_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.
[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.]