66 SOURCE OF THE OXYGEN AND NITROGEN. 



forms of combination in which this element may enter into their circula- 

 tion, in uncertain quantity — yet that all-pervading water is the main 

 and constant source from which the hydrogen of vegetable substances is 

 derived. 



§ 4. Source of the oxygen of plants. 



We can at once perceive, and without difficulty, the various sources 

 of the oxygen of plants ; though it is difficult in this case also to say 

 how much they derive from each. 



1°. The water which they imbibe so largely consists in great part of 

 oxygen, and is easily decomposed, [eight-ninths of the weight of water 

 are oxygen.] This alone would yield an inexhaustible supply. 



2°. The atmosphere contains 21 per cent, of its bulk of oxygen, and 

 the leaves of plants in certain circumstances are known to absorb this 

 oxygen. The air in which they live, therefore, might be another 

 source. 



3°. Carbonic acid contains 72 per cent, by weight of oxygen, and 

 this gas is also known to be absorbed in large quantity from the atmos- 

 phere by the leaves of plants — while its solution in water is admitted 

 readily by the roots. 



From any one of these sources an ample supply of oxygen might 

 readily be obtained, and it may be considered as a proof of the vast im- 

 portance of this element to the maintenance of animal and vegetable 

 life, that it is everywhere placed so abundantly within the reach of 

 living beings. It is from the first of these sources, however, from the 

 water they contain, that plants are believed to derive their principal 

 supply. The reasons on which this opinion is founded will appear 

 when we shall have considered the functions of the several parts of 

 plants, and die chemical changes to which the food is subjected in the 

 course of the vegetable circulation. 



§ 5. Source of the nitrogen of plants. 



The quantity of nitrogen present in plants is very small, compared 

 with that of any of the other elements which enter into their constitu- 

 tion. Of this you will be reminded, by a reference to the analyses of 

 hay, oats, and potatoes, exhibited in the second lecture (page 30), which 

 shew that the nitrogen contained in these several crops, when perfectly 

 dried at 240° F., is respectively 1^, 2^, and 1^ per cent. In the state 

 in which they are usually given to cattle they contain a still less per 

 centage of nitrogen, in consequence of the quantity of water still present 

 in them. Thus raw potatoes as they are given to cattle contain only i 

 of a per cent, of nitrogen, hay 1^ per cent., and oats ly^,^* per cent., or a 

 hundred pounds of each contain 5 ounces, 3 pound 5 ounces, and 1 pound 

 14 ounces respectively. 



It would appear at first sight as if this small quantity of nitrogen 

 could be of little importance to the plant, especially since, as we shall 

 hereafter see, it does not enter as a constituent into those vegetable sub- 

 stances, such as woody fibre, starch, sugar, and gum, which plants pro- 

 duce in the greatest abundance, and of which their own stems and 



* 0-33, 1-29, and 1 87 per cent, —the potatoes containing also 72 per cent, of water, the hay 

 14, and the oats 15 per cent. 



