Pleurodiscus purpurea

8 9 10
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The New York
Botanical Garden

Pleurodiscus borinquenae Tiffany sp. nov. (pi. 2, f. 30-39). -L

Cellulis vegetativis 18—26 |j latis, 16—65 p longis, meinbrana saepe
incrassata; filamentis simplicibus vel ramosis, interdum rhizoidibus
instructis; zygosporis ovoideis vel globosis vel late ellipsoideis, 22—32 |j
latis, 26—32 ^ longis, mesosporio foveolis 3—5 [j latis ornato.

In rivulo prope ‘ ‘ Hacienda Catalina, ’ ’ ad Palmar oppidnm; prope
Marieao et Jayuya oppida; specimina in collectionibus auctoris et
Herb. Hort. Bot. Noveboracensis conservata sub numeris 750b, 788,
836, 837, 1086a, 1790.

The proper disposition of the algal genns Pleurodiscus Lagerheim,
as well as its alleged inclusion of Zygnema purpureum Voile, has been
a subject of considerable uncertainty among algologists for a number
of years.

Recently Skuja (1932) has given a very complete review of the
historical aspects of the situation, and it will suffice here merely to
mention the viewpoints of some of the contributors. Wolle (1885,
1887) applied the name Zygnema purpureum to a fruiting conjugate,
collected in New Jersey and in Florida, whose chief characteristics
(to him) were the purple color and evident geniculation of the fila-
ments. Lagerheim (1895) discovered in Norway vegetative filaments
of an alga whose disc-shaped chromatophores seemed to warrant the
establishment of a new genus among the Zygnemataeeae: Pleurodiscus.
Due to the presence of a brownish-purple cell sap Lagerheim concluded
that his specimens were identical with Wolle’s Zygnema, and hence the
binomial Pleurodiscus purpureus (Wolle) Lagerheim.

Czurda (1932) includes Pleurodiscus with Zygnema. Both Skuja
(1932) and Transeau (1933) place P. purpureus as a synonym of
Zygogonium ericetorum Kuetzing. G. M. Smith (1933) recognizes the
validity of Pleurodiscus, but he is not positive of the inclusion or
exclusion of Wolle’s species. Transeau (1934) recognizes and com-
pletely describes Pleurodiscus as one of the ten genera constituting
the Zygnemataeeae.

It seems very clear, as Skuja (1932) points out, that Lagerheim
had little foundation in fact for the association of his Pleurodiscus

Tiffany: Puerto Rican Fresli-water Algae

171

purpureus with Wolle’s Zygnema purpureum. The purple color
which Lagerheim deemed sufficient to associate the two plants occurs
in several genera of the Zygnemataeeae. As a matter of fact, a
purplish color may appear in the filaments of most any member of the
family when the plants begin to decompose. Wolle’s statement,
“Primarily yellowish-green, but soon changes to a purple color,” may
mean after collection and improper. preservation. Wolle’s figures,
moreover, give no indication of the character of the chromatophores
of his species. The evidence for including Lagerheim’s Pleurodiscus
purpureus in the synonymy of Zygogonium ericetorum appears to the
writer to be quite inconclusive. The alga has never been certainly
reported except in the vegetative state. It appears, therefore, to be
to the best interests of the taxonomy of the group to relegate both
Zygnema purpureum Wolle and Pleurodiscus purpureus Lagerheim
ad species incertas sive haud satis descriptas.

The Puerto Rican collections contained both Zygogonium and
Pleurodiscus definitely separable (Transeau 1934). The alga de-
scribed above as Pleurodiscus borinquenae has two disc-shaped chro-
matophores, is frequently branched, and possesses distinctly pitted
zygospores. On the basis of such definite and clear-cut characters it is
thought advisable to establish a new species rather than to attempt the
dubious task of forcing the alga into an emended P. purpureus.

Collected in Puerto Rico in January, February, and March.

Tiffany : Fresh-water Algae

Brittonia 2, No. 2

02014816

02014816