LKSSON 2G.] ITS CHEMICAL COMPOSITION. 159 



it is dissipated into air. But a little ashes remain : these represent 

 the earthy constituents of the plant. 



452. They consist of some potash (or soda if a marine plant was 

 used), some silex (the same as Hint), and probably a little lime, al- 

 umine, or magnesia, iron or manganese, sulphur or phosphorus, &c, 

 Some or all of these elements may be detected in many or most 

 plants. But they make no part of their real fabric ; and they form 

 only from one or two to nine or ten parts out of a hundred of any 

 vegetable substance. The ashes vary according to the nature 

 of the soil. In fact, they consist, principally, of such materials as 

 happened to be dissolved, in small quantity, in the water which was 

 taken up by the roots ; and when that is consumed by the plant, or 

 flies off pure (as it largely does, 447) by exhalation, the earthy mat- 

 ter is left behind in the cells, just as it is left incrusting the sides 

 of a teakettle in which much hard water has been boiled. As is 

 very natural, therefore, we find more earthy matter (i. e. more 

 ashes) in the leaves than in any other part (sometimes as much as 

 seven per cent, when the wood contains only two per cent) ; because 

 it is through the leaves that most of the water escapes from the plant. 

 These earthy constituents are often useful to the plant (the silex, for 

 instance, increases the strength of the Wheat-stalk), or are useful in 

 the plant's products as furnishing needful elements in the food of man 

 and other animals ; and some mu?t be held to be necessary to vege- 

 tation, since this is never known to go on without them. 



453. The Organic Constituents, As has just been remarked, when 

 we burn in the open air a piece of any plant, nearly its whole bulk, 

 and from 88 to more than 99 parts out of a hundred by weight of its 

 substance, disappear, being turned into air and vapor. These are 

 the organic constituents which have thus been consumed, the 

 actual materials of the cells and the whole real fabric of the plant. 

 And we may state that, in burning, it has been decomposed into ex- 

 actly the same kinds of air, and the vapor of water, that the plant 

 used in its making. The burning has merely undone the work of 

 vegetation, and given back the materials to the air just in the state 

 in which the plant took them. 



454. It will not be difficult to understand what the organic con- 

 stituents, that is, what the real materials, of the plant are, and how 

 the plant obtains them. The substance of which vegetable tissue, 

 viz. the wall of the cells, is made, is by chemists named Cellulose. It 

 is just the same thing in composition in wood and in soft cellular tis- 



