72 BOTANY. 



133, Absorption of carbon dioxide. We have next to 

 inquire where the oxygen comes from which is given off by 

 green plants when exposed to the sunlight, and also to learn 

 something more of the conditions necessary for the process. 

 We know that water which has been for some time exposed to 

 the air and soil, and has been agitated, like running water of 

 streams, or the water of springs, has mixed with it a consider- 

 able quantity of oxygen and carbon dioxide. 



Demonstration 24. 



134. To show the result in boiled water. Boil spring water or hydrant 

 water which comes from a stream containing oxygen and carbon dioxide, for 

 about 20 minutes, to drive off these gases. Set this aside where it will not 

 be agitated, until it has cooled sufficiently to receive plants without injury. 

 Now place some spirogyra or vaucheria, and elodea, or otjier green water 

 plant, in this boiled water and set the vessel in the bright sunlight under the 

 same conditions which were employed in the experiments for the evolution of 

 oxygen. No oxygen is given off. 



NOTE. It can be demonstrated that carbon dioxide is absorbed by the 

 plant while the oxygen is passing off. In the case of aquatic plants the 

 carbon dioxide is mixed with the water, while in the case of the land plants 

 the carbon dioxide comes from the air. In the study of respiration we shall 

 find that carbon dioxide is formed within the plant. Some of the carbon 

 dioxide then which plants use when they are giving off oxygen comes from 

 within the plant itself. For some simple experiments to demonstrate the 

 absorption of carbon dioxide during this process see paragraphs 119-124 of 

 the author's larger "Elementary Botany." 



135. A chemical change of the gas takes place within the 

 plant cell. Since oxygen is given off while carbon dioxide, a 

 different gas, is necessary, it would seem that a chemical change 

 takes place in the gases within the plant. Since the process 

 takes place in such simple plants as spirogyra as well as in the 

 more bulky and higher plants, it appears that the changes go on 

 within the cell, in fact within the protoplasm. We should 

 remember also that this chemical change of the gases in plants 

 Can only take place in the presence of light. 



