LECTURE XIV. 



any increase of the protoplasm of a plant means an accumu- 

 lation of energy in the potential form. 



But it is not so much to an increase of its protoplasm that 

 the gain in organic substance by a plant is due. It is princi- 

 pally due to 'the accumulation of substances which are directly 

 or indirectly the products of the decomposition of protoplasm, 

 such as cellulose, starch, fats, proteids, etc. These represent 

 a certain proportion of the energy which became potential in 

 the construction of the protoplasm from which they were de- 

 rived, and it is in these substances chiefly that the potential 

 energy accumulated by the plant is stored up. 



The proportion of the kinetic energy which is stored up 

 in the form of potential energy in connexion with the produc- 

 tion of organic substance in the plant is large, if we compare 

 plants with animals. If we consider the enormous amount of 

 organic and organised substance formed by a plant from the 

 time of its development from a seed to its death, in the 

 development of an oak from an acorn, for example, and we 

 bear in mind the amount of potential energy which all this 

 organic and organised substance represents, we can form 

 some idea of the amount of kinetic energy which has been 

 stored up in the potential form. The heat which is given out 

 by burning wood or coal is but the conversion into kinetic 

 energy of the potential energy which was stored up by the 

 plant which produced the wood or the coal. The reason of 

 the difference between plants and animals in this respect is 

 that whereas the increase in bulk of an animal is limited, 

 that the animal soon ceases to increase the dry weight of its 

 organised substance, the increase in bulk of a plant goes on 

 during its whole life. The plant therefore produces a very 

 large dry weight of organised substance, and this represents 

 a large amount of energy. 



II. The Dissipation of Energy. We turn now to the loss 

 of energy by the plant. In dealing with this subject we have 

 first of all to shew that the expenditure of energy in con- 

 nexion with growth and movement and with the evolution of 

 heat, light, and electricity, is dependent upon destructive me- 

 tabolism. That this is the case will be fully proved in our 



