340 



ELEMENTARY BIOLOGY 



that it is the excess of air that interferes with life there ; no light is 

 available in such places. If we consider artificial conditions produced 

 by the digging of mines or the use of caissons under water, it is 

 indeed true that these conditions interfere with normal life processes ; 

 but they do this not because there is too much air, but because the 



Fig. 159. The wind as an obstacle to life 



The wind, often helpful to life and growth, is sometimes a hindrance. In the picture the 

 wind, besides making the tree grow one-sided, and bending over the top branches, has 

 blown the earth away from the roots. (Photograph lent by New York Botanical Garden) 



human beings that go into these places are not adjusted to the high 

 pressure} Nor is there any place on earth where there is naturally a 

 scarcity of air, except on the very highest mountain tops ; but in 

 these situations other conditions are sufficiently unfavorable to life, 

 so that we do not usually think of the absence of plants and animals 

 in these places as due to the lack of oxygen. 



1 The distressing disease known as '" the bends," which affects many of 

 those who have to work in the high-pressure atmosphere of the caissons, is 

 very easily avoided by taking sufficient time to enter the working chamber 

 and sufficient time to come out. The disease is not caused by the high pres- 

 sure ; it is caused by the sudden change from high pressure to the normal 

 pressure of the surface atmosphere. 



