In support of the opinion that the attachment of the fuci to rocks is not absolutely necessary for their nourishment, it has been observed of the fucus nodosus, that “this and some other fuci have no dependence on their root for nourishment, and therefore, instead of being ramified, it is merely a disc or button, by the adhesion of which, assisted, perhaps, by atmospherical pressure, the weed keeps an uncommonly firm hold of the rock to which it is attached.” The air vesicles on a plant of course render it more buoyant than those destitute of them. I have found, that detaching the air vesicles from a plant, and placing it in shallow water, it sunk, but where the depth of water was great, the bulk of water was in itself sufficient to keep the weed afloat; the use of the air vesicles is most probably intended for the purpose of bringing some portion of the plant in contact with atmospheric air, or by keeping the plant to the surface of the water, to receive a greater benefit from air and light, or to prevent the young shoots or other parts of the plant being injured by the violent action of the waves. We find in the fucus buccinalis, or trumpet weed of the Cape, that stem is hollow, accommodating itself in length to the depth of water in which it grows; is attached to the ground by ramified roots; the stem terminating in a crown of broad leaves expanded on the surface of the water, and kept in that situation by a broad air bladder, in which the stem terminates under the crown; it may aid the plant also in maintaining itself against the force of the waves in the exposed situations in which it is usually found. Air-bladders are not confined to sea weeds; several plants growing in fresh water are similarly provided. Of these, the _Jussiæa tenella_, found in the rivers of Amboyna, is a remarkable example. Along its stalk are many large oval tubercles, full of air, and each of these is compounded of many others, so that the injury which the plant might sustain from foreign bodies striking against it and breaking the bladders, is obviated.[98]