34 BIOLOGY OF PLANTS. 



triment down between the bark and wood, (in perennial 

 plants,) and assimilates what is nutritious to the livincr tree; 

 and, although most of the matter remains, still there is a quan- 

 tity taken in which is excreted by the roots, or transpired 

 through the leaves as unfit to enter into its composition. 



This power exists in the roots, stem, leaves, juices, flowers 

 and fruit, and presides over all the changes which are carried 

 forward in the vegetable economy. In this case also many 

 of the processes are, in part, purely chemical or mechanical ; 

 but these forces would be of no avail to form the vegdable 

 products, if the living power or vital energy were absent, and 

 hence we may ascribe the effect to that power, just as we do 

 chemical changes to affinity, although many other agents may 

 operate in unison with it, and modify its action. 



As some chemists are disposed to doubt the existence of 

 such a power, attempting to explain the phenomena of vege- 

 tation by chemical and mechanical forces, and particularly 

 by what are denominated " chemical transformations," I will 

 proceed to the 



Proofs of its existence. The existence of such a power in 

 plants and animals is susceptible of the same kind of proof as 

 the power of gravitation or attraction in general. We cannot 

 subject it to the test of the senses, as we can caloric, light and 

 electricity. We infer its existence from the effects which are 

 produced, and which cannot be shown to be caused by those 

 agents which are capable of sensible demonstration. 



That such a power exists in animals might be easily shown. 

 But it is more important for our purpose to exhibit evidence 

 of its existence in plants ; in which this force differs in its 

 operations, in the conditions requisite for its development, and 

 in the instruments, by which its existence is continued. 



I. Vegetables j^ossess the poioer of resisting or counteracting 

 the laws of affinity, gravity, heat and cold. 



1. The process of the absorption of food, and its elabora- 

 tion and assimilation, takes place in opposition to the laws of 



