Phytoplankton of the River Thames 633 
appears, and at the same time a few individuals of Melosira. 
In the next months Melosira and Fragilaria increase in 
numbers, whilst the prevalent Asterionella of the succeeding 
summer months is as yet only slightly represented. In the 
height of the summer this latter form is accompanied by 
Ceratium , Dinobryon , Clathrocystis , and Fragilaria , in sub- 
ordinate numbers. With the autumn a decrease in number 
of individuals again becomes noticeable. Green forms only 
occur in any considerable amount during the summer months. 
To come now to the River Thames (see table), an important 
difference in contrast to the two rivers just discussed at once 
appears. There is a well-marked living Plankton all the year 
round ! The Diatoms, which form such a very large per- 
centage of the organic life of the Thames, are always present 
in appreciable numbers, even though from December to 
February about two-thirds of the individuals are dead and 
only represented by an empty frustule. At the same time, 
however, more or less abundant living representatives of all 
the species mentioned in the table for these months were 
observed, and samples, when examined under the microscope, 
always exhibited a number of live Diatoms in the field of 
view. This difference in the Thames Plankton as opposed 
to that of the Oder and Danube may most probably be 
ascribed to the mildness of our winter in contrast to the 
continental one ; for the sample collected on February 4, 1903 
a few days after the cessation of a heavy frost, showed no 
change in the living element in the river, and it is only rarely 
that we exceed the degree of cold attained on this occasion. 
It is easy to understand how a frost of long duration, con- 
verting the backwaters and other sources of the river’s 
Plankton into a thick sheet of ice, would reduce the organic 
life in the river to a minimum by the temporary congealment 
of the reservoirs, from which it is in the main derived ; this is 
undoubtedly the case with the continental rivers, and it will 
be interesting to see what effect a protracted frost (of say 
three to four weeks), as occasionally occurs, will have on the 
Plankton of the Thames. 
X x 2 
