SIZE AND STRUCTURE OF THE BACTERIAL CELL. 27 



contains relatively less osmotic substances than the cell 

 itself, there is a tendency for the cell fluid and the dissolved 

 substances to pass out into the surrounding water. This is 

 prevented by the outer layer of protoplasm, and hence the 

 osmotic or internal pressure. When, however, the cell is 

 placed in a liquid which is richer in osmotic substances, a 

 2.5 per cent, saltpeter or a 1 per cent, salt solution, the 

 osmotic pressure of the outside liquid overcomes that of 

 the cell fluid. A current is thus established into the celL 

 The protoplasm as a result retracts from the cell wall, and 

 this retraction of protoplasm is known as plasmolysis. 





^ 



Fig. 4. Plasmolytic changes, after A. Fischer, o— Cholera 

 vibrio; b — Typhoid bacillus; ^--Spirillum undula. 



In the above case the salts in the outer liquid, owing to 

 the relatively impermeable wall, do not easily penetrate 

 into the cell. If they did the internal pressure of the cell 

 fluid would soon rise above that of the surrounding- liquid, 

 and as a result the protoplasm would expand and reflU the 

 cell. This actually does occur when a saltpeter solution of 

 double the strength given is employed. When the plasmo- 

 lyzed cell is placed in pure water the protoplasm returns 

 promptly to its original position. 



In the case of the micrococcus the protoplasm contracts 

 to a small globular mass, while in the bacillus two round 

 bodies form, one at each end. As a result the cell-wall 

 between the polar bodies is rendered visible. These plas- 

 molytic changes occur in the living cell. As will be seen 

 later, the flagella or motile organs seem to arise from the 

 outer layer of the cell- wall, in which case this membrane is 

 functionally active and is not a passive structure, as in the 

 case of the cellulose^wall of the ordinary plant cell. 



