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The Life of the Plant. 167 
chlorophyll, and which are not parasitic, is mainly, if not 
exclusively, the, absorption of carbon dioxide (carbonic 
acid gas) by those parts which contain chlorophyll, and its 
decomposition under the influence of light, the carbon 
being retained and the oxygen again given off. It is in- 
disputable that in certain cases the humus can also con- 
tribute a supply of carbon for the nutrition of the plant ; 
‘but the fact that in a soil which has not been affected by 
cultivation, as in a primeval forest, the stratum of humus 
does not diminish, but rather constantly increases, notwith- 
standing its continual decay and its consumption by plants 
and animals, shows that a much greater quantity of carbon 
must be derived from the atmosphere than from the soil. » 
Hydrogen, which, like carbon, is present in every organic | 
compound, enters into non-nitrogenous combinations only 
through the decomposition of water ; in those which con- 
tain nitrogen possibly also by the absorption of ammonia. 
Oxygen is taken up into plants along with the elements 
already mentioned, the substances in which they occur 
being of such a nature that a large portion of their oxygen 
must be given off before they can be of any further use to 
the plant. The decomposition of the carbon dioxide of 
the atmosphere is a remarkable illustration ofthis. But, not- 
withstanding this excess of oxygen in the plant, the oxygen 
of the atmosphere is also absorbed; not, however, in order 
to form a part of any permanent compound, but, on the con- 
trary, in order to decompose the substances contained in the 
plant, to deprive them of at least a part of their carbon, and 
to be again evolved as carbon dioxide. This process, the | 
true respiration of plants, is very important; since it is 
always taking place in all living cells without exception ; 
and the action of the inhaled oxygen is a necessary con- 
dition of the life of the protoplasm, and therefore of the 
plant. In relation to this, the experimental fact is of the | 
greatest importance, that plants placed in pure carbon: di- 
oxide are suffocated, just as animals are. In some flowers 
