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288 mingled with the mud at the bottom;—others again are only discoverable by the microscope, or by the fact that, when they exist in vast numbers, they impart to the water a distinctive colour. But, in whatever situation or habitat these Algæ are found, microscopic analysis reduces them all to the same elements as exist in the higher aerial plants—the vegetable cell—composed of an outer cellulose coat, a primordial utricle, and within this the coloured cell-contents, the endochrome, in which its vital activity is situated.

A comparison of these subaqueous plants with their terrestrial congeners would form a most interesting subject of enquiry, but one of such vast dimensions that I can only venture to touch upon one or two of its most salient points this evening.

Probably the typical form of the vegetable cell is a sphere. In all plants, however, except the very simplest—the unicellular—the spheres by aggregation become changed into various other figures, by mutual compression, and by their growing in the lines of least resistance. Thus we have the free globular cells of the Volvocineæ; the cylindrical ones of the Confervæ, the Zygnemaceæ, etc., corresponding to the elongated cells of vascular and woody tissue;—the quadrangular, polygonal, and irregular cells of Ulvaceæ, Pediastreæ and Desmidiaceæ, which find their analogues in many parts of the epidermis, the expanded portion of leaves, the petals, etc., of the higher plants. Again the markings in dotted, spiral, and glandular vessels, are very similar in appearance, if in nothing else, to the markings in Lyngbyæ, Spirogyræ, Calothrices, etc. It is singular to notice also, how, under some circumstances, the cell appears to endeavour to revert to its typical form, as in pl. XXIII., fig. 10, where the front view of the pediastrum shows a complex geometrical outline, and the side view exhibits four simple circular cells.

Into the question of the modes of combination of these algal cells, and the exquisitely beautiful geometrical figures they often form, or of the siliceous patterns secreted by the Diatoms, it is not my purpose to enter at present, though doubtless they have analogues in the shapes and forms of various flowers, and the arrangements of the elements of many leaf-buds.

The colour of the endochrome of the fresh water Algæ varies nearly as much as it does in flowering plants. In most it is green; in some, as the Oscillatorieæ, it varies from light green through various shades of blue and purple to black; in the Protococci again, we meet with different and often brilliant tints of red, and lastly in some Desmids and the majority of Diatoms with a reddish or yellowish brown hue, although the endochrome of many Diatoms is, in early life, of a brilliant green colour. Taking the fresh-water Algæ altogether, and comparing them with the leaves and flowers of the aerial plants, there appears to be a strong resemblance between the colours exhibited by these two extremes of the vegetable kingdom. The various