Nothing can exceed the vividness of colour or massiveness of the endochrome or soft internal matter of the floating diatoms, that matter which diminishes their specific gravity and makes the plant buoyant which otherwise would be weighed down by its silicious coat. At those periods in which the structural and reproductive phenomena proceed most vigorously, their position in depth must be fluctuating; hence they approach and vanish from the surface. Their growth is perfected by the heat and light which penetrates the sea in calm weather.
Diatoms are social plants crowded together in vast multitudes. Dr. Wallich met with an enormous assemblage of a filamental species of Rhizoselenia, which is from six to twenty times as long as it is broad, aggregated in tufted yellow masses, which covered the sea to the depth of some feet, and extended with little interruption throughout six degrees of longitude in the Indian Ocean. They were mixed with glistening yellow cylindrical species of such comparatively gigantic size as to be visible to the naked eye.
Other genera constitute the only vegetation in the high latitudes of the Antarctic Ocean. Dr. Hooker observes, that without the universal diffusion of diatoms in the South Polar Ocean, there would neither be food for the aquatic animals, nor would the water be purified from the carbonic acid which animal respiration, and the decomposition of matter, produce. These small plants afford an abundant supply of food to the herbivorous mollusca and other inhabitants of the sea, for they have been found in the stomachs of oysters, whelks, crabs, lobsters, scallops, &c. Even the Noctiluci, those luminous specks that make the wake of a boat shine like silver in a warm summer night, live on the floating pelagic diatoms, and countless myriads are devoured by the enormous shoals of salpi and other social marine animals.
The silicious shells of the diatoms form extensive fossil deposits in various parts of the globe, containing species which have long ceased to exist, and others that are identical with those still alive even in their most minute and delicate engravings. The polishing slate of Bilin in Bohemia, which occurs in beds 14 feet thick, and the Tripoli and Phonolite stones on the Rhine consist entirely of the silicious coats of diatoms, while the city of Richmond in Virginia stands upon a marine deposit of the debris of diatoms 13 feet thick, and of unknown extent. Near the Mediterranean, very extensive strata, consisting almost entirely of marine Diatomaceæ, alternate with calcareous strata chiefly formed of Foraminifera, the latter being a race of microscopic mollusca. The fossil Diatomaceæ at Oran in Algeria are particularly perfect and beautiful. In many of these deposits existing species are found.
The trade winds bring over large quantities of dust mixed with diatoms, which sinks through the upper into the lower current, blowing over America, and at last falls in Europe. Professor Ehrenberg found that this dust contained chiefly true American species, many of which were identical with forms existing at the bottom of the Antarctic Ocean, where an area of 4,800 square miles was discovered by Sir James Ross skirting the volcanic coast of Victoria Land, consisting of the remains of these microscopic plants, which have deposited their silicious valves at death for countless generations, producing geological changes of enormous magnitude; while a still greater area of sea-bed in the North Atlantic is the perpetual grave-yard of myriads of microscopic mollusca. Thus the Supreme Being, whose power is stupendously manifested in the motions of the celestial bodies, creates generations of infinitesimal creatures, adorns them with exquisite beauty, and makes them His agents to form future continents.
The Confervaceæ are a numerous tribe of pretty little plants, usually of a green colour, growing in fresh and salt water, on moist ground, wet rocks, and thermal springs. There is scarcely a gently running stream in which they may not be seen, like bunches of green threads, attached to stones and waving in the current; some are so soft as to become almost a mass of jelly when taken out of the water. They are sometimes branched, but more frequently simple, formed of cylindrical cells, joined in a single long row by their flat ends, and they increase in length by the bisection of their terminal cells.
Cell multiplication in Conferva glomerata:—A, portion of filament with incomplete separation at a, complete partition at b; B, the separation completed; C. formation of additional layers of cellulose wall.
In unicellular plants bisection is an act of reproduction; in the multicellular Confervæ it is an act of growth and extension which is accomplished as follows:—The terminal cell of the plant grows to twice its length, the matter within the primordial cell spontaneously divides into two equal parts, and both the film and cellulose coat which cover it, bend round, and form a double layer or cellulose division between them. This cellulose layer extends over the whole exterior of the primordial cell, so that the new cellulose division or septum becomes continuous with a new layer which is formed throughout the interior of the cellulose wall of the original cell. In this manner two perfect cells are formed out of one, and as the extreme cell may undergo the same process, the growth of the plant may be continued indefinitely. Branches are sometimes formed by buds springing from any part of the stem; though apparently so different, it results from the subdivision of the cell which produces the bud.
