Algae. 
541 
lakes or high-alpine; the first group harbours some Myxophyceae 
(i Oscillatoria ), many Diatoms (characteristic are Cyclotellae and Ta- 
bellariae ), few Chlorophyceae (, Sphaerocystis , Botryococcus) , many 
Flagellates ( Dinobryon , Cevatium hirundinella)\ the second group has 
a poor phytoplankton built up by Diatoms and Flagellates. 
5. The phytoplanktons of the Mediterranean lakes, of the 
North American lakes, of the Asiatic lakes and of the tropical 
lakes are rather insufficiently known and are but briefly mentioned. 
The life conditions and adaptation forms of the plankton of each 
geographical area are discussed at length; the chapter ends with a 
summary from which we quote the following:Thefreshwater plank¬ 
ton is characterized by its well-marked cosmopolitanism; the 
freshvvater associations (communities) in contrast to all other asso- 
ciations on land and water, everywhere contain the same types, 
nearly everywhere the same species. The freshwater plankton 
is amongst the oldest associations of the earth. This peculiar 
phenomenon may be explained by its very great power of adaptation 
to various outer conditions. 
The following general chapters, the origin of the freshwater 
plankton and its relation to the Glacial Age, are mainly based 
upon zoological materials. The author emphasizes that only a very 
small portion of the freshwater plankton organisms have immigrated 
directly from the pelagic region of the sea to the pelagic region of 
the lakes; the main part must be considered as littoral and bottom 
forms which have adapted themselves to pelagic life, at least to some 
degree (some periods of their life). 
IV. After a short picture of the ways in which the plankton of 
the Baltic lakes has been transformed in the process of time from 
the Glacial Age until now, the author in his last chapter characterizes 
the different plankton associations of the Danish freshwa¬ 
ter s. The stagnant freshwaters are referred to ten types, mainly after 
their geological development; the plankton associations are different 
in the different freshwater types. Wesenberg-Lund arranges them 
in nine categories, of which we notice here the phytoplankton: 
1. The true lake plankton differs somewhat according to 
depth of the lakes. In deep lakes the Diatoms produce enormous 
maxima in March to November, although in the wärmest period 
Cevatium hirundinella is the main form. Chlorophyceae are very 
rare; Myxophyceae few (only Oscillatoria and Anabaena ßos aquae 
are at times dominant). The shallow lakes are chiefly characterized 
by the water-bloom, which is due to the dominance of various 
Myxophyceae\ the Diatoms are of minor importance, while the 
Chlorophyceae may be common. — The investigations carried out 
by Wesenberg-Lund are mainly made upon true lake plankton. 
2. The plankton of coast-lakes is little investigated, but it seems 
to be very poor (especially the phytoplankton); it contains much detritus. 
3. The plankton of dune-lakes (studied in May and August) 
contains very few Myxophyceae and Diatoms, but Flagellates {Synura, 
Uroglena, Peridinians) in considerable quantities, also many small 
Chlo rop hyceae. 
4. The plankton of heath-lakes are very like no. 3, but Dino¬ 
bryon seems to be dominant. 
5. The pond-plankton is very rieh in species and very 
varying from pond to pond. In contrast to the lake-plankton it is 
rare to find ponds the plankton of which consists mainly of Diatoms. 
Very characteristic planktonts of the ponds are the numerous Pro- 
