THE METABOLISM OF PLANTS. 26 1 



more active in light than in darkness. He ascertained, 

 namely, that when a branch with green leaves is placed in 

 the dark after having been previously exposed to light, the 

 exhalation of carbon dioxide rapidly diminishes until it 

 becomes very inconsiderable, and it then remains approxi- 

 mately constant. After a short re-exposure to light, the 

 activity of the exhalation of carbon dioxide by the branch 

 is perceptibly increased. It might be concluded from this 

 that light affects the destructive metabolism of plants and 

 parts of plants which do not possess chlorophyll otherwise 

 than it affects that of plants and parts of plants which do 

 possess chlorophyll. But such a difference does not really 

 exist : the apparent paradox is susceptible of a simple ex- 

 planation. In a branch, such as those with which Borodin 

 experimented, there is no considerable deposit of reserve- 

 material ; destructive metabolism is carried on at the expense 

 of the organic substances which are being formed in the 

 leaves ; hence the activity of the destructive metabolism is 

 determined by the activity of the constructive metabolism : 

 in the dark the latter cannot be carried on, and hence the 

 activity of the former, as indicated by the evolution of carbon 

 dioxide, must soon diminish. 



We learn, then, that light has very little effect upon the re- 

 spiration of plants ; it appears to be slightly favourable to the 

 absorption of oxygen, but it has no direct effect in promoting 

 the exhalation of carbon dioxide. We may. go on to say 

 that the destructive metabolism of plants, as a whole, is not 

 materially affected by light, though, as we shall shortly see, 

 light has some influence upon certain of the processes of 

 destructive metabolism. 



We may digress for a moment to consider the bearing of 

 the conclusion to which we have just come upon Pringsheim's 

 theory of the function of chlorophyll (p. 157). Pringsheim is 

 of opinion that light promotes the respiration, in other words, 

 the destructive metabolism of plants, and he believes that the 

 function of chlorophyll is to absorb those rays which are most 

 active in promoting destructive metabolism, and thus to 

 render possible the performance of the various constructive 



