"Now, if a fish-globe be not overcrowded with fishes, we have only to throw in a goodly handful of some water-weed,—such as the Callitriche, for instance,—and a new set of chemical operations commences at once, and it becomes unnecessary to change the water. The reason of this is easily explained. Plants absorb oxygen as animals do; but they also absorb carbonic acid, and from the carbonic add thus absorbed they remove the pure carbon, and convert it into vegetable tissue, giving out the free oxygen either to the water or the air, as the case may be. Hence, in a vessel containing water-plants in a state of healthy growth, the plants exhale more oxygen than they absorb, and thus replace that which the fishes require for maintaining healthy respiration. Any one who will observe the plants in an aquarium, when the sun shines through the tank, will see the leaves studded with bright beads, some of them sending up continuous streams of minute bubbles. These beads and bubbles are pure oxygen, which the plants distil from the water itself, in order to obtain its hydrogen, and from carbonic acid, in order to obtain its carbon."[A]
[Footnote A:The Book of the Aquarium, by Sidney Hibbert.]
Thus the water, if the due proportion of its animal and vegetable tenants be observed, need never be changed. This is the true Aquarium, which aims to imitate the balance of Nature. By this balance the whole organic world is kept living and healthy. For animals are dependent upon the vegetable kingdom not only for all their food, but also for the purification of the air, which they all breathe, either in the atmosphere or in the water. The divine simplicity of this stupendous scheme may well challenge our admiration. Each living thing, animal or plant, uses what the other rejects, and gives back to the air what the other needs. The balance must be perfect, or all life would expire, and vanish from the earth.
This is the balance which we imitate in the Aquarium. It is the whole law of life, the whole scheme of Nature, the whole equilibrium of our organic world, inclosed in a bottle.
For the rapid evolution of oxygen by plants the action of sunlight is required. That evolution becomes very feeble, or ceases entirely, in the darkness of the night. Some authorities assert even that carbonic acid is given off during the latter period. So, too, they claim that there are two distinct processes carried on by the leaves of plants,—namely, respiration and digestion: that the first is analogous to the same process in animals; and that by it oxygen is absorbed from, and carbonic acid returned to the atmosphere, though to a limited degree: and that digestion consists in the decomposition of carbonic acid by the green tissues of the leaves under the stimulus of the light, the fixation of solid carbon, and the evolution of pure oxygen. The theory of distinct respiration has been somewhat doubted by the highest botanical authority of this country; but the theory of digestion is indisputable. And it is no less certain that all forms of vegetation give to the air much more free oxygen than they take from it, and much less carbonic acid, as their carbonaceous composition shows. If fresh leaves are placed in a bell-glass containing air charged with seven or eight per cent. of carbonic acid, and exposed to the light of the sun, it will be found that a large proportion of the carbonic acid will have disappeared, and will be replaced by pure oxygen. But this change will not be effected in the dark, nor by any degree of artificial light. Under water the oxygen evolved from healthy vegetation can be readily collected as it rises, as has been repeatedly proved.
Why carbonic acid is, to a limited degree, given off by the plant in the night, is merely because the vital process, or the fixation of carbon and evolution of oxygen, ceases when the light is withdrawn. The plant is only in a passive state. Ordinary chemical forces resume their sway, and the oxygen of the air combines with the newly deposited carbon to reproduce a little carbonic acid. But this must be placed to the account of decomposing, not of growing vegetation; for by so much as plants grow, they decompose carbonic acid and give its oxygen to the air, or, in other words, purify the air.
It has been found by experiment, that every six pounds of carbon in existing plants has withdrawn twenty-two pounds of carbonic acid gas from the atmosphere, and replaced it with sixteen pounds of oxygen gas, occupying the same bulk. And when we consider the amount of carbon that is contained in the tissues of living, and of extinct vegetation also, in the form of peat and coal, we may have some idea of the vast body of oxygen which the vegetable kingdom has added to the atmosphere.
And it is also to be considered, that this is the only means we know of whereby free oxygen is given to supply the quantity constantly consumed in respiration, combustion, and other vast and endless oxygen-using processes. It follows, therefore, that animals are dependent upon plants for their pure oxygen, as well as for their food. But the vegetable kingdom might exist independently of the animal; since plants may derive enough carbon from the soil, enriched by the decaying members of their own race.
There is, however, one exception to the law that plants increase the amount of oxygen in the air. During flowering and fruiting, the stores of carbon laid up in the plant are used to support the process, and, combining with the oxygen of the air, both carbonic acid and heat are given off. This has been frequently proved. In large tropical plants, where an immense number of blossoms are crowded together, the temperature has risen twenty to fifty degrees above that of the surrounding air.
As most of the aquatic plants are cryptogamous, or producing by spores, and not by flowers, it seems probable that the evolution of carbonic acid and heat is much less in degree in them, and therefore less in the water than in the air. We may, therefore, venture to lay it down as a general principle, that plants evolve free oxygen in water, when in the sunlight, and remove the carbonic acid added to the water by the respiration of the animals.