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TAXONOMIC CHARACTERISTICS 9
gametic segregates. In the few instances in which such progeny
have been studied, individual filaments inherit the factor for cell
diameters, forms of receptive gametangia, zygospore size, form,
and wall markings independently. They are expressed through
the cytoplasm of the female gametes. So long as these segregates
reproduce by cell division, by akinetes, by aplanospores, and by
conjugation between cells of the same haploid filament, the gene
complex of the progeny is identical and the filaments are uniform.
Doubtless many of the "species" that are collected and named are
taxonomic units, or clones, that have originated in this way. Other
species seem to have originated by polyploidy and by mutation,
but the evidence for this statement is purely circumstantial.
The bases for the separation of the genera are shown in
the following key. Three genera (Hallasia, Pletirodiscus, and
Entransia) are tentatively and purposely defined to emphasize
the need for further study of the development and reproduction
of these species. All the forms described in this key are designated
as "species." Many variations have been seen in the collections
studied but until more is known about hybrid segregates, isolated
clones, and the effects of the various environmental factors, it
seems unprofitable to assign "variety" and "form" names to every
variant. Many alleged "varieties" are not even closely related to
the species to which they have been assigned.
Key to the Genera of ZYGNEMATACEAE
I. Vegetative cells usually with 2 axile chromatophores, which are
round, radially branched, pillow-shaped, or disclike, with nucleus
contained in the cytoplasmic bridge between them
I. Chromatophores usually a pair of axile stellate or globose radi-
ally symmetric bodies, each with a central pyrenoid
a) Zygospores compressed-globose or ovoid in the tubes or in
one of the gametangia; aplanospores cylindric-ovoid occupy-
ing all or most of the cell; gametangia not filled with a
dense refractive colloid after conjugation, and without cytoplasmic
residues i. Zygnema
b) Zygospores quadrangular-ovoid, or round pillow-shaped,
formed in the extremely wide but shallow conjugating tubes;
aplanospores ellipsoid or asymmetrically ovoid; successive
layers of cellulose and pectic colloids added inside the gam-
etangial walls during conjugation; similar changes occur in