The plants most suitable for the aquarium are those which grow with their leaves entirely submerged. Large plants like calla, arrowhead, etc., which grow with their leaves in the air, do not act upon the water so effectually as those small plants whose leaves obtain all their carbon from the water itself. Indeed, the plants which are most effective in aerating the water of the aquarium are probably those minute confervas which are so apt to cover the rock and glass with a green coating.
Among the plants which are most useful are the following: Vallisneria spiralis, whose slender tape-like leaves sometimes reach a length of six feet in some of our rivers. It readily accommodates itself to the aquarium, and is not only an excellent aerater, but it harbors hosts of animalcules on which the fish feed greedily. Water millfoil is another excellent plant for our purpose. It may be found in the spring growing in dense masses in deep water. The "seed," a rather peculiar form of bud, may often be found floating in the water in very early spring. This bud, which looks like a round green seed, about half an inch in diameter, will, if placed in the aquarium, soon begin to grow and send out a long stem densely clothed with narrow leaves. It forms no roots, but grows floating in the water, and forms a pleasant shade for the animals in the tank. The anacharis alsinastrum, sometimes called water-thyme, is another excellent plant for this purpose. It is the plant which has caused so much trouble in England by choking up the rivers and canals. Nitella is also a good plant, and is frequently cultivated for the purpose of showing the circulation or rather cyclosis of the sap under the microscope, though a good young leaf of vallisneria is perhaps as good as any for this purpose. The duckweeds, frogbits, and starworts, are also great favorites.
The fact is, however, that the young collector can hardly go wrong. Any of the smaller plants found growing naturally under the water of our ponds and streams answer well for the aquarium; and it is a good lesson in botany to gather them, watch their growth, and learn their names.