THE METABOLISM OF PLANTS. 143 



Pfeffer, Godlewski, and Holle, that when a green plant is 

 exposed to sunlight in a closed glass vessel containing air, 

 the volume of the air remains approximately constant. It 

 must be borne in mind, however, that the interchange of 

 gases going on between a green plant and the outside air 

 is not merely the absorption of carbon dioxide and the 

 evolution of oxygen. We have seen in a previous lecture 

 (P 75) tnat plants also absorb oxygen from the air and 

 give out carbon dioxide. The change in the constitution 

 of the atmosphere in a closed vessel in which a green plant 

 has been exposed to light, is then the resultant effect of the 

 operation of these two processes. The green parts of the 

 plant have absorbed carbon dioxide and given out oxygen, 

 the parts which are not green have absorbed oxygen and 

 given out carbon dioxide. The fact that the volume of the 

 oxygen exhaled is frequently slightly smaller than that of 

 the carbon dioxide absorbed, is doubtless to be attributed 

 either to the exhalation of a small quantity of carbon 

 dioxide by the parts of the plant which are not green, this 

 exhaled carbon dioxide not having been subsequently decom- 

 posed in the green parts, or to the retention of a small 

 quantity of oxygen in the plant in the form of highly oxidised 

 organic compounds. 



We accept, then, the fact upon which this view is based, 

 but we have yet to assure ourselves of the correctness of the 

 inference. Does the equality in the volumes of the gases 

 absorbed and exhaled warrant the conclusion that the starch 

 which appears in the chlorophyll-corpuscles is connected 

 with the gaseous interchange in the manner indicated by 

 the equation given above? It is easy to point out that a 

 gaseous interchange of this nature might accompany the for- 

 mation of a substance altogether different from starch ; for 

 instance, methyl or formic aldehyde might be formed accord- 

 ing to the equation 



This very suggestion has, however, been brought forward 

 in support of the view that the starch formed in chlorophyll- 

 corpuscles when exposed to light is derived from carbon 



