NATURE AND USES OF PRODUCTS. 133 



these should be the effect of so small a number of princi- 

 ples, varying only in the proportions in which they are 

 united. 



The aliments of plants are water, air and manures: 

 these substances absorbed by the leaves, the fruits, or the 

 roots, furnish by analysis, carbonic acid, hydrogen, a lit- 

 tle azote, and some earthy and saline principles : it is 

 from these materials that the almost endless variety of 

 widely differing products of plants is formed by their or- 

 gans. 



During the progress of vegetation these products are 

 found to undergo successive changes; that which is first 

 acid becomes sweet ; that which is tender becomes hard, 

 and all is owing wholly to the constant changes taking 

 place in the proportions of the constituent principles ; and 

 one is astonished at finding that the most exact analysis of 

 substances possessing the most opposite characteristics, 

 detects no other difference than some hundredths more or 

 less in the proportions of their elements. 



When a plant has completed or terminated its various 

 stages of vegetation, the dead remains, if exposed to the 

 action of the same agents, such' as air, water, and heat, 

 suffer a succession of retrograde changes ; they are grad- 

 ually decomposed, and their constituent principles enter 

 into combination with those of the bodies by which they 

 are acted upon ; thus the dead plant is entirely governed 

 by those invariable physical and chymical laws, which in 

 the living plant are governed and modified by the laws of 

 vitality, the action of which regulates that of all external 

 agents, and produces results which we can neither explain 

 nor imitate. 



Though great caution should be used when endeavouring 

 to establish an analogy between two modes of existence 

 differing so widely as those of animals and vegetables, it 

 must be perceived that there is a resemblance in the manner 

 in which both are nourished. 



Animals inhale air by their lungs, or absorb it by glands 

 scattered over their bodies ; they are nourished by solid 

 aliments received into their stomachs, or into some analo- 

 gous organ : plants absorb air by their leaves and fruits, and 

 imbibe through their roots the nutritive juices contained in 

 the earth. In animals, the juices circulate through every 

 part, and pass into all the various organs, in which they are 

 elaborated, in order to form all the products whiph belong 

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