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Freshwater Algæ.

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Freshwater Algæ.^[1]

By A. W. Wills, F.C.S.

We must now turn to the very important question of the modes of multiplication and reproduction of these singular plants.

In the simplest forms of unicellular Algæ, (Palmellaceæ and Volvocineæ,) the only mode by which most of the genera seem to be perpetuated consists of the conversion or breaking up of the contents of certain cells into small oval bodies, (zoospores,) which move freely through the water for a While by means of delicate lashes or cilia, and then, after lasing these motile organs, undergo cell-division into fresh individuals, identical with the parents, [Plate I., Fig. 2.)

In Desmidiaceæ, to this process is superadded that of reproduction by the formation of large "resting-spores" resulting from the commingling of the contents of two separate individuals in the space between their adjacent sides. [Plate II, Fig. 7.] Such spores ore capable of retaining their vitality for a long period, and then producing from their contents a fresh generation of plants of their specific kind; while, on the other hand, zoospores undoubtedly perish, unless placed in conditions favourable to their immediate germination.

In many of the higher order of Algæ (freshwater and marine) both these phenomena can be traced, and if appears probable that in all cases the development of spores alone constitutes a tine sexual reproduction of the species, comparable to that which prevails among Phanerogams, whereby its indefinite perpetuation is secured; while the so-called "reproduction" by zoospores is rather to be compared to the increase of flowering plants by buds or offsets, or artificially by cuttings, a process by which, as is believed by most florists, while the multiplication of individuals is accomplished, the continuation of the species is not secured beyond certain limits.

It is probable that even in those genera in which no such sexuality or polarisation of cell-contents has been observed, it has yet to be discovered, much as the fine researches of Messrs. Dallinger and Drysdale "On the Life History of Monads" have proved, that after the succession of many generations of asexual individuals among the lowest Infusoria, a sexual process suddenly intervenes.

The production of spores in Desmidiaceæ is a most interesting phenomenon, and will be best understood by those to whom it is

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Description or Figures in Plate II.
- Fig. 7.—Formation of resting-spore of Micrasterias rotata, from contents of two plants.
- Fig. 8.—Vaucheria ornithocephala.
- Fig. 9.—Vaucheria racemosa, showing "horns" and flask-shaped cells, the latter containing spores.
- Fig. 10.—Formation of zoospores from contents of single cells inŒdogonium.
- Fig. 11.—Formation of spores in Œdogonium, the contents of the enlarged cell being fertilised by spermatozoids developed in dwarf parasitic male, or antheridial plants.

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Description or Figures in Plate II.

Fig. 7.—Formation of resting-spore of Micrasterias rotata, from contents of two plants.

Fig. 8.—Vaucheria ornithocephala.

Fig. 9.—Vaucheria racemosa, showing "horns" and flask-shaped cells, the latter containing spores.

Fig. 10.—Formation of zoospores from contents of single cells inŒdogonium.

Fig. 11.—Formation of spores in Œdogonium, the contents of the enlarged cell being fertilised by spermatozoids developed in dwarf parasitic male, or antheridial plants.

[2] Fig. 7.—Formation of resting-spore of Micrasterias rotata, from contents of two plants.

[3] Fig. 8.—Vaucheria ornithocephala.

[4] Fig. 9.—Vaucheria racemosa, showing "horns" and flask-shaped cells, the latter containing spores.

[5] Fig. 10.—Formation of zoospores from contents of single cells inŒdogonium.

[6] Fig. 11.—Formation of spores in Œdogonium, the contents of the enlarged cell being fertilised by spermatozoids developed in dwarf parasitic male, or antheridial plants.

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