Download PDF

CHAPTER TWO
THE GENUS ZYGNEMA C. A. AGARDH 1824
The plants classified as species of Zygnema consist of unbranched
filaments of short cylindric cells usually covered by a
pectose sheath. To the field collector they are less slippery than
the Spirogyras but more slippery than the Mougeotias. There are
exceptions, of course, because the sheaths vary in thickness from
those which are barely visible under the microscope to those which
are thicker than the cells themselves. In regions of low tempera-
ture and alkaline water, the sheaths are denser and highly stable.
Similar sheaths are found on filaments living on wet shaded soil
on pond margins. The thicker sheaths often have visible struc-
tural lines at right angles to the filaments.
Zygnemas live as annuals, and may complete their life cycle
in a few weeks and then disappear. They are most abundant and
more frequently found reproducing in temporary ponds and
ditches. In permanent ponds one may find vegetative filaments
throughout the year. This is not to be construed as evidence that
individual plants are perennial. It is far more probable that the
germination of spores and akinetes occurs throughout the year.
In cold temperate regions the most abundant germination starts
in autumn and winter, and culminates in early spring. As ponds
become shaded by the growth of marginal shrubs and trees, reproduction
decreases and finally ceases entirely, but the Zygnemas
are perpetuated for a subsequent period of years by the overwin-
tering of akinetes and fragments of filaments. They survive cold
and dry periods in the ooze and silt of the pond bottom. During
the drought period between 1930 and 1935 a small pond near
Columbus, Ohio, was dry for three and a half years. When the
rains finally restored the pond, filaments of Zygnema sterile became
abundant within three weeks. This species reproduces by
akinetes only.
Zygnemas have been collected on all the continents from sea
level to alpine summits, and from the torrid to the frigid zones.
13