56 MORPHOLOGY 



determines the osmotic pressure. Osmotic pressure, it 

 may be said in passing, depends upon the amount of crys- 

 talloids (solutes) in solution. Pressure is produced by 

 molecules and ions pounding against the semipermeable mem- 

 brane. There is a great difference in the way in which various 

 substances diffuse through this or any semipermeable mem- 

 brane. The osmotic pressure will be directly proportionate 

 to the number of molecules of nonelectrolytes (sugars and 

 other carbohydrates, urea, etc.). In the case of the elec- 

 trolytes (salts) the ions in solution produce the osmotic 

 pressure. 



If bacteria are accustomed gradually to the solutions of 

 high osmotic pressure (hypertonic solutions) by passage 

 through fluids of increasing density, there will be no plas- 

 molysis. This shows that the cell is so constituted that the 

 solutes such as salts and crystaloidal nonelectrolytes diffuse 

 slowly through the ectoplasm and become incorporated in the 

 cell sap or entoplasm. The ectoplasm is easily permeated by 

 solvents and in time is permeable to certain solutes. 



It is possible to change the contents of a bacterial cell by 

 placing it in a hypotonic solution, that is, a solution of less 

 density and consequently lower osmotic pressure than the 

 cell sap. The classical experiment is to place bacteria which 

 have been accustomed to growing on or in culture media con- 

 taining salts into a solution of distilled water. The distilled 

 water passes readily through the cell wall and the semiperme- 

 able ectoplasm. The cell increases in size consequent to the 



