64 PHYSIOLOGY. 



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 or but little is given off. There 

 must be then some other requisite for the evolution of the oxygen. 



132. The gases are interchanged in the plants. We will now 

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

 by leading CO 2 from a gas generator into the water. Broken 

 bits of marble are placed in the generator, acted upon by hydro- 

 chloric acid, and the gas is led over by glass tubing. 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. 



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

 plant cell. This leads us to believe then that CO 2 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. 



134. 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. 



^2. Where Starch is Formed. 



We have found by these simple experiments that some 

 chemical change takes place within the protoplasm of the green 

 cells of plants during the absorption of carbon dioxide and the 

 giving off of oxygen. We should examine some of the green 

 parts of those plants used in the experiments, or if they are not 



