444 PLANTS AND THEIR RELATION TO MAN 



about 250 tons of water each spring and summer. Because 

 trees give off moisture, they are a great blessing to a com- 

 munity in a hot, dry summer ; they take water from the deep 

 underground soil which their long roots penetrate, and give it 

 out to the parched air through their leaves. 



Leaf pores or stomata are an extremely important part of a 

 plant. Through them carbon dioxide enters the plant, and 

 surplus oxygen and water escape from the plant. If these 

 pores become clogged, the plant is deprived of the carbon 

 which it requires for food making and is overburdened with 

 oxygen and water which it cannot use. The hedges along 

 dusty roadsides wither partly because their pores are clogged 

 with dirt. The trees along our city streets lose the fresh 

 green beauty in a dry summer, partly because smoke and dust 

 clog their pores. 



Plants need air. Everything which lives, whether it be 

 plant or animal, requires oxygen for life. But the amount of 

 oxygen which a plant takes from the air during respiration is 

 much less than the amount which the plant returns to the air 

 from the carbon dioxide. Thus plants keep the air around them 

 fresh and rich in oxygen. 



Public parks and city trees. Because green plants remove 

 carbon dioxide from the air and supply oxygen to it, they are 

 much needed in large cities where thousands of men are con- 

 stantly breathing out carbon dioxide and constantly requiring 

 oxygen. Parks and squares with grass, shrubs, and trees 

 should be thickly scattered throughout our large cities, and 

 thriving trees should line our sidewalks. The more abun- 

 dant our grass plots and trees, the purer is the air breathed by 

 the inhabitants, and the healthier are the lives of the citizens. 

 Parks and public squares are cared for by the community, but 

 the planting and care of sidewalk and garden trees are usually 

 left to the individual householder. The expense of planting a 



