212 A YEAR IN SCIENCE 



The amount of available water varies from a very small 

 supply in deserts to the abundant supply in swamps and 

 lakes. The character of the soil also has a very 

 important effect on the water supply. Some soils retain 

 water and others do not. The structure of the plants 

 varies according to the water supply. Plants living 

 under water differ greatly from those living on land. 



Permission of Chicago, Milwaukee and St. Paul Railroad. 

 Fig. 84. Type of vegetation found on a mountain slope. 



Those which are so constructed that they lose a great 

 amount of water can live only- under conditions where 

 the supply of water is equal to the loss. In some 

 regions plants must store water to tide them over long- 

 periods of drought. This is especially true of desert 

 plants, some of which, such as cactuses, when cut open, 

 give out enough water for men and other animals to 



