418 Proceedings of Royal Society of Edinburgh. [sess. 
of plankton so insignificant that one may decide as to the original 
colour of our fresh-waters : to determine the colour of the water at 
any other season it would he necessary to filter it. This probably 
applies to all the lakes of the Central European plain, but, as far as 
I am aware, the colour of the water in all these lakes has never 
been determined from filtered samples ; and if so, it must be 
remembered that such determinations may have been greatly 
influenced by a foreign factor, viz., the colour of the chromato- 
phores of the plankton-organisms in greatest profusion at the time. 
Until the colour of the water has been determined from filtered 
samples, we cannot, in my opinion, directly compare the colour of 
the water in these lakes with that of the water in the alpine lakes, 
in which the amount of plankton, especially in the surface layers 
of water, is altogether insignificant as compared with our lakes. 
In winter a great many plankton-organisms totally disappear 
from the water : this is the case with certain species which in 
more southern latitudes occur all the year round ( Ceratium 
hirundinella), but with us they produce their resting organs in 
autumn and disappear. I think it is very probable that those 
resting organs which, before winter sets in, are precipitated to the 
bottom in the deepest parts of the lakes, never rise to the surface 
again, but sooner or later die off, not finding the necessary 
conditions for germination. In my opinion, the plankton-organisms 
of the following year are mostly derived from those resting organs 
which were deposited in shallower water nearer the shore, where 
the waves during the spring gales sweep the bottom, carrying 
away the resting organs and scattering them over the lake. In 
our lakes the resting organs of the different plankton-organisms 
are most plentiful in April and May, after the heavy storms ; and 
I have shown in my Plankton paper that many plankton-organisms 
are in May most abundant near shore, and that their distribution 
over the whole lake does not take place till later in the year. 
I may here remark that very probably — though direct observa- 
tion is very difficult — various plankton-organisms, especially 
certain Diatoms ( Tabellaria fenestrata, Diatoma elongatum), may 
have alternately a fixed littoral stage and a free-swimming or free- 
floating pelagic stage, and these two stages may be restricted to 
certain seasons, the shape of the colonies in the littoral stage 
