8o ELEMENTS OF BIOLOGY. 



upon the life of its elemental factors, but actually being 

 kept up by their constant destruction and as constant re- 

 newal. Only in a very limited sense, therefore, can we say 

 that the life of the organism is the sum total of the lives of 

 these structural units. 



Lastly, in plants as in animals, the vital processes are 

 carried on by forces which we cannot as yet refer to known 

 chemical and physical forces, and which, therefore, we are, 

 in the meanwhile, compelled to speak of as " vital." In 

 the case of plants, for example, it is quite true that certain 

 known chemical and physical forces are concerned in their 

 vital processes, and are, indeed, absolutely necessary for 

 their due performance. Thus it is absolutely certain that 

 no plant can convert inorganic matter into organic com- 

 pounds, or, in other words, can digest, unless it be supplied 

 with solar light; whilst the solar heat is equally essential 

 to the performance of other of its vital processes. On this 

 subject Dr Carpenter expresses himself as follows : 



" Plants form those organic compounds at the expense of 

 which animal life (as well as their own) is sustained, by 

 the decomposition of carbonic acid, water, and ammonia ; 

 and the light, by whose agency alone these compounds can 

 be generated, may be considered as metamorphosed into 

 the chemico-vital affinity by which their components are held 

 together. The heat which plants receive, acting through 

 their organised structures as vital force, serves to augment 

 these structures to an almost unlimited extent, and thus to 

 supply new instruments for the agency of light and for the 

 production of organic compounds. Supposing no animals 

 existed to consume these organic compounds, they would 

 all be restored to the unorganised condition by spontaneous 

 decay, which would reproduce carbonic acid, water, and 

 ammonia, from which they were generated. In this decay, 

 however slow, the same amount of heat would be given off 

 as in the more rapid processes of combustion ; and the 

 faint luminosity which has been observed in some vegetable 



