68 BACTERIA AND SOIL FERTILITY 



fate from the point of high concentration to that of low is 

 known as osmotic pressure. By appropriate means this pressure 

 can be measured, and it is found to be dependent upon the num- 

 ber of particles of the substance in solution. We have found that 

 a cell is covered by a membrane which is readily permeable to 

 water but only slowly to salts. Furthermore, w^e have found that 

 the concentration of protoplasm in this cell is about the same as a 

 0.9 per cent solution of common salt. A cell dropped into such 

 a solution neither loses nor gains in weight, but if it is dropped 

 into a more concentrated salt solution it shrinks, and a careful 

 microscopic examination of it shows that the inner ectoplasm has 

 been torn from the cell wall. This we call plasmolysis. The 

 salt in the solution bathing the cell is more concentrated than that 

 on the inside. The salt molecules cannot get out to equalize 

 pressure; hence, the water passes in to dilute the solution. The 

 plasmolysis of a cell may cause its death. Therefore, we find 

 that the placing of bacteria into strong salt or sugar solutions 

 causes their death. Advantage is taken of this in the making of 

 jellies, syrups, and preserves. Moreover, it is one of the factors 

 entering in alkali soil. We find that when the osmotic pressure 

 of the soil reaches about 8 atmospheres the nitrifiers are depressed 

 in their activities, but are not entirely stopped until the pressure 

 reaches from 15 to 20 atmospheres. The ammonifiers are inter- 

 fered with when the osmotic pressure reaches 9 atmospheres, but 

 they are not entirely stopped until it reaches from 25 to 30 atmos- 

 pheres. The nitrogen-fixing organisms are even more resistant to 

 osmotic changes than are the ammonifiers. 



Pressure. — Bacteria are extremely resistant to great pressure, 

 as they are found at great depths in the ocean where the pressure 

 would be tremendous. However, successful attempts have been 

 made to preserve fruit and vegetables by exposing them to high 

 pressure. Apple juice subjected to from 4,000 to 8,000 atmos- 

 pheres' pressure for thirty minutes did not develop gas. Peaches 

 and pears exposed to this pressure did not spoil during five years. 

 Those vegetables on which are found resistant spores could not be 

 preserved by such pressure. 



