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filamentous kinds which everyone would recognise as first cousins of the familiar green seaweeds, an immense number of minuter forms, the relationship of which to their more robust kindred is not so evident, and at least two vast groups, totally different in external aspect, rich in the tenderest colours and midst exquisite shapes.
But the whole order is especially attractive to the Botanist, not only by reason of its singular gracefulness and beauty, but because in no other can he watch so easily the mysterious fundamental processes of cell-division and of reproduction. "To penetrate everywhere to these first rudiments of structure, to follow out from them the course of the development of the tissues of all parts, and to make out the laws according to which the ¢ell-formation progresses to produce the various arrangements on which the structure of the plant essentially depends, is one of the most different, but at the same time most profitable tasks."[1]
The facility with which these plants can be kept alive for a length of time—often long enough to enable the observer to trace in one individual its entire life-history—the translucency of their coll walls, which lays open to his observation under the microscope the active processes going on within; the many points in their morphology still awaiting solution, and the comparative ease with which they may be preserved for an indefinite period with little loss of their natural form; all these are grounds upon which they possess a high degree of interest, and challenge a more extensive study than they generally receive.
2nd.—Where are the Freshwater Algæ to be sought?
One is almost tempted, from their universal diffusion, to reply, "everywhere." It is, at any rate, safe to answer, "wherever moisture oy fresh water is to be found—on the pots and walls in a greenhouse, on the shady sides of tree-trunks, on damp banks, on the moist faces of old walls, in the dripping from water-taps, in every ditch, in the hoof-holes where cattle have trodden tu marshy ground, on thatched roofs, in hogs, on moist moorlands; above all, in every clear pool, lake, and mountain tarn, in cold springs and hot springs, flouting on the surface of water wherever it is found, clinging as parasites to submerged roots, sticks, or larger water-plants, or entangled among bag mosses and the like.”
In describing the principal families we shall revert to their habitats and give some hints as to the signs by which their presence may be recognised.
3rd.—Let us now proceed to consider briefly the principal orders into which they naturally fall. Omitting, however, for convenience, that vast group of minute brittle siliceous organisms, the Diatomaceæ, whose vegetable character was so long disputed, and is not even now universally admitted, with which every Microscopist is familiar, inasmuch as their amazing variety of form and the great beauty of their sculptured markings lave long caused them to stand foremost among the preparations of dealers and the objects of popular exhibition.
We begin, therefore, with the large and universally distributed division of Unicellular Algæ, which form (with the exception of the
- ↑ A. Braun. "The Phenomenon of Rejuvenescence in Nature," p. 123, (Ray Society, 1853.)