On the Food of Plants. 
513 
Hence — • 
CuUic iiiclies. 
Gain of oxygen 12'4 
nitrogen 1*03 
Oxygen assiniiluted . , . . 7*02 
Tn this last case the conversion of carbonic acid into oxygen 
was not quite completed even after the lapse of eighteen days ; 
but this is easy to understand, since the change in question is 
effected by the agency of the green matter of the leaf, which in 
the pines has but little development in comparison with the 
extent of surface possessed by many plants. 
Water-plants decompose carbonic acid and evolve oxygen by 
the aid of light. The carbonic acid is in this case derived, in 
})art, from the decomposition of organic matter at the bottom of 
the brook or pool, which is dissolved by the water, and thus 
brought within the sphere of activity of the plant. The same 
observation has r^uite recently been extended to plants growing in 
the sea, by M. Aime, of the French College of Algiers. 
These experiments, and a great number of others which might 
be cited, leave no doubt of the fact that plants possess, to a very 
great extent indeed, the power of effecting the decomposition in 
question. 
This action is peculiar to the green parts of plants, or those 
which are capable of becoming green by exposure. It proceeds 
with the greatest rapidity under the direct rays of the sun : it 
occurs even in diffuse daylight, but ceases immediately on the 
withdrawal of the light altogether, when other and completely dif- 
ferent phenomena commence. It appears that in the dark, not 
only does the evolution of oxygen cease, but an actual absorption 
of that gas occurs ; and carbonic acid itself is given off in return, 
just as in the respiration of an animal, although to a far more 
limited extent. 
The emission of carbonic acid by plants has indeed been 
ascribed to a real respiratory process, taking place both by day 
and night, and whose effect is the production of a certain quantity 
of that gas, while the oxygen, so abundantly liberated under the 
influence of sun-light, has been referred to a kind of digestion 
taking place only under those circumstances, in virtue of which 
carbonic acid is decomposed, the carbon assimilated, and the 
oxygen set free, so that during the day the effect of the proper 
respiration is masked by the more extensive process then going on. 
This explanation is a very ingenious one, but it is not com- 
pletely borne out by what we know of these processes in the 
animal system. However true it may be that the assimilation of 
* Ann. Chim. et Phys., August, 1841. 
2 L 2 
