1-'L\ER STRUCTURE OF THE BACTERIAL CELL 9 



tcrial cell is nevertheless considerable, namely from two to three atmospheres. 

 In stronger solutions (~ } per cent, saltpetre) the plasmolysis is rapid but 

 ceases in a few minutes, and the protoplasm expands and fills out the cell 

 again. This retrogression of the process is caused by the penetration of the 

 molecules of the salt. Even in weak solutions (2-5 per cent, saltpetre) this 

 takes place in a few hours, showing that the protoplasm of bacteria is far 

 more easily permeable than that of the higher plants, a peculiarity which is 

 shared by other low organisms such as flagcllata, cyanophyccac, and floridcac. 

 A most important result of this greater permeability is the case with which 

 bacteria are able to adapt themselves to changes of concentration in the 

 medium they live in, and it must greatly facilitate the absorption of food- 

 stuffs as well as the excretion of the products of metabolism such as toxins 

 and enzyms. Finally, it must be mentioned that the motile forms retain their 

 power of movement during plasmolysis, from which fact certain conclusions 

 can be drawn as to the nature of the organs of locomotion. 



In the usual methods of preparation the liquids in which bacteria are 

 suspended contain a considerable amount of soluble salts (the ordinary nutri- 

 tive media hold 0-7 per cent. NaCl), and when a drop evaporates upon the 

 cover-glass the liquid becomes more and more concentrated, so that before 

 the bacteria become dry they are plasmolysed. It is such plasmolysed cells 

 which we see in most cover-glass preparations, many species such as 

 B. typhi, Vibrio cliolcrac, and Spirillum nndnla frequently showing strongly 

 stained granules (Polar-granules) at each end of the cell, the central space 

 being empty (Fig. 6). The meaning of these appearances is evident. The 

 general conclusion to which all these observations lead us is that the 

 bacterial cell-contents are a mass of protoplasm representing an osmotic 

 system precisely like that of the cells of the higher plants, but, unlike them, 

 having no nucleus. 



The membrane or cell-wall of the bacteria is in most cases thin, delicate, 

 colourless, and without a perceptible finer structure. Unlike the cell-wall 

 of plants, it consists of a protein substance (no doubt a modification of the 

 protoplasm) and not of cellulose*. It would seem therefore that the degree 

 of division of labour which characterizes the higher plants, where there 

 is an outer very permeable membrane of cellulose, and an inner more or 

 less impermeable protoplasmic layer, has not been reached by the bacteria 

 where communication between the organism and the outer world is regulated 

 by two layers of medium permeability. 



As is the case in many algae and cyanophyceae, the cell- wall of some 

 bacteria secretes a jelly-like substance which appears optically as a delicate 



* Analysis of the cell-wall of the /). tuberculosis shows that it contains a large amount of true 

 cellulose, so large indeed that it has been delected in tuberculous animal tissues, and was thought to 

 be a product of metabolism (Freund;. 



