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CHAPTER FIVE
THE GENUS ZYGOGONIUM KtJTZING 1843
The discovery and description of 14 species belonging to this
genus have demonstrated the distinctive characteristics of the
genus clearly and have justified its separation from the genus
Zygnema. The small pillow^-shaped, or compressed-globular,
chromatophores of the Zygogoniums are quite different from
those of the Zygnemas. Under certain conditions these chroma-
tophores may have a fevi^ short irregular appendages. In all species
that have been described, how^ever, the chromatophores occupy
but a small fraction of the cell lumen. The small nucleus lies in
the bridge between the chromatophores.
The filaments are often branched, sometimes with horizontal
branches and rhizoids in the soil surface, and with erect branches
arising from them. A regular feature of the species growing on
soil is the formation of thick, lamellate vegetative cell walls and
akinetes. Indeed in situations which frequendy are dry, the filaments
appear to be chains of akinetes.
A common mode of reproduction is by aplanospores. These
globose or cylindric-ovoid spores are formed within vegetative
cell walls and are usually much smaller than the cells. Cyto-
plasmic residues are present regularly in the cell lumen outside
the spore, a feature not found in Zygnema.
Reproduction by zygospores is less frequently seen. Conjuga-
tion may be scalariform or lateral. In either event there are
cytoplasmic residues left in the gametangia after the union of the
gametes. All the 14 species that have been found reproducing
sexually have the zygospores in sporangia formed by the greatly
enlarged conjugating tubes. The two gametes unite directly in
the tube, and a sporangium wall is immediately formed around
them which separates them from the gametangia. The spores are
discharged by the breaking of a distinct equatorial suture in the
sporangium wall.
All the species are amphibious and grow on wet, acid mineral
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