Phytoplankton of the River Thames. 6 37 
equal numbers, being accompanied by a development of 
Asterionella gracillima about the time of the New Y ear ; in May 
a very abundant development of Melosira varians , succeeded 
in the summer months again by a great increase in species of 
Synedra ; in the height of the summer and the autumn con- 
siderable prevalence of any particular form is not noticeable 
(according to last year’s results, see Fritsch, ’ 02 ). This 
periodicity in the course of a year may be summarized thus : — 
mixed Plankton (with Asterionella~pha.se) — > Melosira-^- 
Synedra — > mixed Plankton. 
Asterionella , it should be observed, cannot be said to in- 
dividualize the Plankton to the extent that Melosira and 
Synedra do in later months. In the past season, at least, 
it merely formed a minor phase in the development of the 
river’s Plankton, and the characteristic outward form of this 
species alone makes its occurrence so striking and readily 
noticeable ; there are probably a number of such minor 
phases \ which are not great enough to give a definite stamp 
to the Plankton, and most of which would be overlooked in 
the course of a single year’s observations (cf. also p. 631). 
If we compare the periodicity of the Plankton of the Thames 
with that of the Oder and Danube, which was mentioned 
above, it will at once be perceived that, although we have the 
same dominant forms, their distribution in the different seasons 
of the year is not at all identical. Synedra , which is a spring 
and autumn form in the two continental rivers, attains its 
maximum during the summer in the waters of the Thames ; 
whilst Asterionella , which plays some part in the winter- 
Plankton of the latter, abounds in the Oder and Danube 
during the summer months, a time at which it is not at all 
or scarcely represented in the Thames. According to Brunn- 
thaler, Melosira (but together with Fragilaria) abounds in 
the Danube in spring, that is to say, at the same time as it 
does in the Thames ; apparently this Diatom does not occur 
1 The relative abundance of Synedra Acus, var. delicatissima in March may 
possibly turn out to be another such minor phase. 
