Flint-making, like Chalk-making, is not only an old-world and long-past operation, but a present one. Diatoms lived and died in ages gone by, and their remains are found hardened into flint. Diatoms live and die in these days; and their remains may yet, in centuries to come, be transformed into the same substance.
From Diatoms one might range upward through an enormous variety of Ocean-weeds, of all kinds and descriptions.
Though they are the “last and lowest of all the tribes of plants,” yet it is a wide step from the undermost to the uppermost of even this humble Vegetable Tribe.
At least five or six thousand species of Sea-weeds are known to Naturalists. Each ocean, each lesser sea, has its own particular vegetation, largely affected by the varying degrees of saltness, the warmth or coldness of the water, and the faster or slower currents which happen there to prevail.
Sea-weeds of all sizes are found, from the invisible diatoms to enormous growths in Pacific waters, reaching to yards upon yards in length, with solid trunk-like stems, and huge fronds like those of a tropical palm.
Near Tierra del Fuego immense growths have been seen, with stems between three and four hundred feet in length. Great sub-ocean forests of kelp are there, and floating fucus-islands, with leaves or fronds seven or eight feet from base to tip, covered with living animals, and having air-vessels several inches long.
In the matter of colour no great variety exists. Sea-weeds are generally either grass-green, olive-brown, or red. The green are usually close to land, and they never grow beyond touch with direct sunlight. The olive-brown are the more abundant; and the red belong, perhaps, more often to deep water.
A large majority grow attached either to stones or to the shallow sea-bottom, but some kinds float unattached. To this last class belong the masses of weed in the Sargasso Sea—the centre of the North Atlantic currents.
Round that huge collection of weeds and drift, which reaches over something like two hundred and sixty thousand square miles, the entire North Atlantic slowly revolves. And of these weeds—called “Sargassum”—none are attached to rock or ground, but all float loosely in mid-ocean. They seem to flourish thus, though the fact that fructification is not found upon them points to a somewhat unusual condition of things.
Sea-weeds more often multiply by means of spores, which is a form of vegetable-growth inferior to that of seeds. All spore-growing plants, whether on land or in the sea, take a lower rank than seed-growing plants.