It is an interesting fact that the Sargasso Sea affords the most remarkable example of an aggregation of plants belonging to one single species. Nowhere else, according to Humboldt, neither in the savannas of America, nor on the heaths or in the pine forests of Northern Europe, is such a uniformity of vegetation found as in those boundless maritime meadows.
"The masses of sea-weeds," says Meyen, "covering so vast an extent of ocean have ever since the time of Columbus been the object of astonishment and inquiry. Some navigators believe, that they are driven together by the Gulf Stream, and that the same species of Sargassum plentifully occurs in the Mexican Sea; this is however perfectly erroneous.
"Humboldt was of opinion that this marine plant originally grows on submarine banks, from which it is torn by various forces; I for my part have examined many thousands of specimens, and venture to affirm that they never have been attached to any solid body. Freely floating in the water, they have developed their young germs, and sent forth on all sides roots and leaves, both of the same nature."
Thus the Sargassum seems to be the indigenous production of the sea where it appears, and to have floated there from time immemorial. Its swimming islands afford an abode and nourishment to a prodigious amount of animal life. They are generally covered with elegant sertularias, coloured vorticellas, and other strange forms of marine existence. Various naked or nudibranchiate mollusks and annelides attach themselves to the fronds, and afford nourishment to hosts of fishes and crustaceans, the beasts of prey of this little world.
Similar aggregations of sea-weeds are also met with in the Indian and Pacific Oceans, in the comparatively tranquil spaces encircled by rotatory currents. Their rare occurrence on the surface of the sea may serve as a proof of the restless motion of its waters. Were the ocean not everywhere intersected by currents, it would most likely be covered with sea-weeds, opposing serious, if not invincible obstacles to navigation.
The Red sea-weeds, Rhodosperms or Florideæ, are by far the most numerous in species, and undoubtedly the most beautiful and perfect of all the algæ. They love neither light nor motion, and generally seek the shade of larger plants on the perpendicular sides of deep tide-pools removed from the influences of the tides and gales. They mostly grow close to low-water mark, and are to be seen only for an hour or two at the spring-tides, during which, as is well known, the deepest ebbs take place. To this group belong the wonderfully delicate polysiphonias, callithamnias, plocamias, and delesserias, whose elegant rosy scarlet or purple leaves are the amateur's delight, and when laid out on paper resemble the finest tracery, defying the painter's art to do justice to their beauty. It likewise numbers among its genera the chalky corallines and nullipores, which on account of the hardness of their substance were formerly considered to be polyps, but whose true nature becomes apparent on examining their internal structure.
The Chondrus crispus, or Carrigeen, which grows in such vast quantities on the coasts of the British Isles, also belongs to the rhodosperms, though when growing, as it frequently does, in shallow tide-pools, exposed to full sunlight, its dark purple colour fades into green or even yellowish white. When boiled it almost entirely dissolves in the water, and forms on cooling a colourless and almost tasteless jelly, which of late years has been largely used in medicine as a substitute for Iceland moss. Similar nutritious gelatines, which also serve for the manufacture of strong glues, are yielded by other species of rhodosperms, among others by the Gracillaria spinosa of the Indian Ocean, which the Salangana (Hirundo esculenta), a bird allied to the swallow, is said principally to use for the construction of her edible nest.
The steep sea-walls along the south coast of Java are clothed to the very brink with luxuriant woods, and screw-pines strike everywhere their roots into their precipitous sides, or look down by thousands from the margin of the rock upon the unruly sea below. The surf of incalculable years has worn deep caves into the chalk cliffs, and here the Salangana builds her nest. Where the sea is most agitated whole swarms are observed flying about, and purposely seeking the thickest wave-foam. From a projecting cape, on looking down upon the play of waters, may be seen the mouth of the cave of Gua Rongkop, sometimes completely hidden under the waves, and then again opening its black recesses, into which the swallows vanish, or from which they dart forth with the rapidity of lightning. While at some distance from the coast the blue ocean sleeps in undisturbed repose, it never ceases to fret and foam against the foot of the mural rocks, where the most beautiful rainbows glisten in the eternally rising vapours.
Who can explain the instinct which prompts the birds to glue their nests to the high dark vaults of those deep, and apparently so inaccessible, caverns? Did they expect to find them a safe retreat from the persecutions of man? Then surely their hopes were vain, for where is the refuge to which his insatiable avidity cannot find the way? At the cavern of Gua-gede, the brink of the precipitous coast lies eighty feet above the level of the sea at ebb-tide; the wall first bends inwards, and then, at a height of twenty-five feet from the sea, throws out a projecting ledge which is of great use to the nest-gatherers, serving as a support for a rotang ladder let down from the cliff. The roof of the cavern's mouth lies only ten feet above the sea, which, even at ebb-tide, completely covers the floor of the cave, while at flood-tide the opening of the vast marine grotto is entirely closed by every wave that rolls against it. To penetrate into the interior is thus only possible at low water, and during very tranquil weather; and even then it could not be done, if the rugged roof were not perforated and jagged in every direction. The boldest and strongest of the nest-gatherers wedges himself firmly in the hollows, or clings to the projecting stones, while he fastens rotang ropes to them, which then depend four or five feet from the roof. To the lower ends of these ropes long rotang cables are attached, so that the whole forms a kind of suspension bridge throughout the entire length of the cavern, alternately falling and rising with its inequalities. The cave is 100 feet broad and 150 long as far as its deepest recesses. If we justly admire the intrepidity of the St. Kildans, who, let down by a rope from the high level of their rocky birthplace, remain suspended over a boisterous sea, we must needs also pay a tribute of praise to the boldness of the Javanese nest-gatherers. Before preparing their ladders for the plucking of the birds' nests, they first offer solemn prayers to the goddess of the south-coast, and sometimes deposit gifts on the tomb where the first discoverer of the caverns and their treasures is said to repose. Thus in all zones and in every stage of civilisation, man is directed by an inward voice to seek the protection of the invisible powers when about to engage in a great and perilous undertaking.