacterium into 

 the cells through the cell-wall. 



I think it will be sufficient for me to state that the strictest methods 

 of sterilisation have been carefully observed throughout the work, and 

 all sections of turnip were prepared as described in my former paper. 



To observe the action of P. destrudans upon a living cell, a small 

 section of sound turnip was suspended in a hanging drop upon a 

 Strieker's warm-stage, the lower opening of which was closed with a 

 plate of glass cemented to the stage, a little water being introduced 

 immediately before placing the cover-slip in position. The thermo- 

 meter enabled me to tell the temperature of the preparation (which 

 varied between 15° and 20° C), and the two tubes leading into the 

 central cavity supplied the requisite amount of air for the growth of 

 the bacterium. Into the hanging drop a small fragment of turnip, 

 inoculated with a pure culture of P. destrudans, was introduced before 

 the cover-slip was inverted over the Strieker's stage. 



The effect of introducing the Pseudomonas was most striking and 

 manifested a rapidity of action for which I was hardly prepared. The 

 swelling of the wall could be recognised almost immediately, very soon 

 the position of the middle lamella became visible as a much darker 

 line, and contraction of the protoplasm quickly set in. 



To take one particular case, a section was mounted at 10.30 A.M. 

 and a cell selected for observation, which was uninjured by the razor 

 and at the same time near the edge of the section. A wall common to 

 this cell and the adjacent one was measured and found to be 2*5 in 

 thickness ; at 10.45 the bacteria were hovering round the wall ; at 11.0 

 the wall measured 4*3 /x, and the track of the middle lamella was 

 distinctly defined; at 11.20 the wall measured 6*5 \x- } at 11.45 the 

 two parts of the cell commenced to separate, and at 12 o'clock a gap of 

 2*5 n separated the two walls. 



The first signs of contraction of the protoplasm appeared at 11.15, 

 and by 12 o'clock all the protoplasm had separated from the cell- wall 



* < Boy. Soc. Proc.,' vol. 67. 



