LESSONS WITH PLANTS 



319 



starch in tubers, roots, root-stalks, and other parts as a 

 reserve supply. Seeds also contain reserve food, sometimes 

 in the form of starch as in grains, sometimes in the form of 

 oils as in many nuts, and in other forms. It is chiefly in 

 this form that man utilizes the food 

 obtained from plants. 



As a by-product in starch-making, 

 plants give off oxygen gas. They 

 absorb, for their food the carbon di- 

 oxide which is harmful to animals, 

 and return to the air the oxygen which 

 the latter need. Animals, on the other 

 hand, in breathing give off carbon 

 dioxide. 



The oxygen given off by plants 

 during starch-making may be shown 

 by the following pretty experiment : 



CoUect a mass of green pond scum 

 or other water plants. Place in an 

 aquarium in a sunny window. Set a 

 glass funnel over the plants in the water, and over the tube of 

 the funnel slip an inverted test-tube filled with water. Let 

 stand several days and note that the bubbles of gas given off 

 from the green plants rise into the test-tube. After a consid- 

 erable quantity has collected, the tube may be removed care- 

 fully, closing the opening with the thumb to prevent the 

 escape of the gas. Then with a glowing taper the gas may 

 be tested. If the taper bursts into a flame when it is thrust 

 into the test-tube the gas is oxygen. 



The above experiments on the function of a leaf may be 

 performed in any of the grammar grades; even in the sixth 



Fig. 121. Giving off Oxygen. 



