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124
ZYGNEMATACEAE
insists that cell lengths are of no taxonomic importance since
they may be mocJiiied in some species by environmental condi-
tions. Nevertheless there are some species in which the cells are
relatively short (one-half to two diameters), and others in which
they are relatively long (eight to twenty-five diameters). In these
species at least, the cell lengths may be contributory evidence to
their identification. Cells 6oom long have been seen in at least
5 species, the diameters of which are between 201^ and 45/^
The chromatophores are ribbonlike or troughlike, with or
without a median ridge, arranged in a left-handed, or counter-
clockwise spiral in the parietal cytoplasm. The number of spirals
in a cell may vary from one to sixteen. In some species the number
of chromatophores is uniform. In others there are occasional cells
in filaments with one more than the usual number— particularly
in the one-spiraled species. How these arise is not known. In
still other species the number regularly varies within certain limits.
In determining the number of chromatophores in a given
specimen, counts should be made only in filaments attached to
the sporangia or gametangia being studied, as there may be vege-
tative filaments of nearly the same dimensions but of another
species present in any collection. In most species the number of
spirals is readily determined by focusing just below an upper-half
turn of the spiral, counting this turn as one and adding to it the
number of optical intersections made by the spirals on the oppo-
site side of the cell. In very large species and in those with tightly
coiled chromatophores this may be impossible. In such species
and in those with nearly straight chromatophores, the numbers
are best determined by counting the ends of the spirals near the
cross walls.
Each chromatophore has from several to many disc-shaped
pyrenoids spaced at regular intervals, and in certain species interrupting
the median ridge. The margins of the chromatophores
may be nearly smooth, or variously crenulate. Just after cell
division the spiral pattern is continuous from one cell to the next,
interrupted only by the thin partition wall. Chromatophores are
examples of direct cytoplasmic inheritance. Through aplanospores
they are derived directly from the chromatophores of the sporog-
enous cells. Through zygospores they are derived from the chromatophores
of the receptiue gametangia.
The cells of a filament are enclosed in a pcctic sheath im to 17M