6 INTRODUCTION : MORPHOLOGY 



it is easy to foresee what will happen. The pressure on the outside of the 

 protoplasmic sac being now greater than that within, the protoplasm is 

 pressed inward away from the cell-wall. The sac shrinks until the concentra- 

 tion of the cell-sap is such that the osmotic pressure it exerts is just equal to 

 that of the fluid outside. Since the shrinking of the protoplasm commences 

 directly the pressure of the surrounding medium exceeds that of the cell- 

 sap, we have in this phenomenon, which is known as Plasmolysis, a means 

 of measuring the pressure within the cell. The gist of the whole matter 

 lies in the fact that the protoplasm is easily permeable to water both within 

 and without, but presents a more or less impassable barrier to the molecules 

 of the osmotic substances. As a result, the cells may lie for a long time in 

 the plasmolysing solution without returning to their original state. If they 

 be removed from the solution and placed in pure water again, the pressure 

 of the molecules inside the protoplasm immediately reasserts itself, and the 

 sac dilates until the original condition of turgescence is restored. All these 

 phenomena are only observable in the living cell. At death the protoplasm 

 loses its impermeability, and presents no hindrance to the molecules of 

 substances in solution. In spherical cells the protoplasm is contracted by 

 plasmolysis to a ball, but in long cylindrical cells, such as hairs for instance 

 (Fig. 4), or algal cells, the elongated sac generally becomes constricted 

 into two or three portions. These are at first connected by strands of 

 protoplasm (Fig. 4, #),'but finally separate and form distinct masses. The 

 commonest case is where two round or oval lumps of protoplasm are left, 

 one at each end of the cell (Fig. 4, c). When, by transference to pure water, 

 the process is arrested and reversed, these masses melt together again 

 as soon as they touch and continuity is restored. This de-plasmolysation 

 must not be too rapidly performed, or the fragments of the protoplasmic 

 sac may burst, in which case the protoplasm invariably dies. 



The application of plasmolysis to the study of bacterial cell-structure 

 may now be discussed, but it will be advisable first to consider the results 

 which other methods of investigation have yielded, notably those of fixing 

 and staining. If we examine bacteria under a very high power objective 

 (2,000 diams. and over) very little can be seen. The cell has a fairly sharp 

 outline, but it is not possible to distinguish a separate membrane enclosing 

 the cell-contents. The latter appears as a pale homogeneous mass with 

 occasional granules of stronger refringency, and in the case of some of the 

 larger bacteria (Spirillum, Cladothrix) sap-vacuoles differentiate themselves 

 from the protoplasm by their watery appearance. These meagre details 

 are all that can be seen in the fresh unprepared state, — of the organs of 

 locomotion nothing can be seen, — but by the customary methods of fixing and 

 staining a number of other particulars can be made visible. If a minute 

 speck of a pure culture be mixed with a fixing fluid (e.g. osmic or chromic 

 acid), spread upon a cover-glass, and allowed to dry, the bacteria adhere 



