214 



PHYSICAL INFLUENCES 



the polar sea and of a lagoon near the equator will very probably 

 have different optimum temperatures because of the acclimatization 

 and selection which has been taking place for centuries. 



The great majority of bacteria and related organisms, in fact of all 

 living organisms, except in a few instances, has its optimum tem- 

 perature between 20° and 40°. The optimum temperature of an 

 organism is generally somewhat higher than the average temperature 

 of its natural habitat. 



The following table shows the data obtained for a few microor- 

 ganisms. 



The minimum temperature or the lowest hmit of growth is usually 

 farther from the optimum than the maximum temperature. It will 

 vary with the organisms just as do the other cardinal points. But 

 there is a natural limit drawn by the freezing-point of the nutrient 

 liquid. Not all organisms can grow at such low temperatures, in fact 

 the greater number does not develop below 6° to 10°. Those that can 

 grow at the freezing-point will be inhibited by the solidification of the 

 water in the nutrient medium, for if the water is frozen, food cannot 

 diffuse into the cells and therefore, all life processes are checked. If 

 freezing is prevented by adding salts or other soluble substances which 

 , lower the freezing-point, growth may continue even below 0°. Milk 

 freezes at about — 0.5°. Bacteria are found to multiply in it as long as 

 it is not entirely solid. A certain yeast multiplied slowly in salted but- 

 ter kept at about — 6°. 



