Notes . 
165 
Thames during the winter months 1 . There is an abundant growth of Algae along 
the banks of the river (mainly Cladophora with epiphytic Oedogonium ). — The Plankton 
of the Cam was gathered three days previously, and the samples were for the most 
part taken from the part of the river immediately below Cambridge. The current 
was scarcely noticeable, and the material obtained was practically free from mud. 
Large numbers of aquatic plants (Sagiltan'a, Oenanthe , Potamogeton , Lemna , &c., 
especially the first of these) grow in the river, and from this point of view a Thames 
backwater is recalled ; these plants are covered with a more or less dense investment of 
Algae, whilst small floating" masses (a mixture of Conjugates and Oscillarid) occur 
quite commonly on the surface of the water. These two points make Plankton- 
collecting a difficult matter, for it is almost impossible to prevent the net’s coming in 
contact with the water-plants, and consequently to prevent the enclosure of some of 
the attached Algae in the sample 2 . I do not therefore regard my collections from 
this river as perfectly pure Plankton, although I consider it very probable that all the 
true Plankton forms develop amid the protection of the aquatic plants occurring in 
such a river (cf. below). For the sake of comparison I have chosen samples of 
Thames Plankton collected between Maidenhead and Cookham on August 19, 1902, 
i. e. from a part of the river sufficiently far removed from the estuary as to put 
a probability of marine influence out of the question 3 . 
Comparing in the first place the Trent with the Thames, it is noticeable that 
as regards the number of different species there is little to choose between the two 
rivers; but if we look at the constitution of the Plankton from the point of view 
of number of individuals, we find that eight species occur commonly (c) or very 
commonly (vc) in the Thames, whereas in the Trent one is not able to talk of any 
species as common. Altogether, a glance at the table will show at once that the 
majority of species are rarer than in the Thames, the exceptions being the two 
species of Scenedesmus, Synedra Acus, Ktz., and a few forms (e. g. Volvox globator, 
Bacillaria paradoxa , Ceratium hirundinella) which were not observed in the Thames. 
This quite agrees with the observations which have been hitherto made on the 
Plankton of rivers ; for in the rapidly-flowing Danube Brunnthaler found a Plankton 
very poor in number of individuals 4 . If the commoner species of the Plankton of 
the Trent (i. e. those designated rather common) are picked out, we shall find that 
a number of the common forms in the Thames Plankton are not included; thus 
Scenedesmus quadricauda,Closlerium moniliferum, Me lo sir a varians, Cyclotella operculata , 
Fragilaria virescens , Synedra Acus, S. Ulna may be called the dominant forms of the 
Trent Plankton, of which only the first and last but one play no important part in 
the Thames at the corresponding time of the year. Yet to make the list of dominant 
forms in the Thames complete we must add Pediastrum Boryanum , Eudorina elegans , 
1 Cf. Fritsch, Further observations on the Phytoplankton of the River Thames; Ann. of Bot., 
vol. xvii, 1903, pp. 633, 634. 
2 The leaves of the Sagittaria , for instance, are covered with a mass of the species of Navicula 
observed in the Plankton. 
3 The distance of Maidenhead from the estuary of the Thames is approximately the same as that 
of Nottingham from the mouth of the Trent. 
4 Cf. Brunnthaler, Plankton-Studien. I. Das Phytoplankton des Donaustromes bei Wien ; 
Verhandl. d. k. k. zool.-bot. Gesellsch. in Wien, Jahrg. 1900, p. 309. 
