PHOTOSYNTHESIS: GENERAL 25 



laboratory and breathed into the apparatus 'in order/ he explained, 

 Ho be certain that you were not taking an x for a y and could really de- 

 termine such small amounts of carbon dioxide.' " 



Analysis of Photosynthesis. — To facilitate our understanding 

 of this most important process, the subject of photosynthesis will 

 be analyzed and discussed under the following heads: 



a. Methods of demonstrating. 



b. The gaseous exchange — taking in of carbon dioxide. 



c. Absorption of energy as light. 



d. The use of the energy to synthesize elementary organic compounds. 



e. The condensation of the synthate into more complex substances. 



f . The gaseous exchange — the elimination of oxygen. 



Only a and b will be discussed in this chapter, the others being 

 reserved for the following chapters. 



Demonstrations. — To demonstrate photosynthesis it is a 

 simple matter to compare the dry weight of a plant in the morning 

 and the evening. In the morning cut a hundred disks from the 

 leaves of a healthy plant. Let the plant remain in the sunlight 

 all day, and in the evening remove a hundred more disks from the 

 same leaves. Dry in the oven and compare the weights of the 

 two sets of disks. It will be found that those cut in the evening 

 are somewhat heavier than those removed in the morning due 

 to the weight of the food manufactured during the day. 



An experiment similar to Boussingault's can be performed. The 

 plant may be placed in a closed chamber with removable glass 

 tubes attached so that a sample of the air can be easily taken for 

 analysis at convenient intervals. If analyses are made in the morn- 

 ing and evening it will be found that during the day the carbon 

 dioxide has diminished in amount and the oxygen has increased. 



To show the evolution of oxygen, Elodea or other water plants 

 may be used. Invert a funnel over the mass of plants and cover 

 the end with an inverted test tube filled with water. If placed 

 in the sunlight, bubbles of gas will be seen escaping from the 

 plants and rising into the test tube where they collect and dis- 

 place the water. When full of the gas, the test tube may be re- 

 moved and the gas tested by a glowing splinter or twine. It 

 will be found to support combustion and give all the tests for 

 oxygen. 



That carbon dioxide is removed from water by hydrophytes 

 may be shown very nicely by placing in the water with the plants 

 a little indicator such as phenolphthalein. As the carbon dioxide 



