540 
Algae. 
stages are deposited on the bottom or not, and if the resting stages 
for all the species are resting spores, microspores, auxospores or 
only special, not morphologically distinct resting generations. The 
author believes that the resting stages are deposited on the bottom 
and in spring and autumn through storms are raised from the bot¬ 
tom, into the pelagic region, where the}? - germinate and begin the 
intense cell-division favoured by the rieh nutrition material and the 
light in the uppermost layers. In spring and autumn the rate of 
sinking is so slight that they persist in the upper layers, while in 
Summer the temperature is so high that most of the species sink. 
II. The v ariation-statistic al in vestigations on Ceratium 
hirundinella . The results of Wesenberg-Lund’s in vestigations are in 
the main points in accord with those of G. Entz, but some diffe- 
rences occur. In Danish lakes Ceratium is a summer form, which 
appears in April and disappears in September-November. In the 
early spring it is in general small, increases considerably in May- 
June, but begins in July to decrease and has in August reached its 
minimum size, at which it is stationary in the autumn until the dis- 
appearance. In some of the Danish lakes a special summer form 
simultaneously occurs in May together with, but without transitions 
to, the ordinary form; the large maxima are due to these summer 
forms. The occurrence in the same sample of different forms has 
by some authors been put forward as a refutation of the explanation 
of the variations of C. hirundinella as seasonal variations, but Wesen¬ 
berg-Lund urges that this is not a deciding proof as the sample 
may happen to be taken on the point of appearance of one form 
and the disappearance of another. In Denmark C. hirundinella winters 
as resting-cy^st, which the author has found in the plankton in the 
autumn. 
III. In a large chapter Wesenberg-Lund gives a summary of 
the present knowledge of the freshwater plankton of the Earth, 
its conditions of life and forms of adaptation. The plankton 
of the different geographical areas is analysed by help of the many 
scattered papers, and further the author himself has studied the 
plankton personally in different countries (Scotland, the Alps, 
Northern Central-Europe, Southern Scandinavia). The plankton 
of each area is characterised in some summarising sentences of which 
I may give the botanical parts: 
1. The arctic freshwater phytoplankton is very poor. My- 
xophyceae are wanting, Diatoms ( Tabellariae and Melosirae ) are abun¬ 
dant, Chlorophyceae and Flagellates are few, only Dinobryon is of 
importance; there is a great mixture of littoral and pond forms. 
2. The phytoplankton of the North European lakes is richer. 
Myxophyceae are still of secondary importance (main form: Ana- 
baena ßos aquae)\ Diatoms are abundant (especially Tabellariae)\ 
tycholimnetic species are common; tycholimnetic Chlorophyceae are 
characteristic, especially?' Desmids; Flagellates are rather important 
(characteristic is Peridinium Willei). 
3. The Baltic freshwater phytoplankton is very rieh. The 
Myxophyceae occur in enormous quantities; among the Diatoms the 
more important are Fragilaria crotonensis, Asterionella and Melosirae 
which all occur in great masses, while Tabellariae and Cyclotellae 
are rare; Protococcoideae are numerous, but Desmids rare. Ceratium 
hirundinella is common; with Dinobryon rather unimportant. 
4. The phytoplankton of the Central European alpine lakes 
is different according as the lakes are somewhat low-lying large 
