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CHAPTER SIX
THE GENUS PLEURODISCUS LAGERHEIM 1895
This genus was established by Lagerheim to classify purple-
colored vegetative filaments, the cells of which had two distinct
disc-shaped chromatophores, one on either side of a central
nucleus (P. purpureus). Skuja (1932) questioned the validity of
the genus and suggested that the chromatophore is merely an
expanded form of the Zygogonium chromatophore due to environmental
conditions. In 1936 Tiffany described a species P. borinquenae
from Puerto Rico, the first and only specimens found
in a fruiting condition. The chromatophores certainly were disc-
shaped and oriented at various angles to each other. The proc-
esses of conjugation and spore formation resemble those of
Zygogonium.
Specimens resembling Lagerheim's figure, collected near
Eaglesmere, Pennsylvania, and those collected by Randhawa in
India, were associated with smaller vegetative filaments of Zygogonium;
there were no intergradations. Environmental factors
do not seem to account for the differences in chromatophores in
these collections. When Zygogonium filaments are growing lux-
uriantly, the pillow-shaped chromatophores are larger and there
is a fringe of several stringlike or radial projections, very different
from the smooth-edged disc, or saucer-shaped, bodies of Pleurodiscus.
Further study of the algae growing in the drainage from
wet acid rocks and soil will probably uncover additional species
and clarify the status of this genus.
Description of Species
Pleurodiscus borinquinae Tiffany 1936. Brittonia. 2, p. 169,
Figs. 31-39-
Vegetative cells 18-26 /a x 16-65 /a, pectic sheath sometimes thick;
filaments either simple or branched and having rhizoids; zygospores
ovoid to ellipsoid, within a sporangium partly formed by the tube
papillae and partly by a collar between them; zygospores 22-32 /^ x
26-32 ju. with a scrobiculate spore wall; pits 3 to 5 /a in diameter. (PI. XII,
Figs. 12-15.)
Puerto Rico, Palmar, January to March (Wille Coll.).
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