The miscellaneous botanical works of Robert Brown/Volume 1/On the origin and mode of propagation of the Gulf-weed
ON THE ORIGIN
MODE OF PROPAGATION
ROBERT BROWN, ESQ.,
PRESIDENT OF THE LINNEAN SOCIETY.
ORIGIN AND MODE OF PROPAGATION
OF THE GULF-WEED.
Read before the Linnean Society, May 7, 1850.
Read a letter, dated May 19th, 1845, addressed by the President to Admiral Sir Francis Beaufort, for communication to Baron Alexander von Humboldt, "On the Origin and Mode of Propagation of the Gulf-weed." The letter is as follows:—
"My dear Captain Beaufort,—I am vexed to have kept Baron Humboldt's letter so long, and now in returning it, that it should be accompanied by so little satisfactory information on the only one of its queries with which I could have been supposed to deal, namely, that which relates to the origin and mode of propagation of the Gulf-weed.
"On this subject it appears that M. de Humboldt (in his Personal Narrative) first supported the more ancient notion, that the plant, originally fixed, was brought with the stream from the Gulf of Florida, and deposited in what Major Rennell calls the recipient of that stream. More recently, however, Baron Humboldt has adopted the opinion, also held by several travellers, that the Gulf-weed originates and propagates itself where it is now found. To the adoption of this view it appears that he has been led chiefly by the observations of the late Dr. Meyen, who in the year 1830 passed through a considerable portion of the great band of 78] Gulf-weed, and who ascertained, as he states, from the examination of several thousand specimens, that it was uniformly destitute both of root and fructification; he concludes, therefore, that the plant propagates itself solely by lateral branches; he at the same time denies that it is brought from the Gulf of Florida, as, according to his own observation, it hardly exists in that part of the stream near the great band, though found in extensive masses to the westward. I have here to remark that, as far as relates to the absence of root and fructification, Meyen has only confirmed by actual observation what had been previously stated by several authors, particularly by Mr. Turner (in his 'Historia Fucorum,' vol. i, p. 103, published in 1808), and Agardh (in his 'Species Algarum,' p. 6, published in 1820). But Meyen materially weakens his own argument in stating that he considers the Gulf-weed (Sargassum bacciferum of Turner and Agardh), and the Sargassum natans, or vulgare, specifically distinguished from it by these authors, as one and the same species; adding, that he has observed among the Gulf-weed all the varieties of Sargassum vulgare described by Agardh; and finally, that on the coast of Brazil he has found what he regards as the Gulf-weed in fructification. Now, as Sargassum natans has been found fixed by a discoid base or root, in the same manner as the other species of the genus, and as according to Meyen the Gulf-weed has been found in fructification, the legitimate conclusion from his statements seems to be, that this plant is merely modified by the peculiar circumstances in which it has so long been placed. I am not, however, disposed to adopt Dr. Meyen's statement that he actually found the true Sargassum natans, much less all its supposed varieties, mixed with the Gulf-weed, having reason to believe that at the period of his voyage his practical knowledge of marine submersed Algae was not sufficient to enable him accurately to distinguish species in that tribe. It is not yet known what other species of Sargassum are mixed with the Gulf-weed, what proportion they form of the great band, nor in what state, with respect to root or fructification, they are found; though, in reference to the questions under discussion, accurate information on these points would be of considerable importance.
"That some mixture of other species probably exists may be inferred even from Dr. Meyen's statement, and indirectly from that of Lieut. Evans, who, in his communication published in Major Rennell's invaluable work on the Currents of the Atlantic, asserts that he found the Gulf-weed in fructification, which he compares with that of Ferns, a statement which would seem to prove merely that he had found along with the Gulf-weed a species of Sargassum with dotted leaves, the real fructification of the genus bearing [79 no resemblance to that of Ferns, though to persons slightly acquainted with the subject the arranged dots on the leaves might readily suggest the comparison.
"With regard to the non-existence of roots in the Gulf-weed as a proof of specific distinction, it is to be observed that the genus Sargassum, now consisting of about sixty species, is one of the most natural and most readily distinguished of the family Fucacæe, and that there is no reason to believe that any other species of the genus, even those most nearly related to, and some of which have been confounded with it, are originally destitute of roots; though some of them are not unfrequently found both in the fixed and in considerable masses in the floating state, retaining vitality and probably propagating themselves in the same manner (see Forskål, Fl. Ægypt.-Arab., p. 192, n. 52). It is true, indeed, that a Sargassum, in every other respect resembling Gulf-weed, has, I believe, not yet been found furnished either with roots or fructification, neither Sloane's nor Browne's evidence on this subject being satisfactory. But the shores of the Gulf of Florida have yet not been sufficiently examined to enable us absolutely to decide that that is not the original source of the plant; and the differcnces between the Gulf-weed and some other Sargassa, especially S. natans, are not such as to prove these two species to be permanently distinct. The most remarkable of these differences consists in the leaves of the Gulf-weed being uniformly destitute of those dots or areolæ so common in the genus Sargassum, and which are constantly present in S. natans. These dots, in their greatest degree of development, bear a striking resemblance to the perforations or apertures of the imbedded fructification in the genus. But as the receptacles of the fructification, as well as the vesicles, are manifestly metamorphosed leaves; and as the production of fructification is not adapted to the circumstances in which the Gulf-weed is placed, it is not wholly improbable, though this must be regarded as mere hypothesis, that the propagation by lateral branches, continued for ages, may be attended with the entire suppression of these dots.
"That the Gulf-weed of the great band is propagated 80] solely by lateral or axillary ramification, and that in this way it may have extended over the immense space it now occupies, is highly probable, and perhaps may be affirmed absolutely without involving the question of origin, which I consider as still doubtful.
"My conclusion, therefore, is somewhat different from that of Baron Humboldt, to whom I would beg of you to forward these observations, which will prove that I have not been inattentive to his wishes and to your own, though they will at the same time prove that I have had very little original information to communicate."
- Histoire de la Géographie du Nouveau Continent, vol. iii, p. 73, and Meyen, Reise, vol. i, p. 36–9.
- See Sloane's Jam. i, p. 59. I have examined Sloane's specimens in his Herbarium; they belong to Gulf-weed in its ordinary form, and are alike destitute of root and fructification; hence they are probably those gathered by him in the Atlantic, and not those which he says grew on the rocks on the shores of Jamaica. Browne's assertion to the same effect is probably merely adopted from Sloane.