Comparative Physiology. 467 



matter) in the soil is thus hkely to be a constant source of 

 carbon to plants, in the uhnate of ammonia absorbed by the 

 water and introduced by the roots. To the contrary argument 

 of its being seldom found in analysis of the soil, he replies, 

 that, in soil where it is spread and mixed, the mass used for 

 analysis is generally too small to admit of the minute quan- 

 tity contained being perceptible; the soil also, he says, acts 

 chemically on it when not directly taken up by the roots of 

 plants, and absorbs the humus again till again acted on by 

 ammonia. The excrements of plants probably contain nitro- 

 gen, from the fungi found parasitical thereon ; and, as it will 

 probably be in the state of ammonia, this may account for the 

 action of the spongioles of plants on their food, so generally 

 believed in, but which has hitherto eluded demonstration. This 

 belief is strengthened by the fact that nitrogen is found to 

 abound more in the roots than in any other part of the plant. 

 At all events, there seem good grounds for believing that much 

 of the carbon of plants is furnished to them in the state of ulmic 

 or humic acid, which will be decomposed in the leaf, and give 

 oiF oxygen similar to carbonic acid. It at least appears pre- 

 mature to assert that carbonic acid is the sole source, and much 

 more so to confine it wholly to absorption by the leaves. 



Nitrogen is so far indispensable to plants that no vegetable 

 organ can be formed without it. Dumas says : " The researches 

 of Payen have proved that all the organs of the plant, without 

 exception, begin by being formed of an azotated matter analogous 

 to fibrine, with which at a later period the cellular, ligneous, and 

 amylaceous tissues are associated. This azotated matter, the 

 real origin of all the parts of a plant, is never destroyed ; it is 

 always to be found, however abundant may be the non-azotated 

 matter which has been interposed between its particles. This 

 azote, fixed by plants, serves therefore to produce a concrete 

 fibrinous substance, which constitutes the rudiment of all the 

 organs of the vegetable." Liebig says : " When the newly ex- 

 pressed juices of vegetables are allowed to stand, a separation 

 takes place in a few minutes. A gelatinous precipitate, com- 

 monly green, is deposited, and this, when acted on by liquids 

 which remove the colouring matter, leaves a greyish white sub- 

 stance, the vegetable Jibrine.^'' Ammonia is to be found, he says, 

 in an uncombined state in the juices of the plant, wherever life 

 is most active. The preparation of this vegetable fibrine, so 

 essential in the formation of all the organs, will be indispensably 

 necessary before growth can proceed ; and for this purpose nas- 

 cent carbon, hydrogen, oxygen, and nitrogen will be required. 

 It is probable that carbonic acid, water, and ammonia will, by 

 their decomposition, form the pi'incipal source of these substances. 

 Any organic matter absorbed in a semidecomposed state will 



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