58 ASTRID CLEVE-EULER, QUANTITATIVE PLANKTON RESEARCHES IN THE SKAGER RAK. 



may turn out to be, I am compelled from my present experience to deny any lasting 

 dwelling of theirs on the bottom of the Skager Bak. Still, I intend to continue the exa- 

 mination of bottom detritns at the Swedish shore for at least a year, in order to procure 

 still more reliable information concerning this question about the spores. 



2. The spore will meet, especially in deep basins, witli a water, den se enougli to 

 prevent further sinking. The species then has simply gone över from a form, fit for 

 pelagic life in a less dense water, to one adapted for pelagic life in a denser water. As a 

 sinking means withdrawing from light, this last mode of pelagic life must be a more or 

 less passive one, until the spore, by drifting continually, happéns to meet again with 

 conditions, congenial to a more intense activity of life. 



There are, I think, several circumstances speaking in benefit of this view, that 

 rednces also the spores to an essentially pelagic mode of life, above all its generality. 

 Then out from it, we are capable of envisaging the biological changes of all pelagic algae 

 from one common point of view. A continual chain of types can be formed, from regularly 

 spore-producing species, as many Chaetoceras and Thalassiosirae, then more seldom and 

 accidentally sporal diatoms, as other species of Chaetoceras, över to species, never form- 

 ing spores, but having chosen another, less radical method of increasing their power of 

 sinking at the beginning of resting periods, namely a general thickening of the walls and 

 passing över to a more clumsy or stout structure. Fine instances of this latter kind are 

 provided by several limnetic, pleomorphous species of Melosira (A. Cleve-Euler, 

 1912 II), and also by jDelagic diatoms as Corethron and Eucampia balausticum (Karsten). 

 Gran has drawn attention to the variations between slender and robust races of the 

 forms lacking spores, as Chaetoceras decipiens and Biddulphia. aurita. Going still further, 

 we find some eupelagic forms, getting out of play simply by making their cells heavy with 

 heaped assimilates. This I have observed in Leptocylindrus danicus and Bhizosolenia 

 Shrubsolei. The last link to add is a group of eminently pelagic diatoms, wbose more 

 or less needleshaped cells do not display any visible changes whatever, but where the 

 floating power is modified, when needed, by the forming or dissolving of colonies. In 

 the sea of Skager Rak, this group is chiefly represented by Thalassiothrix ■nitzschioides. 

 Under special circumstances, the last quoted type gains quite the upper hand, as I could 

 show for the Värtan by Stockholm, a rather large and deep, almost isolated bay of the 

 Baltic, with a feebly brackish water, stagnant in the depth (1912 I). The reason is not 

 difficult to find out; indeed, the Diatoma-f ovm&tiow of Värtan presents itself as the result 

 of a natural selection, allowing only such organisms to persist, as are not subject to sink- 

 ing easily at any stage of their life, but remain coiistantly, to the highest percentage 

 possible, floating in the only habitable, thin surface-layer. (In the main water-masses 

 beneath the thin surface-cover, the plankton is killed by lack of air. ) 



I think we can take this particularly clear case as a starting point for trying to 

 understand something of the regenerating problem in the Skager Rak. Taking first the 

 Thalassiosirae and similar discs of the winter-plankton into consideration, we know, that 

 their centre of distribution is the great Arctic Ocean, and may consequently expect their 

 general features of evolution to be in special harmony with the physical and hydrographic 

 conditions of the Arctic Sea. A formation of resting spores, characteristic to these forms, 



