146 VEGETABLE LIFE AND WORK. LSEOTION 16. 



The essential constituents of the organic fabric are those which are dissi- 

 pated into air and vapor in complete burning. They make up from 88 to 

 99 per cent of the leaf or stem, and essentially the whole both of the cellu- 

 lose of the walls and the protoplasm of the contents. Burning gives these 

 materials of the plant's structure back to the air, mainly in the same condi- 

 tion in which the plant took them, the same condition which is reached 

 more slowly in natural decay. The chemical elements of the cell-walls (or 

 cellulose, 402), as also of starch, sugar, and aU that class of organizable 

 cell-material, are carbon, hydrogen, and oxygen (399). The same, with 

 nitrogen, are the constituents of protoplasm, or the truly vital part of 

 vegetation. 



449. These chemical elements out of which organic matters are com- 

 posed are supplied to the plant by water, carbonic acid, and some combina- 

 tions of nitrogen. 



Water, far more largely tlian anything else, is imbibed by the roots ; also 

 more or less by the foliage in the form of vapor. Water consists of oxygen 

 and hydrogen; and cellulose or plant-wall, starch, sugar, etc., however 

 different in their qualities, agree in containing these two elements in the 

 same relative proportions as in water. 



Carbonic acid gas (Carbon dioxide) is one of the components of the atmos- 

 phere, — a small one, ordinarily only about i^^ of its bulk, — sufficient 

 for the supply of vegetation, but not enough to be injurious to animals, as it 

 would be if accumulated. Every current or breeze of air brings to the leaves 

 expanded in it a succession of fresh atoms of carbonic acid, which it absorbs 

 through its multitudinous breathiug-pores. This gas is also taken up by 

 water. So it is brought to the ground by rain, and is absorbed by the roots 

 of plants, either as dissolved in the water they imbibe, or in the form of 

 gas in the interstices of the soil. Manured ground, that is, soil containing 

 decomposing vegetable or animal matters, is constantly giving out this gas 

 into the interstices of the soil, whence the roots of the growing crop absorb 

 it. Carbonic acid thus supplied, primarily from the air, is the source of the, 

 carbon which forms much the largest part of the substance of every plant. 

 The proportion of carbon may be roughly estimated by charring some wood 

 or foliage ; that is, by heating it out of contact with the air, so as to decom- 

 pose and drive olf all the other constituents Oi the fabric, leaving the large 

 bulk of charcoal or carbon behind. 



Nitrogen, the remaining plant-element, is a gas which makes up more 

 than two thirds of the atmosphere, is brought into the foliage and also to 

 the roots (being moderately soluble in water) in the same ways as is car- 

 bonic acid. The nitrogen which, mixed with oxygen, a little carbonic acid, 

 and vapor of water, constitutes the air we breathe, is the source of this 

 fourth plant-element. But it is very doubtful if ordinary plants can use 

 any nitrogen gas directly as food ; that is, if they can directly cause it to 

 combine with the other elements so as to form protoplasm. But when com- 

 bined with hydrogen (forming ammonia), or when combined with oxygec 



