THE METABOLISM OF PLANTS. 289 



mann, " in every chemical process in which stronger affinities 

 are saturated than were saturated before its occurrence, po- 

 tential energy becomes kinetic." The simpler and the more 

 stable the waste-products of any destructive process, the 

 greater is the amount of energy evolved. For instance, in the 

 self-decomposition of protoplasm (p. 188), if the decomposi- 

 tion were as complete as it would be on combustion, and 

 the only waste-products were carbon dioxide, water, and some 

 comparatively simple nitrogenous substance or even nitrogen 

 itself, a relatively large amount of energy would be evolved ; 

 whereas, as a matter of fact, though carbon dioxide and water 

 are formed, yet various more or less complex substances are 

 formed as well, so that only a relatively small amount of 

 energy is set free. 



We have now especially to consider what becomes of 

 kinetic energy in the plant. The matter may be briefly 

 stated thus : a portion of the kinetic energy is stored up by 

 the plant in the form of potential energy : the remainder is 

 lost to the plant ; it is either spent in the performance of 

 mechanical work in connexion with growth or movement, 

 or it is given off most generally in the form of heat, occasion- 

 ally in the form of light, and possibly in the form of elec- 

 tricity. We shall speak of the storing up of energy in the 

 plant as the accumulation of energy ; and of the loss of energy 

 as the dissipation of energy. 



I. The Accumulation of Energy. We know already that 

 the accumulation of energy is the necessary accompaniment 

 of constructive metabolism, that the formation of more and 

 more complex organic substances involves the conversion of 

 kinetic into potential energy. We know also that construc- 

 tive metabolism is being constantly carried on in the plant. 

 We learned in a previous lecture (p. 202) that so long as 

 a plant is living its protoplasm is undergoing active decom- 

 position. Hence, if the life of the plant is to be maintained, 

 the repair, or as we may term it the Nutrition, of its proto- 

 plasm must be carried on at least as actively as its decompo- 

 sition. This continual construction of protoplasm involves 

 a continual conversion of kinetic into potential energy : hence 

 V. 19 



