26 OF THE CONSTITUENT ELEMENTS OF PLANTS. 



life has ceased to exercise its influence, this element 

 resumes its chemical character, and materially assists 

 in promoting the decay of vegetable matter, by es- 

 caping from the compounds of which it formed a 

 constituent. 



Oxygen, the only remaining constituent of organic 

 matter, is a gaseous element, which plays a most 

 important part in the economy of nature. It is the 

 agent employed in effecting the union and disunion 

 of a vast number of compounds. It is superior to 

 all other elements in the extensive range of its af- 

 finities. The phenomena of combustion and decay 

 are examples of the exercise of its power. 



Oxygen is the most generally diffused element on 

 the surface of the earth ; for, besides constituting 

 the principal part of the atmosphere which surrounds 

 it, it is a component of almost all the earths and 

 minerals found on its surface. In an isolated state 

 it is a gaseous body, possessed of neither taste nor 

 smell. It is slightly soluble in water, and hence is 

 usually found dissolved in rain and snow, as well as 

 in the water of running streams. 



Such are the principal characters of the elements 

 which constitute organic matter ; but it remains for 

 us to consider in what form they are united in plants. 



The substances which constitute the principal mass 

 of every vegetable are compounds of carbon with ox- 

 ygen and hydrogen, in the proper relative propor- 

 tions for forming water. Woody fibre, starch, sugar, 

 and gum, for example, are such compounds of carbon 

 with the elements of water. In another class of sub- 

 stances containing carbon as an element, oxygen and 

 hydrogen are again present ; but the proportion of 

 oxygen is greater than would be required for produc- 

 ing water by union with the hydrogen. The numer- 

 ous organic acids met with in plants belong, with 

 few exceptions, to this class. 



A third class of vegetable compounds contains car- 

 bon and hydrogen, but no oxygen, or less of that 

 element than would be required to convert all the 



