GREEN PLANTS GIVE OFF OXYGEN 



85 



for animals as well ; and the food is stored in such a stable form 

 that it can be kept and sent to all parts of the world. 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 consider that in 1924 in the 

 United States the total value of all farm crops was more than 

 $12,000,000,000, and when we realize that these products came 

 from the air and soil through the energy of the sun, we may realize 

 why the study of plant biology is of great 

 importance. 



Green Plants give off Oxygen in Sun- 

 light. — In still another way green plants 

 are of direct use to animal life. During the 

 process of starch making, oxygen is given 

 off as a by-product. This may easily be 

 proved by the following experiment. Place 

 any green water plant in a battery jar 

 partly filled with water,i 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 may be 

 obtained by displacement of the water to 

 prove, by the rapid oxidation of a glowing 

 splinter of wood in the gas, that oxygen is 

 present. 



That oxygen is given off as a by-product 

 by green plants is a fact of far-reaching 

 importance. 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. 



Experiment to show that 

 oxygen is given off by green 

 plants in sunlight. 



^ Water contains air in solution, including some carbon dioxide, but the amount 

 may be too small. Immediate success with this experiment will be obtained if the 

 water has been previously charged with carbon dioxide. 



H. NEW CIV. BIOL. — 7 



