As the forest must be planted before its denizens can luxuriate in its shades, so the submarine shrubbery of the Aquarium must be perfected before the aquatic animals can be introduced. For it has been shown, in tracing the history of the experiments which resulted in the establishment of the principles that regulate the formation of Aquaria, that it is by plants only that a supply of oxygen can be kept up, sufficient for the health and existence of all forms of animal life beneath the water.[1] It is necessary, also, that the rays of sunlight should fall upon the foliage directly through the surface of the water; and when an Aquarium, with its plants, is placed in a position to receive the light in this manner, their fronds may be observed giving forth the gas in small silvery bubbles and corruscations, which have a brilliant and gem-like appearance. [Footnote 1: Analogous principles are at work in our fields and forests, but we have now only to do with the submarine production of oxygen.] Some marine plants appear to succeed much better than others, but I believe that happens only from their treatment being imperfectly understood; and I believe, not only that all the exquisitely beautiful marine _Algæ_ of our own shores may be successfully grown, but also that the more splendid varieties of the tropical seas may be made to thrive in properly-heated Aquaria, and thus form one of the most attractive features of our hot-houses—one that has not yet been dreamed of. In ordinary Aquaria, such as I am now treating of, I shall name first those species of sea-weed recommended by Mr. Gosse and others as most easily cultivated, but I shall also point out many other species, which I feel convinced may be successfully grown under proper management; and they certainly deserve every effort that can be made to establish them in Aquaria, as they are among the most beautiful of their tribe. If not purchased of dealers, the plants must be very carefully collected by the amateur himself, taking care to detach a portion of the substance to which they are growing, and packing them in damp refuse sea-weed, keeping them out of their native element as short a time as possible.