8 



INTRODUCTION : MORPHOLOGY 



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evolution. The tingibility of the cell-contents in bacteria is not so great 

 when one takes into consideration the amount of stain taken up by the cell- 

 membrane, and even though in some cases an unusual amount may be 

 absorbed there is certainly nothing that warrants us in assuming the existence 

 Of a specific reaction between the anilin stains and nuclei. The bacterial cell 

 then, interpreted in the light of the above facts, is a simple protoplast 

 enclosed within a cell-membrane but devoid of a nucleus. 



The phenomena of plasmolysis lend additional strength to this view. 

 To plasmolyse bacteria it is only necessary to place upon a slide a drop 

 of water containing bacteria, lay upon it a cover-glass (with a few fibres 

 of cotton-wool between to prevent crushing), and 

 allow the plasmolysing fluid to run in at the edge. 

 Spherical and very short rod-shaped bacteria become 

 under this treatment more highly refractive. This 

 increase of density is the outward expression of the 

 contracting of the cell-contents, and in these very 

 minute forms it is the only change perceptible. In 

 the larger bacteria, however, such as typhoid, cholera, 

 spirillum, cladothrix, &c, the details of the process 

 are readily followed. Solutions of 2-5 per cent, 

 saltpetre or 1 per cent, common salt (fresh blood 

 serum contains 07 per cent. NaCl) are sufficient 

 to set up plasmolysis. As soon as the solution 

 reaches the bacteria its effect is visible, and one 

 sees the protoplasm gradually raised away from the 

 cell-wall and pushed inward. Just as in elongated 

 plant-cells, it frequently breaks up into two or three 

 highly refractive rounded masses which expand and 

 fuse together again if the plasmolysing solution be 

 replaced by pure water (Figs. 4 and 6). In short 

 rods the protoplasm generally contracts to a single 

 spherical or egg-shaped mass which lies sometimes 

 in the centre of the cell, sometimes at one end. With a low amplifica- 

 tion plasmolysed bacteria often look as though they had broken up into 

 a row of granules (Fig. 6, a), but careful examination under a high power 

 reveals the delicate membrane of the cell-wall still intact (Fig. 6, b). 

 Two facts of fundamental importance are brought to light by these pheno- 

 mena. They show in the first place that the protoplasm of the bacterial 

 cell is not, as is the case with the ' pellicula ' of infusoria, attached to the 

 cell-wall, but lies free within it just as does the protoplasm of plant-cells, 

 and furthermore that the osmotic pressure in the bacterial cell is com- 

 paratively low, only about half that of the cells of higher plants which need 

 solutions -twice as strong to plasmolyse them. The pressure inside the bac- 



'FlG. 6. Plasmolysis of bac- 

 teria, a, Vibrio choleras from 

 an agar culture (Bouillon + 1 % 

 peptone +1 % grape sugar) in 

 1-25 / NaCl. They are plasmo- 

 lysed but still living ; the proto- 

 plasm is broken up into refrin- 

 gentgranules. b, Tnesamemore 

 highly magnified. c,Vibriocho- 

 lerae plasmolysed with ciliuni. 

 </,Typhoid bacilli in x-$ % NaCl, 

 stained ; cell-contents in various 

 positions, to the right a cell with 

 the protoplasm arranged as -in 

 the plant cell (Fig.4, b). e,Sfiril- 

 lum undula plasmolysed by the 

 evaporation of stagnant water ; 

 the structure of the clumps of 

 protoplasm is well seen. Proto- 

 plasm black in all figures. 

 Magn. a 300, b~e 1500. 



