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Nutrition of Vegetables . 
j impregnated with these matters, which are eminently injurious 
j to vegetation, the perfect developement of plants is prevented, 
j This excretion from the roots is evident from the surrounding 
earth, which becomes unctuous, and sometimes of a darker co- 
lour. In several of the euphorbiums and cicoraceous plants it is 
very perceptible, and milky. It may be observed too, that roots, 
when they multiply under water, become covered with a glairy 
matter, which deserves to be examined ; and which no doubt the 
earth would have absorbed, had the roots remained buried in it. 
It is to these excretions from the roots perhaps we must frequent- 
ly ascribe that kind of antipathy between certain plants, which are 
never found together. Thus the thistle is injurious to oats, 
euphorbium and scabious to flax, elecampane to carrots, fleabane 
and darnel to wheat, Sec. 
It would certainly be wrong, to ascribe the fertility of land 
pared and burned to the charcoal produced in this operation ; for 
Mr. Chaptal has shown, that dry charcoal, alone or mixed with 
earths of little solubility, does not penetrate into the vessels of 
vegetables. 
To add to the proofs, that vegetables have no need of drawing 
carbon from the earth, I might mention high trees, loaded with 
fruit, that grow and thrive on rocks or old walls, totally destitute 
of vegetable mould ; and those vast forests, the soil of which is 
pure sand extending far beyond the roots. 
I have now to examine the opinion, that vegetables absorb 
their carbon from the small quantity of carbonic acid contained in 
the atmosphere. Sennebier first announced this decomposition ; 
and T. Saussure afterward endeavoured to prove, that this very 
small quantity would be sufficient for all the vegetables that 
exist. But though this philosopher was persuaded of the utility 
of carbonic acid in vegetation, he satisfied himself, that plants 
could grow in an atmosphere freed from it j and he ascribed this 
growth to the carbonic acid produced by the plants themselves, 
which they decomposed after having formed it.* To prove this 
he exposed to the sun closed receivers, in which plants were 
growing, and suspended quicklime to the upper part of them. 
The plants soon grew yellow, and at the expiratioxi of five days 
* It is obvious, that the carbonic acid formed by the plants could not 
furnish them with more of its base than it had previously taken from 
them. Tr. 
