THE STAGE SETTING ., 



The uses to which water is put by an organism are nianit'old. It 

 is necessary as a solvent for foods within the body. In HvIiik tissues 

 it becomes a medium of exchange between different parts of tlio body, 

 while in higher animals it carries off body heat, thus helping in tiic 

 regulation of their temperature. In air it causes humidity. In soil 

 it carries the raw food materials of green plants. In many alkali 

 lakes, such as Great Salt Lake, fish life is practically absent and the 

 numbers of insects and crustaceans inhabiting such water are greatly 

 reduced because of the high mineral content of the water. On the 

 other hand certain crustaceans, such as the brine shrimps, are only 

 found in water containing a high concentration of salts. Acid lakes 

 and streams contain only certain types of fish, and according to in- 

 vestigation by Jewell ^ are lacking in snails, possibly because of the 

 absence of lime from which snails build their shells. 



Temperature 



Differences in climate (which after all are largely differences in 

 temperature and water supply) are accompanied by changes in the 

 appearance and kinds, of plants and animals. The life processes of 

 organisms proceed between certain maximum and minimum limits 

 of temperature. Somewhere between these is an optimum temi^era- 

 ture at which the life processes function most normally. In i)lants 

 optimum temperatures vary greatly for different species, and are 

 largely instrumental in determining what plants will grow in a gi\-cn 

 locality. For example, apple-raising regions must have a mean 

 summer temperature of not more than 70° F. The optimum of 

 most tropical plants ranges over 90° F., while alpine species require 

 a temperature slightly above freezing. The temperature of plants 

 changes rapidly, depending on the amount of external heat they re- 

 ceive. This has an important bearing on horticulture. Lemons on 

 the trees, for example, freeze at a temperature of 28° F., and oranges 

 at 26° F. They are often kept from freezing by means of heaters. 

 Plant injuries caused by freezing are due to the rapid withdrawal of 

 water from the soft parts, therefore plants with a high water con- 

 tent are more easily injured. This accounts for the freezing of the 

 young tips of trees. Seeds which have a small water conteut are 

 capable of withstanding very low temi)eratures. 



In animals, as in plants, the lif(> processes proceed best at oi)timuiii 

 temperatures which differ with the species. Mast p rotozo a divide 



1 Jewell, •• The Fishes of an Acid Lake." Tran.. Amer. M \ol. XLIII, 1924. pp. 77-84. 



