52 PHYSIOLOGY. 



minutes, these gases are driven off. We should set this aside 

 where it will not be agitated, until it has cooled sufficiently to 

 receive plants without injury. Let us now place some spirogyra 

 or vaucheria, and elodea, or other 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. 



121. Can it be that this is because the oxygen was driven 

 from the water in boiling? We shall see. Let us take the vessel 

 containing the water, or some other boiled water, and agitate it 

 so that the air will be thoroughly mixed with it. In this way 

 oxygen is again mixed with the water. Now place the plant 

 again in the water, set in the sunlight, and in several minutes 

 observe the result. No oxygen is given off. There must be 

 then some other requisite for the evolution of the oxygen. 



122. The gases are interchanged in the plants. — We will 

 now introduce carbon dioxide again in the water. This can be 

 done by blowing into the water through a glass tube in such a 

 manner as to violently agitate the vvater for some time, when the 

 carbon dioxide from the "breath" will become mixed with the 

 water. Now if we place the plant in the water and set the vessel 

 in the sunlight, in a few minutes the oxygen is given off rapidly. 



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

 plant cell. — This leads us to believe then that CO^ is in some 

 way necessary for the plant in this process. 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. 



124. Gases as well as water can diffuse through the proto- 

 plasmic membrane. — Carbon dioxide then is absorbed by the 

 plant while oxygen is given off. We see therefore that gases as 

 well as water can diffuse through the protoplasmic membrane of 

 plants under certain conditions. 



