except by one of its own aspect ; for its nearest and, indeed, very near ally, is a native of New Holland; whilst in size, luxuriance and beauty of growth, the present surpasses not oidy all other species of the genus, but almost the whole of the group Cystoseirea.
We are accustomed to regard the ocean as so ever-active and powerful an agent in facilitating migration, and its uniform temperature is so conducive to the general diffusion of species, that it seems almost wonderful that Alga should have limits to their distribution, especially in waters which gird the globe on the same parallel of latitude, and whose unchecked swells and currents literally extend over every degree of longitude. The remarkable increase in temperature of the tropical over the polar seas of the Atlantic may, and probably alone does, check the progress of the Macrocystis in its course from Cape Horn to the Equator in that ocean, for, as I shall afterwards show, the same sea-weed can float with the colder currents of the Pacific from the same Cape to Behriug's Straits; but no such obstacle prevents the fullest interchange of Cystoseirea between New Zealand and the temperate seas of South America. It, however, is the fact, that whilst this group literally abounds in certain latitudes and longitudes, which are those of New Holland and the West Pacific, they are nearly absent from analogous positions in the longitude of South America.
Throughout all latitudes the two tribes Fucoidea and Cystoseirea form that prevailing marine vegetation to which the name sea-weed is commoidy applied; and the different genera so far arrange themselves within geographical limits as to present, with such few exceptions as the Scytothalia Jacauinotii, a most harmonious assemblage. Thus, in the opposite colder and frigid zones the waters are inhabited by certain genera of Fucoidea which are in a great measure representatives of one another; as, in Fucus proper, and "1 are represented in analogous 1 B'Urvillea, Himanthalia, southern zones, by J SarcopJ/ycus, and Kiitz.
None of these genera approach the tropics, for the Fucoidea abound towards the poles, and there attain their greatest bulk, diminishing rapidly towards the Equator, and ceasing some degrees from the Line itself. The representatives of the Cystoseirea in the higher latitudes of the opposite hemisphere, are equally appropriate with those of Fucoidea, for we have in
f Cystoseira, and "| represented in the f Blossevillea, and Halidrys, [ south cool zone, by 1 Scytothalia •
whilst the immense genus Sargassum finds its maximum in lower latitudes, and under the Equator itself.
Such are the salient featm - es of the distribution of these tribes, which are not influenced by the minor divisions, chiefly local assemblages of small genera, affecting exclusively certain coasts or bays.
3. LESSONIA, Bory.
1. Lessonia fuscescens, Bory, in Buperrey Yoy. Bot. Crypt, p. 75. t. 2. f. 2. et t. 3. Post, et Ritppr. Elust.AIg. p. 2. t. 3 et p. 4. t. 39. f. 14-18. L. flavicans, IfUrviUe, in Mem. Soc. Linn. Paris, vol. iv. p. 594. (Tab. CLXVIL, CLXYIH. A., and Tab. CLXXI. B.)
Hab. Hermite Island, Cape Horn, and Falkland Islands ; most abundant, always far beyond low-water mark.
Christmas Harbour, Kerguelen's Land ; rare The fructification of the species of Lessonia occurs, as in Macrocystis, upon the surface of the fronds, and there forms large patches. In the present species the sori are situated beyond the middle of the leaf, they are oblong and nearly as broad as the lamina, of which they carry away the upper part when decaying, causing their broad apices to be two-horned. In none of our specimens is the point perfect, all the spores we have seen being situated on the edges of the sorus, which has itself fallen away from the frond. The air-cells are less numerous, and the spores are smaller, shorter, more densely packed than in the following species, and covered
