THE FOOD OF THE PLANT 



57 



glass jar containing water. The tumbler serves as a 

 support for an inverted funnel which is now placed 

 over it. The end of the stem of the funnel must be 

 below the surface of the water. A test-tube full of 

 water is inverted over the end of the 

 funnel stem and kept in position by 

 a clamp (Fig. 28). 



Observations. — Bubbles are given 

 off from the pieces of Elodea, especi- 

 ally from the cut ends. The bubbles 

 rise through the water and gradually 

 displace the water in the test-tube. 



When enough gas has collected it 

 may be tested with a glowing splint 

 and proved to be oxygen. 



Inference. — When a plant is ab- 

 sorbing carbon-dioxide it gives out 

 oxygen. 



It can easily be shown that oxygen 

 is given out only during the absorp- Fig. 28 



tion of carbon-dioxide. If the condi- 

 tions are such as to render the latter process impossible 

 no bubbles will be given oif. This can be proved by 

 placing the apparatus in the dark, by using parts of 

 the plant that are not green, or by placing the plants 

 in distilled water that contains no carbon-dioxide. 



Thus the amount of carbon-dioxide that is being 

 absorbed can be roughly estimated by watching the 

 rate at which the oxygen bubbles are given off from a 

 green, submerged plant, such as Elodea canadensis. 

 In the warm, full sunlight there is a constant stream of 

 bubbles ; in subdued light they come off more slowly ; 

 in the dark, they altogether cease. 



Conversion of Starch into Sugar. — It may, at first, 

 be somewhat puzzling to find that starch can only be 

 formed in the presence of chlorophyll, since, as everyone 

 knows, there is a large quantity of starch in a potato, 

 a broad-bean, and in many other parts of plants that 



