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CHAPTER THREE
THE GENUS ZYGNEMOPSIS (SKUJA) TRANSEAU 1934
The vegetative filaments of the 24 species here classified are
usually indistinguishable from those of Zygnema. They may have
a thin, or a thick, pectic sheath and they have been found floating
in ponds, ditches, and slow flowing streams, not infrequently
associated with other species of Zygnemataceae.
At the beginning of the reproductive phase, however, they
may be distinguished by the partial replacement of the usual cell
contents by a transparent refractive gel. Whether the cells become
gametangia, or aplanosporangia, some of them lengthen and suc-
cessive layers of cellulose are deposited as the protoplast contracts
to the middle of the cell. At maturity the zygospores have four,
and the aplanospores two, lamellate solid appendages attached to
them. As seen in the illustrations, these may be short and stubby,
or may be very much longer than the original vegetative cell.
All the 19 zygosporic species have isogamous gametes and
scalariform conjugation. After the papillae unite in sexual repro-
duction the tube is exceedingly narrow, but soon broadens in the
plane of the filaments. The zygospore resulting from fusion of
the gametes is a compressed, more or less quadrangular pillow-
shaped, body which may subsequently become lenticular or irregular
in outline. Both the tube development and spore forms
resemble those of the quadrangular-spored species of Mougeotia
more than those of any species of Zygnema. The outer or firstformed
wall bounds the colloidal gel and is composed of cellulose.
The median wall is chitinous and in at least one species consists
of two layers. Not infrequently yellow or brown granules are
deposited between the outer and median walls and may obscure
the surface features of the median wall. The innermost wall is
transparent and thin and can be seen only when the spore is
crushed. Parthenospores are not infrequent in some collections.
They have walls similar to those of the zygospores, but are ovoid,
smaller, and laterally placed in the gametangia. Aplanospores are
common in 5 of the species. They vary in form from ovoid to
ellipsoid and have walls similar to those of the zygospores.
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