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. [Illustration: 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.