1 18 SOURCES OF THE ELEMENTS OF VEGETABLE STRUCTURE. 



Starch, which, as we shall hereafter see (Chap, ix.), performs 

 offices of great importance in the vegetable economy, is precisely 

 the same. On the other hand, the substances into whose com- 

 position Nitrogen enters, though very generally diffused through 

 the tissues of the Plant, do not seem to undergo organisation, 

 but to form part of the contents of the cells, vessels, &c., of 

 which these tissues are composed. It is curious to remark, that 

 precisely the reverse is the case with Animals; their tissues 

 being composed of a substance containing nitrogen ; and sub- 

 stances which are destitute of it being never found in their 

 bodies in an organised state, but only existing there in the 

 cavities of their cells, tubes, &c. (See ANIM. PHYSIOL., 

 . 16, 154). 



164. Of all these, CARBON is by far the most abundant. It 

 is, as already mentioned, nearly identical with charcoal; which 

 consists of the carbon of the wood, mixed up with a small 

 quantity of earthy matter. If this charcoal be burned, it passes 

 off in the form of carbonic acid gas, leaving a minute portion 

 of white ash, which is principally of a mineral nature. It is 

 chiefly to the carbon which it contains, that the hardness and 

 solidity of wood are due. In so large a proportion does it exist 

 in that tissue, that, when the other elementary bodies (the oxygen 

 and hydrogen) have been separated, the carbon retains the form 

 of the tissues in great beauty and perfection ; so that a section 

 of a piece of charcoal, will indicate the character of the wood 

 from which it was made, nearly as well as would a section 

 of an unburat branch. On the other hand, in proportion as 

 the tissues of the Plant are deficient in carbon, do we find 

 them deficient in firmness of structure. 



165. When we consider the large quantity of carbonic acid, 

 extricated by the respiration of Animals, and by the immense 

 amount of combustion of Coal, which is constantly going on in 

 our large towns, there would seem no difficulty in understanding 

 how it may be supplied to Plants ; but so vast is the extent of 

 the atmosphere, through which the carbonic acid has to be dif- 

 fused, that any given bulk of air only contains about l-1000th 

 part of this gas. Hence it might be supposed impossible, for 



