shades of green, from dark olive to emerald, of red, and of blue, from purple to sky blue, which we find in the Algæ, are very much the prevailing tints of the leaves and flowers of aerial plants. And again the reddish and yellowish brown hues of some Batrachospermeæ, Lyngbyæ, Desmids and Diatoms, correspond closely with the shades assumed by the leaves of trees, shrubs, and herbs, after they have lost their summer verdure, and to which is due the picturesqueness of their autumnal foliage. Many Confervoids, which when young have green endrochrome, assume in more adult age a yellowish or brownish hue, and the analogy between this change and that which occurs in the leaves of the oak, the ash, the elm, and other trees, is at least both striking and suggestive.
Many Algæ exhibit colours which cannot be referred to autumnal influences, or to the effects of age. The prevailing tints among some of the Volvocineæ and Oscillatorieæ have strong points of resemblance with those of the flowers of phanerogamous plants. In the latter the diversities of colour appear to be connected in some way (of which various explanations have been advanced) with the multiplication of the plant, and so we find that the flowers are the parts which are most liable to variations of colouration. On the other hand, in these simple organisms there is no division into stem, leaves, and flowers, almost every portion is concerned in the process of reproduction—each filament or frond represents a perfect herb, shrub, or tree, and every sporiferous cell is the analogue of a flower. In the phænogamic class the floral colours are useful as attractions to insects of various kinds, which, visiting them for food, carry away the pollen to other flowers, and so conduce to their fertilization. Although, so far as I am aware, no observations have been made on the subject, is it not something more than possible that the multitudes of Infusoria, Rotatoria, Paramecia, etc., which we continually meet with seeking their food amongst the Algæ, may assist in the same way as insects in conveying antheridial spores from one plant to another; and that the varying colours of the filaments may be attractive to them as those of flowers are to insects; and that thus may be reproduced in the subaqueous world some of those phenomena with which we are familiar in the aerial? Should future observation verify this conjecture we shall see amongst the Algæ the exact analogue of the entomophilous fertilization of flowers, and also be able to understand why the various and beautiful tints they exhibit are, to a certain though much less extent, reproduced in the filaments and fronds of the fresh-water Algæ.
In submerged vegetation anemophilous fertilization is of course out of the question, yet even here a substitute appears to have been afforded by the provision of cilia to the androspores and zoospores, to enable them to perform the requisite movements through the water which is their home,
