LECTURE XIV. 



METABOLISM (continued}. 



9. The Expenditure of Energy. 



IN entering upon the subject of the expenditure of energy 

 by the plant we must bear in mind that it is with kinetic 

 energy that we are directly concerned, and we must therefore 

 begin by ascertaining what are the sources from which kinetic 

 energy is obtained. We have already learned that the income 

 of all plants includes a certain amount of kinetic energy in 

 the form of heat, and that, in the case of plants possessing 

 chlorophyll, it includes a large amount of kinetic energy in 

 the form of light. But in addition to the kinetic energy 

 absorbed from without, there is a constant evolution of kinetic 

 energy going on in every actively living plant in connexion 

 with the decomposition of more or less complex organic sub- 

 stances which have either been formed in the plant, or, as in 

 the case more particularly of plants destitute of chlorophyll, 

 have been absorbed as food, in connexion, that is, with de- 

 structive metabolism. It was pointed out in a previous 

 lecture (Lecture I. p. 5) that as the constructive metabolism 

 of plants involves the conversion of kinetic into potential 

 energy, so their destructive metabolism involves the conver- 

 sion of potential into kinetic energy. For the processes of 

 destructive metabolism consist in the decomposition of rela- 

 tively complex and unstable compounds into others which are 

 relatively simple and stable, and, to quote the words of Her- 



