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14
ZYGNEMATACEAE
In higher latitudes and altitudes the species are not well known
because they rarely have been found with zygospores.
In warm regions it is not improbable that some species are
perennial; that is, some of the filaments form spores and die,
while others merely break into segments which continue vegeta-
tive growth. In well-aerated running water the Zygnemas are
rarely found fruiting, but in the shallow pools adjoining such
streams one can usually collect the same species in a fruiting
condition.
The average length of the filaments of Zygnemas is a few
inches, much shorter than that of the Spirogyras. Apparently
before the filaments become very long the pectic sheath breaks
and the cells separate. Cell diameters vary from 8 to 58 /^ but more
than half the species have cell diameters between 20 and 30M.
Usually there are two axial chromatophores in each cell with
the nucleus contained in the cytoplasmic bridge connecting them.
Each chromatophore consists of a round body, with irregular short
branches radiating in all planes, containing a conspicuous central
pyrenoid. As a result of food accumulation the chromatophores
may become merely two large globose bodies, or may completely
fill the lumen of the cell.
On rare occasions one may find filaments in which each cell
has two axial chromatophores on either side of the nucleus. In
three collections that I have examined there were four chroma-
tophores in the cells of some filaments arranged in the form of
an X with the nucleus in the center.
Sexual reproduction occurs in 82 of the 95 described species.
Twelve others reproduce by aplanospores, and i by akinetes only.
Of the described species reproducing by zygospores 37 are isogamous,
with spores in the conjugating tubes; and 45 are anisogamous,
with spores in one of the gametangia. Of the 95 species,
70 have scalariform conjugation, 9 both scalariform and lateral
conjugation, and 3 lateral conjugation; no conjugation has been
observed in the remaining species. Thirty species have blue
median spore walls when mature. During development these
walls change from colorless to yellow, to brown, to blue. As not
all spores mature at the same time, one may find all these stages
in a collection. If some of the spores are blue, one may be sure
that the other spores in the filament are immature.
Four chromatophores are usually visible in immature zygo-