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CHAPTER FOUR
THE GENUS HALLASIA ROSENVINGE 1924
Among the incompletely known genera of the Zygnemataceae
is Hallasia, proposed by Rosenvinge in 1924 better to classify the
Zygnema reticulatiim described by Emma Hallas in 1895. The
young vegetative cells resemble those of Zygnema in having two
stellate chromatophores, but at the beginning of spore formation
there may be an increase in the number of chromatophores up to
seven. At the same time the cells may elongate, and as the cell
contents contract toward the enlarged middle portion the cell
cavity becomes filled with a cellulose-pectose colloid as in Zygne-
mopsis. The aplanospores also resemble those of Zygnemopsis in
being ellipsoid. When these aplanospores germinate, however,
there may emerge one, two, or three sporelings. This Rosenvinge
interprets as analogous to the germination of parthenospores in
Cosmarium from which two sporelings result. He has suggested
that the genus be placed among the Mesotaeniaceae or between
that family and the Zygnemataceae.
The species has previously been classified in Zyg?iema Hallas
1895, in Debarya Transeau 1925, and in Zygnemopsis Transeau
1934-
Description of Species
Hallasia reticulata Rosenvinge 1924. Rev. Algolog. 1, pp. 209-12.
Hallas. Bot. Tidsskrift. 20, pp. 1-16. 1895 (as Zygnema
reticulatiim^
Vegetative cells 18-20 /x x 35-1 00 /j- with 2 to 7 stellate chromato-
phores in each cell; reproduction by aplanospores which are ellipsoid
and up to ^^5^'' in diameter, 60 /.i long; median spore wall yellow, scro-
biculate or irregularly reticulate; sporogenous cells lengthen up to 240 /a.
At germination the contents of the spores may become divided into
2 or 3 parts, from each of which a new filament develops. Sometimes
only a single plant develops from a spore. (Pi. X, Figs. 1-9.)
Denmark, Copenhagen.
According to Rosenvinge the plant has not been found again at the
site of the original collection.
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