5§4 
Notes. 
abundant Plankton than -does the main stream, and the species found 
are sometimes different. They also differ in that animal life is much 
commoner than in the main stream, which at some points is almost 
destitute of Zooplankton. These backwaters thus form a kind of 
transition from the typical Potamoplankton of the flowing river to the 
Heleoplankton of the ponds of the Thames valley. Thus, if we 
compare the Thames Plankton with that of the stationary Plankton of 
the artificial waters at Kew, we find that in the latter the animals and 
also the green organisms far exceed the Diatoms in number, whereas 
the latter are far the most abundant in the river. It has been shown 1 
that the Potamoplankton approaches in its character more to the 
Heleoplankton of a pond the slower the stream flows. 
One backwater somewhat above Shepperton gave very interesting 
results. The samples were crowded with Melosira varians , Fragi- 
laria virescens and mutabilis, whereas forms like Pleurosigma at- 
tenuatum and the three species of Surirella , so common in the 
samples collected in the main river on that day, were quite absent. 
Pediastrum was wanting, the green and blue-green forms being 
represented by Scenedesmus quadric auda, Seen, acutus , Ophiocytium 
circinatum , Wolle, and Merismopedia glauca , which except for the first 
were not found in the main stream. The chief reason for this 
difference in the Plankton is undoubtedly to be found in the fact that 
the temperature of the water in this backwater was only i5°C., 
whereas in the main river it was 1 8 ^° C. Diatoms flourish well in cold 
water ! 
The results embodied in the present note are, of course, of a 
preliminary nature. The number of species to be found in the 
Plankton of the Thames is undoubtedly considerably greater than that 
mentioned in the table. My present object has been to obtain some 
idea of the distribution of the commoner species, and time was only 
rarely expended on the determination of isolated forms or on possible 
new species and varieties. Such observations as have been made in 
this direction have been retained for future publication after the 
investigations of Thames Plankton shall have been extended over 
a year’s time and the periodicity of the flora has been determined. 
Jodrell Laboratory, Kew, 
A ugust, 1902. 
F. E. FRITSCH. 
1 Cp. Zacharias, loc. cit., p. 46 ; Zimmer, loc. cit., p. 7. 
