338 ELEMENTARY BIOLOGY 



killed. When cooled a few degrees lower, the fish were killed. Frogs 

 have been frozen to a temperature of — 28° C. (18.4° F.) without 

 being killed. Some of the animals without backbones regularly survixe 

 even colder temperatures. Many insects that survive the winter in the 

 adult stage have to be frozen and then thawed out again, although 

 many of them no doubt escape freezing by burrowing into the ground. 



In our experience with winter weather many of us have no 

 doubt frozen an ear or a finger. That did not kill us, but it 

 may have killed some of the cells in the affected part. When 

 frozen cells are thawed out rapidly, they are liable to burst 

 and thus be killed, but with a slow thawing the life of the 

 cells may be saved. That is why a frost-bitten ear is rubbed 

 with snow, to prevent it from warming too rapidly. 



At the other end of the temperature scale some of the 

 simplest animals (ameba) have been found to survive a tem- 

 perature of 122° F. when slowly heated. But most of them 

 died at that temperature. This does not mean that the animals 

 were unaffected in the gradually heated water until they were 

 killed. Long before this temperature is reached- — at about the 

 temperature of our blood — they ceased active motion. 



391. Water and life. We have learned that water is an 

 intimate part of the cell contents, and we can realize that life 

 is impossible without it. Yet the amount of water available for 

 plants and animals is constantly changing (except in the oceans 

 and larger lakes and rivers), so that at one time there is drought, 

 — at least relatively speaking, — whereas at other times there 

 is an injurious excess of moisture. Ponds and brooks dry up ; 

 and, so far as the availability of water is concerned, the same 

 condition arises when the water freezes. The soil dries, or it 

 freezes, and the water supply is thus cut off from the countless 

 plants that inhabit the earth. We know that a dry spring or 

 summer may mean a famine, and that some parts of the earth's 

 surface are quite uninhabitable because of the scarcity of water. 



392. Light and life. We have learned that light is essential to 

 the manufacture of organic food, that it is the ultimate source of 



