LEAVES AND ROOTS ABSORB CARBONIC ACID. 63 



therefore, as well as in spring or in autumn, the plant must be ever ab- 

 sorbing nourishment by these roots, if the soil is capable of affording it — 

 and thus, in the general vegetation of the globe, the increase of carbon 

 in growing plants must, at every season of the year, be partly derived 

 from the vegetable matter of the soil in which they grow. 



§ 2. Form in which carbon enters into the circulation of plants. 



Supposing it to be established that the whole of the carbon contained 

 in plants has originally been derived from the air — we have only to in- 

 (juire in what state .this element exists in the atmosphere, in order to 

 satisfv ourselves as to the form of combination in which it is and has 

 been received into the circulation of plants. In considering the consti- 

 tution of the atmosphere in the jjreceding lecture, it was stated that car- 

 bonic acid, a compound of carbon and oxygen, is always present in it — 

 and that, though this gas is ditFused through the air in comparatively 

 small quantity only, yet it is everywhere to be detected, — while no 

 other compound of carbon is to be found in it u) any appreciable quanti- 

 ty. We must conclude, therefore, that from this gaseous carbonic acid 

 tiie whole of the carbon contained in j)lants has been primarily derived. 

 This conclusion is confirmed by the observation so frequently made, 

 that the leaves of plants in sunshine absorb carbonic acid, and that 

 plants die in an atmosphere from which this gas is entirely excluded. 



But we have seen reason to believe that, under existing circumstan- 

 ces, plants also extract a portion of the carbon they contain from the 

 soil in which they grow. In what state or form of combination do the 

 roots absorb carbon ? 



The most abundant product of the decay of vegetable matter in the 

 soil, is the same carbonic acid which ]ilants inhale so largely from the 

 atmosphere by their leaves. In a soil replete with vegetable matter, 

 therefore, the roots are surrounded by an atmosphere more or less 

 charged with carbonic acid. Hence if they are capable of inhaling 

 gaseous substances, this gas will enter the roots in the aeriform state — if 

 not, it must enter in solution in the water, wiiich the roots drink in so 

 largely, to supply the constant waste caused by the insensible perspira- 

 tion of the leaves. 



During the early fermentation of artificial manures there is also de- 

 veloped in the soil a variable proportion of light carburetted hydrogen 

 (Lecture III., p. 49), which is supposed by some to enter occasionally 

 into the roots. That it does enter, however, is doubtful, — and we are 

 safe, I think, in considering this conipound not only as an uncertain 

 source of the carbon of plants, but as one from which, in the most fa- 

 vourable circumstances, they can derive only a small supply. 



Thus, from the eartli as from the air, the most unfailing supply of food 

 is the gaseous carbonic acid. 



But as the water passes through the soil it takes up inorganic substan- 

 ces — potash, soda, lime, magnesia — and conveys them through the roots 

 into the circulation of the plants. Can it refuse to take up and to perform 

 a similar office to the soluble organic substances it meets with, as it sinks 

 through the soil ? Or do the spongioles of the roots keep a perpetual 

 watch over the entering waters, to prevent the inJ:^rod action of every so- 

 luble form of carboa but that of carbonic acic! Or, supposing such 



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