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TAXONOMIC CHARACTERISTICS 7
walls formed by the conjugating tube and the two gametangia.
This conjugation apparatus is relatively persistent and the zygospore
remains in the enclosure for days or weeks after its own
walls are mature. Zygospores at first have two nuclei and either
one set of chromatophores (Spirogyra) derived from the receptive
gametangium, or two sets (Zygfjema and Mougeotia) derived
from both gametangia, one of which disintegrates later. After the
union of the nuclei, reduction division occurs and four nuclei
result. Of these, three disintegrate and the remaining one becomes
the nucleus of the basal cell of the sporeling on germination.
Only when one contemplates the many successive steps in
conjugation from the development of papillae and adhesion of
the gametangia onward to the maturation of the spore walls and
protoplast, does he realize how very complicated are the chemical,
physical, and biological processes that are integrated in zygospore
development (PI. I, Figs. 3, 8, 9, and 10).
Parthenospores may be formed when the normal union of one
gamete with another fails to occur either by absence of an opposite
gamete, or by sudden changes in environmental conditions. In
some collections gametes are numerous; in others, rare. Partheno-
spores can usually be distinguished from aplanospores by their
position in the gametangial cells and their relation to adjacent
zygospores. It is customary to say that "they resemble the zygo-
spores of the same species but are smaller." Akinetes, aplano-
spores, zygospores, and parthenospores have all been seen germi-
nating by numerous students. New filaments grow from any of
these forms.
The taxonomy of the Zygnemataceae has been centered on the
conjugation apparatus: its development, forms, and zygospores.
The evolutionary history of reproduction in the group, however,
must have begun with propagation and survival by akinetes.
These are frequently seen in other families of the Chlorophyta.
The development of aplanospores must have been a next step in
evolution. This is not a large step either, since it implies merely
the contraction of the protoplast and the growth of a spore wall.
This type of spore also occurs in many other families of the
Chlorophyta.
When aplanosporangia became differentiated chemically and
hormones with plus and minus properties were released which
brought about local growth of cell walls followed by adhesions