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64
ZYGNEMATACEAE
soils, rocks, and bogs. From there they are sometimes carried into
streams and ditches by rains, and continue to grow in the water
medium.
In Zygogojiinm encetorum there is a strong tendency toward
encystmcnt whenever habitat conditions change; this is true even
of the gametes. The gametes may be walled off in the conjugating
papillae before the solution of the wall between the papillae. Sub-
sequently this wall may be dissolved and the gametes may unite.
This was the mode of reproduction first seen and described by
de Bary (1858). Subsequently normal reproduction was described
by the Wests (1894), t>ut in 1918 Hodgetts found and described
the reproduction by encysted gametes and insisted that this is
the normal process. In 1933 Transeau published figures show-
ing both modes of conjugation in the same pairs of filaments.
Since none of the other species exhibits so-called secondary gametangia,
these must be regarded as a peculiarity of Z. encetorum,
and direct conjugation the normal procedure.
The taxonomic characteristics of the genus Zygogonium may
be summarized as follows:
1. The species are terrestrial or amphibious on acid substrates.
2. The filaments may become branched, sometimes with horizontal
filaments on the soil and erect branches arising from them.
3. Reproduction by akinetes and aplanospores is common. All the
vegetative cells of some filaments may be changed to these structures.
4. Reproduction by zygospores is apparently infrequent, and the
zygospores are enclosed in a sporangium wall with an equatorial suture.
5. Cytoplasmic residues are present in sporiferous and gametangial
cells after spore formation.
6. Cell walls and cell contents are often colored purple, and in terrestrial
specimens the cell walls may become very thick, opaque, and
yellow or brown.
7. Accumulation of fat globules and starch in terrestrial specimens
often obscures both the nucleus and the chromatophores.
Key to the Species or ZYGOGONIUM
I. With aplanospores only (zygospores unknown) 7
1. With zygospores (sometimes aplanospores also) 2
2. Median spore wall smooth 3
2. Median spore wall not smoodi, yellow to brown 4
a- Median spore wall not smooth, blue 13. Z. indicum