As this young Volvox increases gradually in size, the hyaline matter is increased, the green threads lengthen, and the angular masses assume the form of a flask the 1⁄3000th of an inch in diameter exactly as in the Protococcus; for the green matter with a few red spots is collected in the thick end, while the hyaline beak is turned towards the circumference of the sphere, which is pierced by their long cilia. Each of them is invested with a pellucid envelope of considerable thickness, the borders of which are flattened against those of similar envelopes. While these ciliated bodies are approaching maturity their endochrome exhibits vacuoles or apparently empty cavities of a spherical form about one-third of its own diameter. Mr. G. Busk discovered that these vacuoles expand and contract at regular intervals of about forty seconds. The contraction, which almost obliterates the cavity of the vacuole, is rapid and sudden; the dilatation is slow and gradual. This action ceases when the body comes to maturity.
When this mass of zoospores connected by green threads is immature and begins to expand into a hollow sphere, then the central globe is continually bisected so as to form 4, 8, 16, 32, 64, or a greater number of equal and similar parts, each of which is ultimately developed into a zoospore exactly the same with the matured green zoospores on the surface of the primary sphere, so that the ‘Volvox globator is a composite fabric made up of a repetition of organisms in all respects similar to each other,’ which Professor Ehrenberg the first to discover, though he did not investigate the development of the plant.
It appears that certain spheres of the Volvox are monœcious, that is, each sphere contains male and female cells, though the greater number of cells are neutral. The germ or female cells are larger and of a deeper green than the others; the male cells resemble them, but the endochrome within them breaks up symmetrically into a multitude of linear particles aggregated into discoid bundles beset with vibratile cilia, which move about within their cells and soon become decomposed into their component corpuscles. Each of these corpuscles has a linear body, thicker at its posterior end, and furnished with two long cilia. The female cell, when fertilized, gets a smooth envelope, and then a thicker one, beset with conical-pointed processes, and the contained chlorophyll gives place, as in Palmoglœa, to starch and a red or orange coloured oil. It appears that the Volvox stellatus and V. aureus are only phases of the Volvox globator.
The Desmidiaceæ are minute green algæ inhabiting fresh-water pools or slow running streams, never those that are muddy. They are free unicellular plants, sometimes triangular, sometimes cylindrical, crescent or bow-shaped, smooth or spined. So varied are their microscopic forms that a description would be tedious. In plants of such extreme minuteness, the only means of ascertaining the nature of their component materials is by chemical tests. A solution of iodine turns starch blue, and cellulose brown, and thus it is found that the interior of the Desmidiaceæ is occupied by a mass of starch granules, covered with chlorophyll, and mixed with a formative fluid. This mass, enclosed in a delicate membrane, constitutes the primordial cell; it has an exterior coat of firm cellulose, and the whole is more or less enveloped in a gelatinous substance. Like other plants, when in bright sunshine, the Desmidiaceæ decompose carbonic acid gas, give off the oxygen, and assimilate the carbon into chlorophyll.
Fig. 9. Various species of Staurastrum:—A, vestitum; B, aculeatum; C, paradoxum; D, E, brachiatum.
These plants are frequently distinguished by projections from their cellulose coat above their surface, these being sometimes short and conspicuous, but often projected in spines, which form a beautiful symmetrical hyaline border round the green internal cell, as shown in [fig. 9]. Another peculiarity of the Desmidiaceæ is the appearance of their being divided into two symmetrical parts by a satural line, as the name implies, though there is no real division.
Many of the Desmidiaceæ, but more especially the genus Closterium, are remarkable for having a double circulation of the internal fluid in opposite directions, maintained by a vital contractile energy. One current flows between the cellulose horny coat, and the thin film covering the chlorophyll, while the other spreads in a broad stream in the contrary direction between the thin film and the chlorophyll mass, carrying from the latter some of its coloured particles to the extremities of the frond, where there seems to be a connection between the two streams.
Fig. 10. Economy of Closterium Lunula:—A, frond showing central separation; D, frond in a state of self-division.