378 



ELEMENTARY BIOLOGY 



416. The forest and the air. Another use of the forest is 

 found in the fact that through photosynthesis fresh supplies of 

 oxygen are thrown into the air, replacing the carbon dioxid. 

 In addition to this, the transpiration may be considered a 



help in that it keeps down 

 the temperature of the 

 plants and so of the sur- 

 rounding air. The shade 

 value of trees is highly 

 appreciated in the sum- 

 mer time even by city 

 dwellers, and the effect of 

 trees in breaking the wind 

 is appreciated in the winter 

 time, especially by those 

 living in the country. 



417. The forest and 

 water. The most impor- 

 tant relation of the forest 

 to man, aside from the 

 direct utility of the forest 

 products, is in its effects 

 upon water. When we 

 compare the action of rain 

 water and snow on a hill- 

 side covered with trees 

 with the action upon a 

 similar hillside devoid of 



vegetation, we can realize the practical importance of the forest 

 in relation to our water supply. On the bare hillside the water 

 soaks down into the soil almost as fast as it falls, or it runs 

 off, carrying particles of earth along in its course. On the 

 covered hillside the force of the falling raindrops is broken 

 by the leaves of the trees, from which the water slides down 

 to the ground along the twigs and larger stems. The rain 



Covered Bare 

 April 2-9 



Covered Bare 

 April 14-17 



Covered Bare 

 April 22-24 



FIG. 197. The relation of the forest to 

 water flow 



In experiments made by government agents a 

 comparison of a covered area with one devoid 

 of trees showed (i) that in a given period the 

 covered area accumulated more snow than the 

 bare area (this is shown by the relative heights 

 of the two columns in each pair), and (2) that 

 in a given period the bare area lost more water 

 than the covered area (this is shown by the rela- 

 tive heights of the shaded portions in each pair) 



