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ZYGNEMA 15
spores, and in parthenospores and aplanospores only two. In
general the zygospores are compressed-globose or ovoid. There is
an equatorial suture often marked by a slight ridge or keel
(carinate), and in some species there are two lesser ridges on
either side of the suture and parallel with it. These may not be
visible in fresh fully distended spores but they become evident
in dried or plasmolyzed specimens. Usually, the polar, or flatter,
sides of the spores lie in the plane of the tubes, but there are
6 species in which the plane of compression is at right angles to
the axis of the tubes.
The most abundant and generally distributed of the brown-
spored isogamous species is Z. pectinatum, of the blue-spored
isogamous species, Z. synadelphum. Of the anisogamous brown-
spored species Z. stellintim is both abundant and widely distrib-
uted; Z. peliosporum is its counterpart among the blue-spored
species. As might be expected, all these species are highly variable
in dimensions, and local varieties and forms are apparently common.
It is not improbable that they have been the mutating forerunners
of nearly related species found locally wherever the
Zygnemas have been studied intensively.
There is no good evidence that the taxonomic characteristics
of species are changed materially by environmental conditions.
There are alterations in chromatophores, accumulated foods, and
thickness of walls due to exposure on soil, restricted photosyn-
thesis, low temperatures, and mineral deficiencies. Reproductive
capacity may be decreased or increased by external conditions.
The mode of reproduction by zygospores or aplanospores, and the
placement of the zygospores in the tubes, or in one of the gametangia,
are the results of hereditary rather than environmental
factors. In any one species the position of the spores, whether in
the tubes or in the gametangia, does not change from season to
season (see Fritsch and Rich, New Phytologist, 26 [1927]).
There are several species in which the spores occur in either the
gametangia or the tubes—even in the same pair of conjugating
filaments.
The identification of a species depends partly upon the dimensions
of the vegetative cells. Vegetative cell diameters should be
measured at the partition walls. Most important are the relative
dimensions of the spores, their form, and the color and ornamen-
tation of the several layers of the spore walls. Obviously only