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8o ZYGNEMATACEAE
occupying only a part of the axis of mature cells. The nucleus is
near the center of the cell, placed on one side of the chromatophore
in those species which have a single chromatophore. In
species with two chromatophores, such as M. protia, the nucleus
lies in the bridge between the chromatophores. Under natural
conditions favoring active growth the chromatophores of M.
capticina may be either ribbonlike or rodlike, and the cells may
have purple cell sap in some filaments instead of the usual color-
less solution. The pyrenoids in most species are arranged in a
linear row; in a few they are scattered throughout the broad plate-
like chromatophores.
Of the 99 species here described, 92 usually reproduce by means
of zygospores, and 7 by aplanospores. Of the 92 zygosporic species,
25 also occasionally produce aplanospores either in the same
filament with zygospores, or in separate aplanosporic filaments.
Scalariform conjugation occurs in 86 species. Conjugation
may be either scalariform or lateral in 4 species, and in only 2 is
it regularly lateral.
Almost all the species are isogamous; only 3 species are strictly
anisogamous, and 2 others somewhat variable even in the same
paired filaments. The distinction between parthenospores and
aplanospores is not always easy to make. In many of the 25 species
reproducing both by zygospores and aplanospores, the form
and placement of the spores are somewhat different. In those
species having aplanosporic filaments the distinction is more
evident. These filaments are quite regularly zigzag with the
spores at the angles, the first facing in one direction and the next
in another direction. Hassall described such filaments in 1842,
and figured an immature one in 1845 {M. notab'iUs). Wittrock
(1878) also discussed this feature of aplanosporic filaments when
he proposed the genus Gonatonema. Paul Petit (1880), the Wests
(1902), and Czurda (1931) have suggested that these spores may
result from internal division of a vegetative cell followed by
lateral conjugation. This speculation still awaits cytological evi-
dence. In the hundreds of developing aplanospores studied by
me and my associates, not a single example of preliminary division
of either the protoplast or the nucleus has been found. There is
good reason to believe that rare instances of lateral conjugation
may be found among the usually scalariform species of Mougeotia
just as in Zygnema and Spirogyra. Such instances, however, can-