From the simplicity of diatoms the vegetation of the sea can be traced upward through larger and more complicated kinds of plants until we reach the enormous algæ that break the gloom of black headlands by their brilliant tints, and furnish a lurking-place under their wide-spreading and dense foliage for hosts of marine animals—some hiding for safety, others to watch for prey. Seaweeds grow in all latitudes, even close to the pole, but mainly along the shore, for below the depth of about one hundred fathoms none but microscopic forms are known. These latter float about, of course, and many of them have been thought to be animals because they seem able to move at their own will. They come to the surface as well as haunt the depths; and the Red Sea takes its name from the fact that a minute carmine-tinted alga occasionally rises to the surface in throngs so dense and wide as to tinge the water for miles at a stretch. The same thing occurs in the Pacific, where the sailors call it “sea-sawdust.” The proper home of the seaweed, however, is a rocky shore between tide-marks or just below them, and it is because the eastern coast of the United States is deficient in rocks—at least south of Cape Cod—that this is poor in algæ, compared with other regions. The seaweed has no roots, and only clings to the rock for support; shifting sand therefore would not hold it, and there are great sandy deserts under the ocean, bare of algæ, as some land regions are sandy deserts naked of terrestrial plants. It often happens, however, that masses of weed will be torn away from their moorings and set adrift. This does not necessarily kill them, for they go on flourishing while afloat, and such is supposed to be the origin of those great areas of “gulfweed” vegetation in mid-ocean called “sargasso seas.” You will remember that a branch of the Gulf Stream, striking over toward the Moorish coast of Africa, is turned southward there, and sweeps down to the equator, then westward again, circumscribing a broad region in the middle Atlantic whose only currents go round and round in a slow whirlpool; and here it is that the gulfweed concentrates in masses sometimes dense enough to impede the progress of a ship—Columbus reported among the wonders of his first voyage the trouble he had in sailing through it—and covering an area between the Azores and the Bahamas as large as the Mississippi valley. This is the Sargasso Sea ordinarily referred to in books, but it is not the only one. A thousand miles west of San Francisco there is a similar collection of floating plants, and others exist under like conditions in the southern oceans. [Illustration: THE MARBLED ANGLER ON ITS GULFWEED RAFT.]