[Illustration: 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_).] [Illustration: FIG. 40.--A cell of _Zygnema_. _S_ Pyrenoid.] [Illustration: FIG. 41.--_Zygnema insigne_, with zygote.] [Illustration: FIG. 42.--Germinating zygote of _Spirogyra jugalis_: the young plant is still unicellular; the end which is still in the wall of the zygote is elongated and root-like; the chromatophore divides and forms the spiral band.] The conjugation among the Zygnemaceæ takes place in the following manner: the cells of two filaments, lying side by side, or two cells, the one being situated above the other in the same filament (Fig. 41), push out small protuberances opposite each other (Fig. 39 _A_, _a_, _b_); these finally meet, and the dividing wall is absorbed so that a tube is formed connecting one cell with the other; the protoplasmic contents round off, and the whole of these contents of one of the cells glides through the conjugation-tube and coalesces with that of the other (Fig. 39 _B_), the aggregate mass then rounds off, surrounds itself with a cell-wall, and becomes a zygote. A distinct difference may be found between the cells in the two filaments, those in the one whose protoplasmic contents pass over being cylindrical, while those of the recipient one are more barrel-shaped, and of a larger diameter. The former may be regarded as a male, the latter as a female plant. The zygote germinates after a period of rest, and grows out into a new filament (Fig. 42). Order 3. =Mesocarpaceæ.= The cell-walls are glabrous, unconstricted in the centre, and united into simple unbranched filaments. The chromatophore consists of an axial chlorophyll-plate, with several pyrenoids. The zygote is formed by the coalescence of two cells (Fig. 43) (sometimes three or four), but the whole protoplasmic contents of the cells do not take part in this process, a portion always remaining behind; the aplanogametes coalesce in the conjugation-canal. The zygote thus formed appears incapable of germination until after 3–5 divisions. Of the cells so formed, only one is fertile, the sterile cells, according to Pringsheim, constituting a rudimentary sporocarp. The germinating cells grow out into a new filament. In this order, conjugation has been observed between two cells of the same filament. The Mesocarpaceæ thrive best in water which contains lime. [Illustration: FIG. 43.--Mougeotia calcarea. Cells showing various modes of conjugation: at _m_ tripartition; _pg_ quadripartition; _s_ quinquipartilion of the zygote.] Class 6. =Chlorophyceæ (Green Algæ).=