SILOS, ENSILAGE AND SILAGE. 55 



and sugar was formed at the expense of the malic acid, 

 as shown in the following table : * 



The real significance of the facts already presented 

 cannot be clearly seen if our attention is confined to the 

 obvious chemical changes taking place at different stages 

 of growth, without taking into consideration the law of 

 the conservation of energy in its relations to organic life. 

 With the progress of biological science, the metamor- 

 phoses of matter in organic processes, which have been 

 the almost exclusive subjects of study until within a few 

 years past, are coming to be looked upon as of less and 

 less importance, while the transformations of energy are 

 being recognized as dominant factors in all vital activ- 

 ities. Heat and light are the main sources of energy 

 concerned in the processes of nutrition and growth, and 

 in general terms, the leading phenomena of plant meta- 

 bolism may be summarized as follows : In the building 

 up of tissues (constructive metabolism), work is per- 

 formed and an expenditure of energy is made at the 

 expense of the heat and light supplied to the plant 

 Step by step comparatively simple food materials are 

 converted into more and more complex organic com- 

 pounds, resulting in the formation of living protoplasm, 

 an essential constituent of every cell, as the final and 

 most complex state of constructive metabolism. 



An expenditure and storing up of energy is involved 

 in every step of this process. This stored-up energy is 

 spoken of as potential energy, that may afterwards be- 

 come active in doing work, or become sensible in the 

 form of heat. 



* Jour. Chem. Soc. XXVIII (1876), 904, quoted by Prescott, Mich. Pom. Rep'fc 

 1877, p. 152. 



