632 Fritsch. — Further Observations on the 
Oder to some extent from this point of view, and observations 
of this kind are at present being carried out on the river 
Volga (cf. Zykoff, '02, p. 60). With regard to the Oder at 
Breslau, Schroder (’ 98 , p. 525, ’ 99 , pp. 22-23), gi ves the 
following data : — 
I. December-February : — nothing or exceptionally Synura 
and Eudorina. 
II. March-May : — Synedra and a few brown Flagellates. 
III. June-August: — Asterionella\ a few green and occa- 
sional blue-green forms. 
IV. September-November : — Synedra and a few brown 
Flagellates. 
During the winter months the only living elements occur- 
ring in the Plankton are a few Rotifers, intermingled with 
large quantities of organic detritus and muddy particles, whilst 
Algae are almost alone represented by detached portions of 
filamentous forms. During March Melosira varians and 
Fragilaria virescens appear in some quantity, but near the 
end of the month Synedra delicatissima is the most abundant 
form ; only a few specimens of green Algae ( Chlamydomonas 
tingens , Pandorina morum, Eudorina elegans , and Volvox 
minor) occur, whilst Flagellates are rather commoner at this 
time of the year ( Synura uvella , Uroglena Volvox , Dinobryon 
sertidaria and Chrysomonas ovata). In the summer months 
Asterionella plays an exceedingly important part, and is 
accompanied by a quantity of other Diatoms ( Diatome tenue , 
Fragilaria capucina and F. crotonensis , Melosira gramdata , 
Stephanodiscus Flantzschianus) ; of the green forms Actin- 
astrum Hantzschii , var .jluviatile, and Dictyosphaerium Ehren- 
bergii , alone are at all abundant. As the autumn comes on 
the Plankton again acquires the spring character, and by 
a decrease in number of individuals gradually merges into 
that of the winter months. 
In the Danube, the Plankton of which has been examined 
by Brunnthaler (’00, pp. 310-311), the periodicity is somewhat 
different. In the clear water, occurring during the winter 
months, life is almost entirely wanting. In February Synedra 
