98 GROWTH AND WORK OF PLANTS 



163. Need of the air relation in this work of leaves. 



The air relation is also necessary in the formation of sugar and 

 starch as well as in respiration. There is an interchange of 

 gases in the plant during the process, and if the leaf is deprived 

 of one of the necessary gases the formation of sugar and starch 

 ceases. The gas which the plant absorbs during this process is 

 carbon dioxide (one part of carbon and two parts of oxygen in 

 the molecule of carbon dioxide = CO 2 ). The air consists of 

 about twenty-one parts of oxygen, about seventy-nine parts of 

 nitrogen and a small fraction of one part of CO 2 . A small per- 

 centage of CO 2 is sufficient since during respiration of animals 

 and plants, and by the burning of combustible material, CO 2 is 

 constantly added to the air. More than about 4 per cent of CO 2 

 in the air is harmful to most plants. While CO 2 is absorbed by 

 plants during this process oxygen is given off. 



164. To show the evolution of oxygen by green plants. 

 For a simple demonstration of the oxygen given off by green 

 plants, water plants are more suitable than land plants, since 

 the gas bubbles can easily be seen as they rise in the water. 

 Nevertheless with proper apparatus and methods of measuring 

 and determining gases it can be demonstrated that the same gas 

 (oxygen) is given off by green plants in the air. A few sprigs of 

 the water weed, Elodea ( = Philotria), with freshly cut stems 

 about 10 cm. (4 inches) long, are inverted and immersed in a 

 bottle of spring or tap water. The bottle is placed in the sunlight. 

 Bubbles of gas will soon begin to rise quite rapidly from the 

 cut ends of the stem. If the bottle is moved into the shade for 

 a moment, the bubbles are given off very slowly. When moved 

 into the sunlight again, the bubbles immediately begin to rise at 

 a rapid rate. In the same way when pond scum (threads of the 

 alga, Spirogyra, for example) are placed in a vessel of spring or 

 tap water in the light, the bubbles of gas are given off. This is 

 interesting as indicating, what is a fact, that this same process 

 takes place in all green plants which have chlorophyll. 



165. To determine that this gas is oxygen. A large quan- 

 tity of fresh Elodea is placed in a large jar of tap water. A 



