MINUTE STRUCTURE OF THE BACTERIAL CELL 55 



metachromatic and polar granules, which will be referred to 

 later. 



There is a great difference in the way the various species of 

 bacteria are affected by solutions of higher and lower density, 

 or osmotic pressure. There is without doubt a great difference 

 chemically and anatomically between the protoplasm of 

 different species of bacteria, and this accounts for the inabil- 

 ity of some species to plasmolyze readily. There is a difference 

 in the permeability of the ectoplasm to the solvents. The 

 ectoplasm in some few species does not prevent the diffusion 

 in and out of the cell of certain particular solutes, while in 

 other bacteria the same solute is unable to pass through the 

 ectoplasmic layer. This is what constitutes a semipermeable 

 membrane, a membrane permeable to solvents but not to 

 solutes. Old bacteria plasmolyze more easily than young 

 bacteria, and this is probably due to the former having a 

 thicker layer of protoplasm or semipermeable membrane. 



The cell sap contains solutes derived from external fluids 

 as well as some generated within. The presence of these so- 

 lutes within produces a great osmotic pressure against the 

 protoplasmic layer and the cell wall, and there is a constant 

 tendency for the cells to become distended, due to the passage 

 into the cell of solvents, such as water, from the exterior. The 

 pressure inside a bacterial cell may equal two or three atmos- 

 pheres. It is due to the rigidity of the cell wall that the 

 bacterial cell is able to withstand an internal pressure which 

 would easily rupture the protoplasmic layer which really 



