20 HENRIK PRINTZ [1920 



PL III, fig. 131 shows an empticd zoosporangium. The length of Ihe 

 zoospores is on aii average 1,5 — 1,8 ^^, and! the}'- are liberalted by 

 the rupturc of the molher meimbrane. In the maiterial examined, the 

 formation of zoospores seems lo be rare. The species propagales 

 very frequently, on the other haiid, in Uie narrowing and at last 

 the dividing of a larger thallus into two or more smaller ones, which 

 independenlly and directly grow further. In addition, i,t foiuns 

 akiiietes through the rounding off and liberatioii of the cells on 

 accounl of the disintegration of the midmost membrane layer. Fig. 

 138—150, 152 — 155 show a series af akinetes. The walls of Ihe 

 akinetes are sculpturless, rather thick, colourless, sometimes 

 stratified. 



The genus Pleurastrum has been fomided by Chodat in 1894, 

 I. c. Laler on, in 1899, Julia Snow has, under the name of 

 Pseudo-Pleurococcus, more accurately drawn and described the two 

 species being hitherto laiown of this genus. As to the systematical 

 position of these piants, great uncertainty has been reigning among 

 the leading algologists. Wille, Heering (in Pascher, Die Siiss- 

 wasser-Flora, 1914) a. o. class them among the Chaetophoraceae, 

 while West, Algae, 1916 p. 192, designates this genus as a «Proito- 

 d6rma-state» of Protococciis (Pleurococcus), a state, which under cer- 

 lain condilions, as too much dampness, may arise from Protococcus. 

 Chodat (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 yeit established, and I therefore class the genus Pleurastrum 

 provisionally among the Chaetophoraceae. It is, by the w'ay, remark- 

 able that a so important question, regarding some of the most com- 

 mon algae of the world, and in spite of this species haYing been 

 brought into cultures many times, has not yet been definitely settled. 

 This probably may be due to the faet that tlie 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 sul>aérial algae, various systematic types will easily adopt 

 a miiform character, Especially the primitive and slightly differen- 

 tiated forms will not be sufficiently morphologically characterized, 

 so that it is not always possiible from the extemal features solely to 

 diiscriminate the forms, and many of them are only to be distin- 

 guished in certain stages, 



It must be made an unavoidable requiirement for all pure 

 cultures of these algae that the m.aterial is derived from a single 

 cell only. Otherwise one will too easily confound sysitematioaLly 

 heterogeneous tj^pes and get a systematicelly impure material, which 

 during the la ter development will go in different directions and 



