PLANTS MAKE FOOD 



95 



the food is stored in such a stable form that it may be sent to all 

 parts of the world in the form of grain or other fruits. Animals, 

 herbivorous and flesh-eating, man himself, all are dependent upon 

 the starch-making processes of the green plant for the ultimate 

 source of their food. When we remember that in 1913 in the 

 United States the total value of all farm crops was over 

 $6,000,000,000, and when we realize that these products came from 

 the air and soil through the energy of the sun, we may begin to 

 realize why as city boys and girls the study 

 of plant biology is of importance to us. 



Green Plants give off Oxygen in Sun- 

 light. - - In still another way green plants 

 are of direct use to us in the city. Dur- 

 ing this process of starch-making oxygen 

 is given off as a by-product. This may 

 easily be proven by the following experi- 

 ment. 1 Place any green water plant in a 

 battery jar partly filled with water, cover 

 the plants with a glass funnel and mount 

 a test tube full of water over the mouth of 

 the funnel. Then place the apparatus in a 

 warm sunny window. Bubbles of gas are 

 seen to rise from the plant. After two or 

 three hours of hot sun, enough of the gas 

 can be obtained by displacement of the 

 water to make the oxygen test. 



That oxygen is given off as a by-product 

 by green plants is a fact of far-reaching Experiment 

 importance. City parks are true " breath- 

 ing spaces." The green covering of the 

 earth is giving to animals an element that 



they must have, while the animals in their turn are supplying to 

 the plants carbon dioxide, a compound used in food-making. 

 Thus a widespread relation of mutual helpfulness exists between 

 plants and animals. 



1 Immediate success with this experiment will be obtained if the water has been 

 previously charged with carbon dioxide. 



to show that 

 oxygen is given off by 

 green plants in the sun- 

 light. 



