1158 Proceedings of Royal Society of Edinburgh. [sess. 
the phytoplankton, especially in summer time, only plays an in- 
significant part in those lakes ; hence the main nutriment of the 
zooplankton for a long time of the year mainly consists of detritus. 
The phytoplankton of arctic and sub-arctic lakes consists in all 
probability mainly of algse with yellowish or yellowish-brown 
chromatophores ; algse with green or blue-green chromatophores are 
almost entirely wanting. As exceptions from this common rule 
we only mention Sjohcerocystis, the semi-limnetic Desmids and a 
few rare Chlorophycese. It is a well-known fact that, in the arctic, 
antarctic, and cold temperate seas, the Phseophycese are the pre- 
dominating algse (see e.g., Schimper, 1898, p. 832); also with 
regard to the marine phytoplankton, so the Diatoms with their 
yellow-brown chromatophores play the greatest part in the colder 
seas (see e.g., Schiitt, 1893, p. 26). 
None of these facts can be explained by us. Further explora- 
tions may probably show whether the optima of assimilation 
for yellow-brown coloured chromatophores commonly lie at a 
lower tp. than those of the green or blue-green chromatophores. 
We are quite aware that these assertions are as yet only proble- 
matic ; still, we do not hesitate to set them forth here, thinking 
that they might be of use as working theories for further explora- 
tions carried on in higher latitudes than at this time has been 
possible. 
Note received October 3 , 1905 
After our paper had gone to press, I received from Professor 
W. West, F.L.S., and Professor G. S. West, M.A., F.L.S., their 
paper : A Further Contribution to the Fresh-water Plankton of the 
Scottish Lochs {Trans. Roy. Soc. Edin., 1905, p. 477). As the 
said paper essentially touches on the same subjects as my 
paper : A Comparative Study of the Lakes of Scotland and 
Denmark ( Proceed . Roy. Soc. Edin., vol. xxv., 1905, p. 401), and 
the present paper, I take the liberty here shortly to mention 
some points in the same. 
In the summary the authors write : — 
1. “ The quantity of plankton is relatively small at any time 
and scarce affects the colour of the water. It exhibits 
