120 MINERAL SUBSTANCES CONTAINED IN PLANTS. 



in wheat ; and it is in part on this account, that wh eaten bread 

 is the most nutritious of ail vegetable substances ordinarily used 

 as food. It is, indeed, on account of their entire deficiency in 

 nitrogen, that gum, sugar, and other similar products, are not fit 

 to maintain animal life by themselves (seeANiM.PiiYSioL..155). 

 Nitrogen constitutes four-fifths of the atmosphere ; but it does 

 not seem to be taken in by the Plant in its simple form. This 

 gas with hydrogen forms ammonia, of which a minute quantity 

 always exists in the atmosphere, being chiefly supplied to it by 

 the decomposition of animal matter ; and this is absorbed by the 

 soil, and taken up by the roots, in the manner hereafter to be 

 described. It is in the supply of ammonia which they yield, 

 that the principal benefit of animal manures seems to consist. 



169. Besides these elementary substances, which all Plants 

 contain, and of which the vegetable tissue may be regarded as 

 essentially consisting, almost all plants contain some mineral 

 ingredients, the presence of which is necessary to their healthy 

 existence. These remain as ashes, when the other parts of the 

 structure are set at liberty by combustion ; the carbon uniting 

 with the oxygen, and with some additional oxygen from the air, 

 passes off as carbonic acid ; of the hydrogen, part unites in the 

 same manner with oxygen, and passes off as watery vapour ; 

 the oxygen is thus entirely carried off; and the nitrogen unites 

 with the remainder of the hydrogen, to form ammonia. Thus 

 there remains nothing of the vegetable tissue, but the incom- 

 bustible matter ; and the nature of this varies in different plants. 

 Thus, in the Grasses (including Corn, Bamboo, Sugar Cane, &c.), 

 the ashes consist principally of minute particles of flint. In most 

 other plants growing inland, we find some compound of the 

 alkali potash ; and it is from this source, that the greatest quan- 

 tity of the pearl-ash that is largely used in various manufactures, 

 such as soap and glass, is derived. On the other hand, in plants 

 growing near the sea, the potash is replaced by soda, which has 

 nearly similar properties. Again, in most plants, there is a 

 small quantity of carbonate of lime, and in others there is a large 

 quantity of lime combined with other acids ; thus, in Rhubarb 

 we find large crystals of oxalate of lime ; arid in the Corn-grains 



