During this voyage it was first observed, as I have previously mentioned, on the 31st of March, in latitude 22° north, and 41° west longitude; and was seen in large quantities nearly the whole distance. From the trade wind being much to the northward, we were driven as far to the westward as 44° 7′, still continuing to have a quantity of weed about the ship; indeed it rather increased than diminished, for large masses were seen even thus far beyond the usual limit assigned to it; on the 12th of April extending to 38° 32′ north latitude, and 34° 30′ west longitude. The utmost western limit I had an opportunity of observing it in being 44° 7′, and the range of the temperature of the atmosphere in its limits being maximum 76°, medium 70°, minimum 59°.
Of the genus Sargassum[95] there are numerous species distributed over the globe; but the S. vulgare, or Fucus natans, and other species, are also described as having their habitat in the particular range I have before mentioned; but I have not been able to find more than one species, some sprigs of which exhibit anomalies, but not specific differences; for although apparently differing in having the leaves broader, and not so serrated at the edges, yet many of these were growing from plants which had not such distinctions. According to Greville, this genus, the most extensive of the Fucoideæ, comprising above seventy species, is nearly confined to the two tropics, and examples rarely occur beyond the forty-second degree in either hemisphere:—
“Flung from the rock on ocean’s foam to sail,
Where’er the surge may sweep, the tempests breath prevail;”
will only now be partly applicable to this weed, as it is tolerably well ascertained to vegetate floating on the water, each sprig becoming, as detached by the violence of the waves, the contact of ships, or other causes, a perfect and large plant.[96]
This species of the Sargassum, found in such immense quantities, floating upon the surface of the ocean, is of a fine yellow colour, lighter or deeper in tint, being when very young of a greenish yellow colour; it is very buoyant, occasioned both by its lightness of structure and body of water to support it, as well as by the number of air vesicles with which it is profusely covered; it has a handsome appearance when seen spread out and swimming on the surface, or when just taken out of the water. On drying, it first retains the yellow, or greenish yellow, afterwards becoming of a reddish brown colour, but turning black, if exposed to the atmosphere when drying. The leaves are long, narrow, scattered, serrated at the edges in an irregular manner; the stems are studded with numerous air-vesicles in different stages of growth, spherical, and attached by a short pedicle to the stem. In many instances a young leaf would be seen emerging from the air-vesicle, and in others many of the leaves would have the air-vesicle (instead of being, as usual, on the stems) at the summit, forming a rounded termination to the leaf; some of the vesicles would have a longer and broader pedicle than usual, which assumed the character of an embryo leaf just forming, similar to what has been just mentioned as occurring.[97] All the weed was more or less profusely covered with parasitical confervæ, displaying much delicacy and beauty.
I succeeded in capturing with the weed numerous specimens of small crabs, and some even of large size, small nereis, together with various specimens of fish, the Syngnathus, or pipe-fish, &c. The Scyllea pelagica was also abundant, clinging to the plants; and also numerous small sepiæ of a beautiful purplish colour.
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.