522 On the Food of Plants. 
of M. Senebier, in which he observed, that ** plants abuw- 
dantly supplied with water which had been impregnated with 
carbonic acid, transpired much more oxygen than when they 
were supphed with common water.” 
Some plants take more carbon than others into their com- 
position; as for instance, the Agaricus quercinus, Agaricus 
antiquus, Boletus versicolor, Boletus igniarius, Boletus str?- 
atus, Boletus perennis, Clavaria hypoxylon, Clavaria pistil- 
éaris, and many others. All these contain, from the result of 
analysis, a quantity of carbon nearly equal to al} their other 
component parts. But the Lichen crispus, Pinaster granu- 
latus, and Lycoperdon tessellatum, contain a very small por- 
tion of carbon. 
Plants do net, however, retain all the carbonaceous mat- 
ter they receive; they obtain more in the day, when exposed 
to light, than they naturally require ; but by the absence of 
hight they part with this surplus, and therefore yield respira- 
ble gas only in the day-time. 
The separation of oxygen from plants by radiant light, 
seems to arise from the chenrical affinity between oxygen 
andlight. For this fact we are indebted to Dr. Ingenhouz ; 
but Humboldt was the first who ascertained that hydrogen 
gas applied to plants, even when excluded from the light, 
occasions a separation of their accumulated oxygen. 
Some plants, as for instance tremella nostoe, the filices, 
musci, aud alg@, retain their oxygen weakly, and part with 
it readily. And it is remarked by Van Uslar, to whom I 
am indebted for many of these observations, that such plants 
as coutain much oxygen, and retain it obstinately, are white; 
as for instance, our endive and celery when excluded from 
the light; while such as contain much oxygen, and part — 
with it easily, are generally green. 
Tf the analysis of plants leads us to consider carbon as one 
of the most essential articles in their composition and sup- 
port, no less does the experience of ages prove to us that 
the principal source fyom which they derive their nutri- 
ment, whatever it may be, is to be sought for in vegetable 
earth, the produce of ahimal and vegetable substances de- 
eayed. Many plans, indeed, require little or no earth for 
their 
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