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.
[Illustration: 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.