Microplankton from the Finnish Waters dwing the Month of Man 1912. 5 



When the low salinity of the Finnish waters was taken intu considération it was 

 probable, that a rather poor plankton would be found. On t hat account this treatise was 

 begun with some scepticisiu, but the résulta were encouraging. Especially Acbnanthes 

 taeniata was very interesting, occurring very abundantly, particularly in the inmost 

 part of the Gulf of Finland, where the River Neva earries much nutritive water into 

 the sea. The maximum nuniber of cells pr. litre was hère four millions. The cells are 



JL 



Pig. 2. Achnanthes taeniata. 



a. Chain without spores. 



b. The first walls of the resting-spores are forraeil. 



c. Chain with resting-spores. 



d. Uesting-spores in front- view. 



950 

 1 



very small compared with many other plankton-organisms; but when the long chains 

 occur in such large quantifies as hère, they can be observée! with the naked eye. 



Achnanthes taeniata of May gave very interesting material for studying the deve- 

 lopraent of the resting-spores. Every phase of development was to be found from the 

 delicately-walled cells to the thick-walled finished resting-spores. The slender-walled cells 

 were often very beautifully preserved with quite distinct H"f° rme d chromatophores. 



During the formation of the resting-spores the contents of the cells will be coloured 

 black by the osmic aeid in the preserving fluid. The spores being formed by cell-division 

 are apparently placed two in each cell. At the same time the chains will twist into 

 spirals. 



The breadth of the chains examined was 13 — 30 <<. 



Both the thin-walled chains and those with resting-spores occurred at every station, 

 the resting-spores for the most part dominating. This species occurred most abundantly 

 in the Gulf of Finland, especially at the inmost station (F 41). The maximal nuniber 

 of cells pr. litre amounting to "2600000 cells without resting-spores and 1305000 



N:o 5. 



