792 PHYSIOLOGY. 



of the same plant. The following table, by Johnston, indicates 

 approximately the relative proportion of the organic and inor- 

 ganic constituents of some of our vegetable food substances in 

 1,000 parts, and of the different elements of which the former are 

 composed. These substances were first dried at a temperature 

 of 230° Fahr :— 



constituents, the soiu'ces from which they are derived, and the 

 state in which they are taken up by plants. 



Carbon is the element which forms the largest proportion of 

 all plants ; its amount varies in different species from 40 to 60 

 per cent. That plants thus contain a large proportion of car- 

 bon may be conveniently proved by taking a piece of wood, the 

 weight of which has been ascertained, and converting it into 

 charcoal, which is impure carbon containing in its substance 

 also a small quantity of the inorganic constituents or ash. 

 The charcoal thus produced is of the same shape as the piece of 

 wood from which it was obtained, and when weighed it will be 

 found to have constituted a large proportion of its original sub- 

 stance. As carbon is a solid substance and insoluble in water, 

 it cannot be taken up in its simple state, for plants, as already 

 noticed, can only take up their food as gas or vapour, or dissolved 

 in water. In the state of combination, however, with oxygen, it 

 forms carbonic acid, wliich is always present in the atmosphere 

 and the soil. Carbonic acid is also soluble to some extent in 

 water. Hence we have no difficulty in ascertaining the source 

 of carbon and the condition and modes in which it is absorbed 

 by the plant ; it is taken up, combined with oxygen in the form 

 of carbonic acid, from the air directly in a gaseous state by 

 the leaves, and in less quantity from the earth, dissolved in 

 water, by the roots. 



Oxygen is, next to carbon, the most abundant organic con- 

 stituent of plants ; and when we consider to what an enormous 

 extent it exists in nature, constituting as it docs about 21 per 

 cent, by volume of the atmosphere we breathe, eiglit-ninths by 

 weight of the water we drink, and at least one-half of the solid 

 materials around us and of the bodies of all living animals, 

 we see that there are abundant materials from which plants can 

 obtain this necessar}^ portion of their food. The whole of the 

 oxygen required by plants as food appears to be taken up either 

 combined with hydrogen in the form of water, or with carbon as 

 carbonic acid. »Some of the oxygen is therefore obtained by the 

 roots from the soil, and some from the air by the leaves. 



