1110 Proceedings of Royal Society of Edinburgh. [sess. 
at 1°-1*5° and at 8°-8'5°; if we must choose the most favour- 
able tp., it must be 4°-5°.* We do not find any tendency to 
relation to the light. Asterionella reaches one max. in the winter 
(November-February) and another in May- June. I think we 
may say that the growth of Asterionella in some degree depends 
upon the tp., but that the limits of the max. tp. are very wide, 
going from 1° to 8°. 
The two Melosirce act in a manner very similar to that of the 
Asterionella , although with some differences. They prefer a some- 
what lower tp. than the Asterionella ; their max. lies in March- 
May at a tp. of l°-5°, and they cannot endure a tp. of 7°-8° 
as well as the Asterionella. They have no distinct relation to 
the light. 
Finally, we take the Peridinium aciculiferum ; it appears 
suddenly in great quantities in the sample of February at 1*5° and 
flowers in the next samples at about the same tp. (l°-2°), but 
when the tp. rises it decreases in number ; consequently it has its 
max. at l°-2° and is dependent on the tp.f Nevertheless we 
have no explanation of its sudden occurrence, and here, I think, 
we must take the light as the moving factor. The remaining forms 
have no distinct max. ( Staurastrum pelagicum seems to follow . 
Sphcerocystis). Some of them occur rather evenly throughout 
the entire year, but not in quantities ; others, e.g., Fragilaria 
construens , show no regularity, and these last mentioned manifest 
thereby that they are only occasional guests in the plankton. If 
we summarise our remarks, they will be in accordance with my words 
quoted above. Tliingvallavatn has a phytoplankton consisting 
mainly of a few species of Diatoms ( Asterionella and Melosirce ), 
Myxophycese are wanting, Flagellates (in the widest sense) and 
Chlorophycese are without greater importance. The phyto- 
plankton has not at all an alpine character, but is very like the 
plankton of the lakes in the Central European lowland during 
ivinter and early spring. It is very poor in species and one of 
its most remarkable features is the number of organisms one might 
* Some authors (Marsson, B. Schroder) have suggested that the number of 
cells in the stellate colonies depends on the seasons, but this is not the case in 
Thingvallavatn, as I have found 4, 5, 6, or a still greater number of cells in each 
colony in all the samples. 
t For its relation to the tp. in other countries see p. 1128. 
