Fig. 38.—Desmidiaceæ. A Closterium moniliferum; B Penium crassiusculum; C Micrasterias truncata (front and end view); D Euastrum elegans; E Staurastrum muticum (end view).
The most frequent genera are:—
A. Solitary cells: Mesotænium, Penium (Fig. [38] B), Cylindrocystis, Euastrum (Fig. [38] D), Micrasterias (Fig. [38] C), Cosmarium (Fig. [36], [37]), Xanthidium, Staurastrum (Fig. [38] E), Pleurotænium, Docidium, Tetmemorus, Closterium (Fig. [38] A), Spirotænia.
B. Cells united into filaments: Sphærozosma, Desmidium, Hyalotheca, Gymnozyga, Ancylonema, Gonatozygon.
Order 2. Zygnemaceæ. Cell-wall without markings. The cells are cylindrical, not constricted in the centre, and (generally) united into simple, unbranched filaments. The whole contents of the conjugating cells take part in the formation of the zygote, which on germination grows out directly into a new filament.
Spirogyra is easily recognised by its spiral chlorophyll band; Zygnema has two star-like chromatophores in each cell (Fig. [40]); both these genera are very common Algæ in ponds and ditches.
Fig. 39.—Spirogyra longata. A At the commencement of conjugation, the conjugation-canals begin to protrude at a and touch one another at b; the spiral chlorophyll band and cell-nuclei (k) are shown. B A more advanced stage of conjugation; a, a’ the rounded female and male aplanogametes: in b’ the male aplanogamete is going over to and uniting with the female aplanogamete (b).
Fig. 40.—A cell of Zygnema. S Pyrenoid.