THEIR ELKMENTAKY CONSTITUENTS. 179 



Sect. II. The Food and the Elementary Composition 



OF Plants. 



321. The Food and tlie elementary composition of plants stand 

 in a necessary relation to each other. Since it is not to be sup- 

 posed that plants possess the power of creating any simple element, 

 Avhatever they consist of must have been derived from without. 

 Their composition indicates Xhaxv food, and vice versa. If we have 

 learned the chemical composition of a vegetable, and also what it 

 gives back to the soil and the air, we know consequently Avhat it 

 must have derived from without, that is, its/oorf. Or, if Ave have 

 ascertained what the plant takes from the soil and air, and what it 

 returns to them, we have learned its chemical composition, namely, 

 the difference between these two. And when we compare the na- 

 ture and condition of the materials which the plant takes from the 

 soil and the air with what it gives back to them, we may form a 

 correct notion of tlie influence of vegetation upon the mineral king- 

 dom. By considering the materials of which plants are composed, 

 we may learn what their food must necessarily contain. 



322. The Constituents of Plants are of two kinds ; the earOnj or in- 

 organic, and the organic. It has been stated (93) that various 

 earth}' matters, dissolved by the water Avhich the roots absorb, are 

 drawn into the plant, and at length deposited in the wood, leaves, 

 &c. These form the ashes which are left on burning a leaf or a 

 piece of wood. Although these mineral matters are often turned 

 to account by the plant, and some of them are necessary in the 

 formation of certain products, (as the silex which gives needful 

 firmness to the stalk of AYheat, and the phosphates wliich are found 

 in the grain,) yet none of them are essential to simple vegetation, 

 which may, to a certain extent, proceed without them. These 

 materials, the presence of which is in some sort accidental, although 

 for certain purposes essential, are distinguished as the earthy, or 

 mineral, or inorganic constituents of plants. This class may 

 be left entirely out of view for the present. But the analysis of 

 any newly formed vegetable tissue, or of any part of the plant, 

 such as a piece of wood, after the incrusting mineral matter has 

 been chemically removed, invariably yields but three or four ele- 

 ments. These, which are indispensable to vegetation, and make 

 up at least from eighty-eight to ninety-nine per cent of every vege- 



