46 ANATOMY AND PHYSIOLOGY 



C. The Exchange of Gases in Photosynthesis: 



Experiment 28. To demonstrate the evolution of gas 

 in photosynthesis. 



1. Observe uninjured branches of Elodea growing in 

 water in direct sunlight. (For individual experi- 

 ments one or two branches in a large test-tube of 

 tap- water will serve.) Describe what you observe, 

 coming from the basal ends, or other parts of the 

 stems. 



2. Shade the plants for a moment by interposing a 

 note-book or other convenient screen between them 

 and the sun. Describe how the process just ob- 

 served is affected. 



3. Make a diagram of the apparatus and material, 

 showing what you have observed. 



4. Observe the bubbles among a mass of any green 

 alga floating in water, and explain their presence. 



Experiment 29. To demonstrate what gas is given off 

 in photosynthesis. 



5. With a rubber band, or other convenient means, 

 fasten together (not too tightly) the cut ends of 10 

 or 15 clean branches of Elodea, and place them into 

 a glass funnel, with the cut ends extending upward. 

 Invert the funnel into a jar of water. The surface 

 of the water should rise an inch or two above the 

 neck of the funnel. 



6. Fill a test-tube with water and invert it over the 

 neck of the funnel, being careful that no air enters 

 the tube. 



7. Place the apparatus in bright sunlight, and when 

 sufficient gas has been collected in the test-tube, 

 test it with a glowing splinter. How is the splinter 

 affected by the gas? What gas does this test indi- 

 cate? The best success of this experiment requires 



