32 THE BACTERIAL CELL 



staining are given by Friedlander, Ribbert, Lceffler, and others. The annexed 

 illustration (-Fig. 8) is drawn from a preparation stained in this way. It repre- 

 sents Bacterium Pasteur ianum, an acetic acid bacterium discovered by Hansen. 

 The chain seen at the left-hand side differs from the remaining portions of the 

 figure. The upper half presents no special peculiarities, and shows (like the 

 remaining chains) three darkened cells held together by the swollen membrane ; 

 whereas in the lower moiety the dark parts are wanting, the three cells formerly 

 present therein having been accidentally crushed in making the preparation, so 

 that only the mucinous envelope remains behind. In a more closely investigated 

 case (not, however, with this bacterium) the substance composing the capsule was 

 identified chemically as related to mucin, or probably identical therewith. 



If the gelatinisation of the cell walls proceeds to a little greater extent, it 

 causes the individual cells to become joined or cemented into a coherent mass, 

 called zoogloea by COHN (V.), the size of the agglomerations being greater or 

 smaller according to the degree of development. 



In a few species of bacteria, colouring matter or ferric oxide is stored in the 

 mucinous envelope, further particulars of which will be found in chapter xxxiv. 



34. Plasmolysis. 



The difficulty of detecting the cell envelope of bacteria can be removed by 

 immersing the organisms in a solution of salt, which exerts a hygroscopic action 

 on the cell contents, in consequence whereof they shrink and retreat from the 

 cell wall, whereupon the latter is readily recognisable, even in unstained 

 preparations. 



This kind of influence, called Plasmolysis by H. de Vries, was first observed 

 in bacteria by De Bary, in connection with his Bacillus megatherium. 



The experiment is simple in performance. An ordinary preparation of the 

 bacteria under examination is made, a few cotton fibres being immersed to pre- 

 vent the escape of the organisms. The salt solution is then allowed to flow in 

 at the edge of the cover -glass, its tranfusion being facilitated by absorbing the 

 water with blotting-paper held at the opposite side. Provided the solution is 

 not excessively concentrated, and that it contains no toxic substance, the resulting 

 plasmolysis in no wise destroys the vitality of the cells. It can be made to 

 disappear again by washing out the hygroscopic reagent. 



The concentration of the cell contents caused by plasmolysis, which is also 

 generally accompanied with an increased refractive power, leads very often, as 

 BUCHXER (I.) pointed out, to agglomerations of material, which, unless more 

 closely examined, might illusively indicate the presence of endogenous spores. 



In such bacteria as are endowed with the power of locomotion, the rate of 

 movement is retarded by increased concentration of the plasmolytic solution. 

 WLADIMIROFF (I. and II.) entitled the smallest percentage capable of arresting 

 locomotion the critical solution. The values of this for the various salts 

 on any determined species of bacteria show an unmistakable regularity, the 

 chloride of any given metal (K, Na, &c.) having the weakest, and the bromide 

 and sulphate the strongest retarding effect, whilst the nitrate occupies an 

 intermediate position. As far as the bases are concerned, potassium, sodium, 

 and ammonium rank in the order given. E. OVERTON (I.) has published several 

 communications with regard to a number of substances which ai^e unable to 

 effect plasmolysis owing to their passing through the plasma as rapidly as 

 water. 



The weakest solution of sodium chloride (NaCl) capable of effecting plasmo- 

 lysis is, according to A. FISCHER (I.), 0.5-0.75 per cent, for Cladothrix dichotoma, 

 and for Crenothrix Kiihniana 0.75-1.0 per cent. These limits are, however, not 



