578 
Notes . 
although I must say that the extremely abundant Melosira was only 
found with reproductive organs in a quiet, almost stationary back- 
water. Undoubtedly it is from these backwaters and tributaries of 
the upper part of a stream’s course that the initial mass of the 
Plankton is derived ; they act, so to say, as reservoirs x , from which 
by means of currents or incidentally during storms new supplies are 
continually being dealt out. It is, however, just as certain that these 
organisms do afterwards multiply still more, and that a great part 
(quantitatively) of the Plankton of the Thames is formed when the 
individuals have already reached the flowing stream. Zacharias holds 
the same view of the matter and adduces the river Schlei as evidence 2 ; 
each year it becomes covered with a dense stratum of Clathrocystis , 
and it is impossible that this can all have been derived from the few 
backwaters of the river. Zimmer 3 places the Plankton of rivers in 
three classes - 
(1) The eupotamic Plankton which flourishes and reproduces either 
in the flowing water of the stream or in stationary water (e. g. Pedi - 
astrum, Eudorina , <$r.). 
(2) The tychopotamic Plankton which only reproduces in stationary 
water, and when swept into the main water of the river continues to 
live, but not to reproduce (probably many Diatoms). 
(3) The autopotamic Plankton which has become adapted to life in 
flowing water (e. g. Aciinastrum Hantzschii , Lagerh., var. fluviatile , 
Schrod., and Synedra Ulna (Nitzsch), Ehrb., var. actinastroides , 
Lemm. 4 ). 
What then becomes of the Plankton as it gets nearer and nearer to 
the river’s mouth ? On most of the Plankton-organisms, found higher 
up the river, the increasing brackishness of the water 5 will act fatally. 
In the Thames, in which the tide extends to Richmond or further, 
this action is noticeable already at Kew. The quantity of Plankton- 
organisms decreases steadily as we approach the river’s mouth. 
Plankton-hauls from the Thames at Kew showed that many of the 
Desmids and Pediastrums are either dead or in a dying state, 
and many of the Diatoms merely consist of the empty frustules. 
These after some time sink to the bottom of the water, and in this 
1 Cp. Zimmer, Das thierische Plankton der Oder. Forschungsber., Plon, 1899. 
Teil vii, p. 4. 
2 Zacharias, loc. cit., p. 47. 3 Zimmer, loc. cit., p. 5. 
4 Cf. Schroder, loc. cit. I have not found either of these varieties in the 
Thames as yet. 5 Cp. foot-note on p. 581. 
