Phytoplankton of the River Thames . 641 
This backwater has a winding course, and penetrates 
between 1 00-150 yards into the land up to Hampton Court 
railway station ; it is deep enough to admit of rowing along its 
entire length. The banks are fairly thickly wooded, pollard 
willows being especially common. This is the only case 
I have as yet observed, in which the Plankton of a backwater 
is relatively poor as compared with that of the main river 1 , 
although the percentage of green forms present in the former 
is even here greater. To some extent also there is a difference 
in the constitution of the Plankton of the backwater and the 
main stream ; Synedra Ulna , which is common in the latter at 
this time of the year, is entirely wanting in the backwater, as 
is also the case with Fragilaria virescens and Pleurosigma 
attenuation ; on the other hand Scenedesmus acutus and 
Closterium acerosum were both only found in the backwater, 
whilst the other species of Closterium is far commoner in the 
main river. Animals are also considerably more abundant in 
the backwater. As far as I am aware, the River Mole is in 
some way connected with this backwater ; and the Plankton 
of the former, except for the occurrence of a number of blue- 
green forms (. Microcystis marginata, Merismopedia glaucd), is 
quite identical with that of the backwater just discussed, being 
rich in green forms and poor in Diatoms relative to the 
main river. 
(ii) Backwater near Sunbury (May 23, 1903). 
This backwater, except in quantity of individuals, differs 
very slightly from the main river. It is very shallow, and 
communicates with the stream by means of a short arm about 
halfway along its length. At this time of the year there 
is little vegetation in it ; Nymphaea is just commencing to 
appear. It has a rich Diatom flora, green forms in corre- 
spondence with the time of the year being rare. Amongst 
the Diatoms Pleurosigma Fasciola and Asterionella gracillima 
1 Bacteria were rather abundant in some parts of this backwater, which seems 
to indicate that refuse of some kind has access to it. This may possibly account 
for the paucity of its Plankton. 
