Fig. 35.—A Cell of Gymnozyga brebissonii, external view showing the distribution of the pores. B A portion of the membrane of Staurastrum bicorne with pores containing protoplasmic projections. C Cell-wall of Hyalotheca mucosa during cell-division: the central part, being already formed, shows the connection with the divisional wall.
The Desmidiaceæ are not able to swim independently, many species, however, show movements of different kinds by rising and sliding forward on the substratum. These movements, which are partly dependent upon, and partly independent of light and the force of gravitation, are connected with the protrusion of a mucilaginous stalk. The mucilage, which sometimes surrounds the whole individual, may acquire a prismatic structure, it is secreted by the protoplasmic threads which project through certain pores definitely situated in the walls (Fig. [35] A, B).
Vegetative multiplication takes places by division. A good example of this is found in Cosmarium botrytis (Fig. [36] A-D). The nucleus and chromatophores divide, and simultaneously the central indentation becomes deeper, the outer wall is then ruptured making a circular aperture through which the inner wall protrudes forming a short, cylindrical canal between the two halves to which it is attached (Fig. [36] C). After elongation the canal is divided by a central transverse wall, which commences as a ring round its inner surface and gradually forms a complete septum. The dividing wall gradually splits, and the two individuals separate from each other, each one having an old and a new half. The two daughter-cells bulge out, receive a supply of contents from the parent-cells, and gradually attain their mature size and development (Fig. [36] B-D). Exceptions to this occur in some forms.
Fig. 36.—Cosmarium botrytis. A-D Different stages of cell-division.
Fig. 37.—Cosmarium meneghinii: a-c same individual seen from the side, from the end, and from the edge; d-f stages of conjugation; g-i germination of the zygote.
Conjugation takes place in the simplest way in Mesotænium, where the two conjugating cells unite by a short tube (conjugation-canal), which is not developed at any particular point. The aplanogametes merge together after the dissolution of the dividing wall, like two drops of water, almost without any trace of preceding contraction, so that the cell-wall of the zygote generally lies in close contact with the conjugating cells. The conjugating cells in the others lie either transversely (e.g. Cosmarium, Fig. [37] d; Staurastrum, etc.), or parallel to one another (e.g. Penium, Closterium, etc.), and emit a short conjugation-canal (Fig. [37] d) from the centre of that side of each cell which is turned towards the other one. These canals touch, become spherical, and on the absorption of the dividing wall the aplanogametes coalesce in the swollen conjugation-canal (Fig. [37] e), which is often surrounded by a mucilaginous envelope. The zygote, which is often spherical, is surrounded by a thick cell-wall, consisting of three layers; the outermost of these sometimes bears thorn-like projections, which in some species are simple (Fig. [37] f), in others branched or variously marked; in some, however, it remains always smooth (e.g. Tetmemorus, Desmidium). Deviation from this mode of conjugation may occur within certain genera (e.g. Closterium, Penium). Upon germination the contents of the zygote emerge, surrounded by the innermost layers of the wall (Fig. [37] g, h) and generally divide into two parts which develop into two new individuals, placed transversely to each other (Fig. [37] i); these may have a somewhat more simple marking than is generally possessed by the species.
