20 HENRIK PRINTZ [1920 
P1. II, fig. 181 shows an emptied zoosporangium. The length of the 
zoospores Is on an average 1,5—1,8 u, and they are liberated by 
the rupture of the mother membrane. In the material examined, the 
formation of zoospores seems to be rare. The species propagates 
very frequently, on the other hand, in the narrowing and at last 
the dividing of a larger thallus into two or more smaller ones, which 
independently and directly grow further. In addition, it forms 
akinetes through the rounding off and liberation of the cells on 
account of the disintegration of the midmost membrane layer. Fig. 
138—150, 152—155 show a series af akinetes. The walls of the 
akinetes are sculpturless, rather thick, colourless, sometimes 
stratified. 
The genus Pleurastrum has been founded by CHopat in 1894, 
I. €. Later on, in 1899, Juri Snow has, under the name of 
Pseudo-Pleurococcus, more accurately drawn and described the two 
species being hitherto known of this genus. Ås to the systematical 
position of these plants, great uncertainty has been reigning among 
the leading algologists. WiLLe, HrErinG (in PAScHER, Die Siss- 
wasser-Flora, 1914) a. o. class them among the Ghaetophoraceae, 
while West, Algae, 1916 p. 192, designates this genus as å «Proto- 
derma-state» of Protococcus (Pleurococcus), a state, which under cer- 
tain conditions, as too much dampness, may arise from Protococcus. 
CHopart (Algues Vertes de la Suisse, 1902, p. 281) is also of the same 
opinion. However, as it has not been proved that the true genus 
Protococcus forms zoospores, which it does not do, at least under 
normal conditions, I consider the last mentioned authors views as 
not yet established, and I therefore class the genus Pleurastrum 
provisionally among the Ghaetophoraceae. It is, by the way, remark- 
able that a so important question, regarding some of the most com- 
mon algae of the world, and in spite of this species having been 
brought into cultures many times, has not yet been definitely setiled. 
This probably may be due to the fact that the material for examina- 
tion in many cases has been heterogeneous, not originating from the 
same species. Under so homogeneous and extreme conditions of life 
as those of subaérial algae, various systematic types will easily adopt 
a uniform character. Especially the primitive and slightly differen- 
tiated forms will not be sufficiently morphologically characterized, 
So that it is not always possible from the external features solely to 
diseriminate the forms, and many of them are only to be distin- 
guished in certain stages. 
ft must be made an unavoidable requirement for all pure 
cultures of these algae that the måterial is derived from a single 
cell only. Otherwise one will too easily confound systematically 
heterogeneous types and get a systematicelly impure material, which 
during the later development will go in different directions and 
