Phytoplankton of the River Thames. 645 
marked differences from the main river outside ; the Plankton 
in the first place is very much richer in number of individuals. 
The Diatom flora is mainly composed of Melosira varians 
and Fragilaria virescens , the latter species especially being 
far commoner here than in the main river. Further, the 
Plankton of the backwater is characterized by the occurrence 
of large numbers of splendid specimens of Nitzschia sigmoidea 
(frequently bearing Amphora minutissima ), which are almost 
absent from the river itself at this point. Green forms were 
better represented in the backwater, whilst a number of 
them (notably Pandorina moram and Eudorina elegans) were 
entirely wanting in the Plankton of the river ; however, 
the relative abundance of the green forms with regard to the 
main river is not so noticeable in this as in some of the other 
backwaters. It is the Diatom-flora here, as in the case of the 
smaller backwater at Walton, that affords the characteristic 
feature. The Plankton of the Wey, which was cursorily 
examined on this occasion, does not differ noticeably from 
that of the Thames at this point, except for a rather frequent 
occurrence of Closterium acerosmn. 
The most important features of the backwaters are thus : — 
(i) Relative abundance of the Plankton in individuals as 
compared with that of the main stream. 
(ii) Relative greater development of green and blue-green 
Algae and of the fauna, compared with the river itself. 
(iii) An often very noticeable difference in the entire specific 
constitution of the Plankton. 
On the whole, though however much the Plankton of the 
backwaters examined may differ from that of the actual 
Thames, its nature is still very different from that of the 
Plankton of a pond, and, so to say, always bears the stamp 
of a river Plankton. As an example, the results of some 
dredging carried out on the Brentford Reservoir near Hendon 
towards the end of October of last year may be mentioned. 
The chief mass of the Plankton consisted of animals, whilst 
Diatoms were only represented by a species of Stephanodiscus 
and a few isolated individuals of Surirella ovalis. A con- 
