OF PLANTS. 3 



The substances which constitute the principal 

 mass of every vegetable are compounds of carbon 

 with oxygen and hydrogen in the proper relative 

 proportions for forming water. Woody fibre, 

 starch, sugar, and gum, for example, are such com- 

 pounds of carbon with the elements of water. In 

 another class of substances containing carbon as an 

 element, oxygen and hydrogen are again present ; 

 but the proportion of oxygen is greater than would 

 be required for producing water by union with 

 the hydrogen. The numerous organic acids met 

 with in plants belong, with few exceptions, to this 

 class. 



A third class of vegetable compounds contain 

 carbon and hydrogen, but no oxygen, or less of 

 that element than would be required to convert all 

 the hydrogen into water. These may be regarded 

 as compounds of carbon with the elements of 

 water and an excess of hydrogen. Such are the 

 volatile and fixed oils, wax, and the resins. Many 

 of them have acid characters. 



The juices of all vegetables contain organic acids, 

 generally combined with the inorganic bases, or 

 metallic oxides ; for these metallic oxides exist in 

 every plant, and may be detected in its ashes after 

 incineration. 



Nitrogen is an element of vegetable albumen and 

 gluten ; it is a constituent of the acids, and of what 

 are termed the " indifferent substances," of plants, 

 as well as of those peculiar vegetable compounds 



