DIVISION II 

 Physical Influences 



CHAPTER I 

 MOISTURE 



Moisture may be called the most important factor of life. Not 

 only bacteria, but every microscopic and macroscopic being requires a 

 considerable amount of moisture. Living organisms contain on the 

 average between 70 per cent and 90 per cent of water, and only 10 per 

 cent to 30 per cent of solid matter. Microorganisms which live 

 entirely submerged in liquids need water not only within but without 

 the cells. Bacteria, yeasts, molds, and some protozoa obtain their food 

 by diffusion through the cell-membrane; their food-substances must 

 be soluble and dissolved. No other liquid can take the place of water. 



The amount of water required by microorganisms cannot be stated 

 briefly. Several factors have to be taken into consideration, as the 

 osmotic pressure, the insoluble and the colloidal substances, the species 

 of organisms, temperature, and perhaps others. 



Osmotic Peessuke. — In the organic world we find very commonly 

 membranes which will allow water to pass through but retain some 

 compounds dissolved in the water. Such so-called semi-permeable 

 membranes are found surrounding the protoplasm of cells. They are 

 not the cell waU, but separate the protoplasm from the cell wall. 

 Siinilar properties are found in parchment paper, pig's bladder, and 

 other organic membranes. 



If a salt solution is poured in water, the two Uquids will mix in a 

 short time and soon every smallest portion of the mixture will have the 

 same concentration. If a salt solution and water are separated by a 

 membrane which does not allow the salt to pass, the water will go 

 through the membrane toward the salt with a certain amount of 

 pressure. This pressure depends upon the nature of- the dissolved 

 substance as well as upon its concentration. 



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