-^ 



can be distinguished in the bacteria. The parts arc : the cell -wall (if, Fig- 4, 

 rt), the protoplasm (/>, Fig. 4, a], the nucleus (k, Fig. 4, ), and the cell-sap 

 (.r, Fig. 4, a). The cell-sap is contained cither in spaces (vacuolcs) scattered 

 through the protoplasm, or in a large central cavity that fills out the greater 

 part of the cell, the protoplasm being reduced to a thin layer coating the 

 cell-wall (primordial utricle}. Now the relation of this protoplasmic lining of 

 the cell to the cell-wall is of great importance in the study of the structure 

 of bacterial cells, and it will be necessary to discuss in some detail the 

 conditions present in the case of plant-cells in general. It will then be seen 

 how far the observable facts are competent to elucidate the finer structure 

 of bacteria. The cell-sap of vegetable cells consists of water in which are 

 dissolved various substances, organic compounds and mineral salts. The 

 protoplasm, which forms a sac enclosing 

 the cell-sap, is during life endowed with 

 properties similar to those of animal 

 membranes, being readily permeable to 

 pure water, but almost or quite imper- 

 meable to the substances held in solution 

 by the cell-sap. As a result, these bodies 

 exert a strong pressure (osmotic pressure] 

 from within outwards upon the proto- 

 plasmic sac, stretching it and pressing 

 it tightly against the comparatively un- 



. 

 yielding Cell-Wall. An explanation Of 



this is afforded by the modern Theory 



* 



Of Solutions which shoWS that the mole- 



CUleS Of a substance dissolved in water 



2-5 / INat-l ; the contents form two separate 



act as though they were in a gaseous spherical masses. Magn. 3 ). 

 state and strive to fly outwards a\vay 



from each other. Consequently, if the membrane is in the form of a closed 

 sac surrounded by pure water and containing a solution, the substance 

 dissolved behaves as a gas, and its molecules, like those of hot air in 

 a bladder, expand and dilate the sac until the pressure inside is equal 

 to that without. In the case of the protoplasmic sac of plant-cells this 

 condition of equilibrium is unattainable, because unlimited dilatation is 

 prevented by the cell-wall, and the result is that the whole of the cell 

 is during life in a permanent condition of strain (turgor}. It will be 

 observed that osmotic pressure can only originate when there is water on 

 both sides of the membrane, and this condition is present not only in water- 

 plants, but also in terrestrial vegetation, inasmuch as the cell- walls are always 

 saturated with water. If we now reverse the conditions and, instead of 

 placing the cell in pure water, lay it in a solution of greater osmotic pressure 

 than that of the cell-sap (5 per cert, saltpetre or 2-5 per cent, common salt) 



FIG. 4. riasmoiysis O f a ceil from a fine hair 



of Ecballiitni elateriiim. a, distribution of the 

 cHl-rontents in natural state Amounted in water); 

 HI, cell-wall; />, protoplasm (primordial utricle); j, 

 cell-sap in central vacnole;*, nucleus. ,in2-5% 

 common salt solution, medium degree of plasmo- 

 b s i s ; th , e .protoplasm has retreated from the cell- 

 wall and is being constricted into two parts, c, 

 after lying about half an hour in the 



