EXPERIMENTS ON THE DECOMPOSITION OF CARBONIC ACID BY PLANTS. 
EXPERIMENTS ON THE DECOMPOSITION OE CARBONIC ACID BY PLANTS* 
By ]\I M. Cloez and Gratiolet. 
f T has been long known that the green parts of plants decompose the surrounding carbonic acid and 
separate it from the oxygen. The submerged aquatic plants possess this property in a very high 
degree, and thus disengage, in a comparatively short time, and in the normal condition of their existence, 
an enormous quantity of oxygen. The knowledge of this fact led us to choose these plants for the 
subject of some experiments which were made with various species of Potomogeton, Naias, Cerato- 
pliyllum, Myriophyllum, and Confervae. The conclusions we have formed may be stated under the 
following heads:— 
1. Influence of light. —The disengagement of oxygen, which takes place very rapidly in solar light, 
becomes insensible in a diffuse light, and is perfectly stayed in darkness. In the latter case, the plants 
of which we speak, contrary to the opinion generally received, do not evolve the smallest trace of 
carbonic acid. We endeavoured to determine comparatively the action of coloured glasses on the 
decomposition of carbonic acid by the green parts of plants; and, as far as the difficult nature of the 
experiments permit us to judge, we have concluded that the activity of the phenomenon is at the 
maximum when plain and unpolished glasses are used. The yellow glass comes next, then the plain 
transparent, the red, the green, and last of all, the blue. We are satisfied that these differences do not 
hold good with a difference of temperature. 
2. Influence of temperature. —The decomposition of carbonic acid by aquatic plants exposed to 
the light in a medium, of which the temperature rises from 4° Centigrade [about 40° F ahr enheit], 
does not commence below 15° [59° Fahrenheit], and appears to attain its maximum at 30 3 
[86° Fahrenheit]. The decomposition of carbonic acid by plants exposed to the light in a medium, the 
temperature of which is lowered from 30°, continues to act until 14° 13' 12", and does not completely 
cease till it reaches 10° [50° Fahrenheit]. This result is, indeed, conformable to the conclusions 
which M. Chevreul has drawn from his observations on the circulation and ascent of the sap. 
3. Influence of the composition of the surrounding medium. —The vegetation of submerged plants 
continues during several months in the water of the Seine, if aerated and renewed every day. In river 
water deprived of air by ebullition, and containing only carbonic acid in the same proportion as the water 
of the Seine, and which is also renewed each day, the decomposition at first is very active, but by and by 
it becomes subdued, and ceases completely at the end of four or five days. After that time the intensity 
of the green colour of the plant is considerably diminished. The phases of this phenomenon are very 
remarkable. Indeed, it may at first be observed that the gas which is produced is mixed with a 
certain quantity of azote, which goes on diminishing to that point at which the decomposition stops, 
when the air that is disengaged is almost pure oxygen. It may also be observed that the total volume 
of azote gas which is disengaged is much more considerable than the bulk or volume of the plant; and 
if the plant is submitted to an elementary analysis, it is found that at an equal weight it contains much 
less azote than a portion of the same plant which has not been submitted to an experiment. These 
facts demonstrate that in the act of vegetation of submerged plants a certain quantity of azote is 
produced by the decomposition of the same elements of the plants; that consequently a reparation is 
necessary, and that the free or combined azote is an indispensable element to the life of aquatic plants. 
From these experiments we were constrained to investigate the influence of ammonia and ammoniacal 
salts, and we have found that ammonia and ammoniacal salts under water in a dose of one ten-thousandth 
in weight, have always been hurtful. The decomposition of carbonic acid diminishes and stops at the 
end of some hours. We have, consequently, a right to conclude that the plant assimilates directly the 
azote gas in a state of dissolution in the water. 
4. The movements of the elements which are absorbed or exhaled by the plant. —Everybody may easily 
observe that whatever the position of the leaves of Potamogeton may be, some carbonate of lime is 
constantly deposited at the superior face of the leaves (we say superior in the sense understood by 
botanists), and never at the inferior face. This fact appears to demonstrate that the absorption of 
carbonic acid takes place essentially by the superior face of the leaves. The oxygen produced by the 
decomposition of carbonic acid has, in the plant, a perfectly defined course. It descends constantly from 
the leaves towards the root. Thus, when a branch or slip of Potamogeton, provided with a few leaves, 
is placed horizontally in water, the escape of the gas always takes place by the section nearest the 
radicular extremity of the plant. 
From the Comptes Rendus. 
VOL. XI. 
2 L 
