LECTURE XIII. 



METABOLISM (continued). 



8. The Supply of Energy. 



OUR consideration of the metabolism of plants will have 

 made it evident that the various chemical processes involve 

 an expenditure of energy : from this it follows that the con- 

 tinuance of these processes, in other words, the maintenance 

 of the life of the plant, is dependent upon a supply of energy. 

 The supply of energy will form the main subject of the 

 present lecture, but we may conveniently consider in con- 

 nexion with it the general relations of light and of heat to 

 plant-life. 



In the case of animals, the food affords the principal 

 supply of energy. It consists for the most part of complex 

 organic substances which represent a considerable amount 

 of potential energy, and when these substances are decom- 

 posed in the body, this potential energy appears in the 

 kinetic form. This holds good also with reference to plants 

 which are destitute of chlorophyll, for their food necessarily 

 includes, like that of animals, complex organic substances. 

 But with plants which possess chlorophyll the case is entirely 

 different. We have learned (p. 121) that their food consists 

 of simple inorganic substances which do not represent any 

 considerable amount of potential energy ; from these simple 

 substances green plants build up complex organic substances 

 which do represent a considerable amount of potential energy, 

 substances which serve directly or indirectly as the food of 

 all living organisms whatsoever which do not contain chloro- 



