SOURCES OF THE ELEMENTS OF VEGETABLE STRUCTURE. 119 



tlie gigantic mass of carbon contained in the wood of a wide- 

 spreading forest, to have been derived chiefly, if not entirely, from 

 this source; and yet such will be seen to be the case. For, 

 although the soil may contain carbon, none of it is taken up in a 

 solid form ; and its quantity rather increases than diminishes in 

 the course of years. 



166. OXYGEN is contained largely in Plants ; and the pre- 

 sence of it in the air which surrounds them, is very necessary to 

 their healthful existence, chiefly as affording the means by 

 which, as already explained, the superfluous carbon is removed. 

 This element is equally necessary to animals ; and it constitutes 

 about a fifth part of the air we breathe. A portion of this air 

 is dissolved, as it were, in water ; and it is in this manner, that 

 fishes and other -aquatic animals, as well as aquatic plants, are 

 supplied with oxygen. Most, if not all, however, of the oxygen, 

 which is contained in vegetable substances, is taken up by them, 

 either in combination with carbon, or in union with hydrogen, 

 a body which, with it, forms water. 



167. HYDROGEN is also contained largely in Plants ; end, in 

 most of the substances into whose composition it enters, it is com- 

 bined with oxygen, nearly in the same proportion as in water. 

 Although it is probable that a small quantity is introduced with 

 nitrogen in the form of ammonia (the pungent gas which gives 

 strength to hartshorn, smelling salts, &c.), we may regard the 

 water introduced into the substance of plants by their roots, and 

 also in part absorbed by their general surface, as the chief source 

 of this element, as well as of the oxygen contained in the 

 vegetable structure. 



168. NITROGEN has not been commonly regarded as an im- 

 portant element of the Vegetable structure; but it has been lately 

 shown to exist largely in certain parts of plants; and there seems 

 reason to believe its presence to be essential to the increase of 

 their fabric, by the formation of new parts. It is an important 

 ingredient in the substance called gluten, which is identical in its 

 chemical composition with the Albumen, which is the basis of 

 the Animal tissues (ANIM. PHYSIOL. . 153) ; this gluten exists 

 largely in the seeds of the various kinds of corn, and most of all 



