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



within wide limits of salinity, as they are submitted to be passively brought into an un- 

 favourable medium by slnking and by movements of the sea. But a look at their curves 

 of frequency will soon make clear, what special exigenees most of tbem have for 

 salinity, as well as for temperature, to enable tbem to »flower». The different asso- 

 ciations of plankton, living in the Skager Rak and spöken of above, have tbus been 

 shown to requirc, each of tbem, a special kind of water for propagating. The Sira- 

 plankton grows in a cold and dilute water of nearly 30 % , the LeptocyUndrus-a,sso- 

 ciation in a somewhat warmer water of abont the same salinity, the Guinardia- 

 association in a rather warm and salt (34 — 35 %o) medium, etc. Duringapreviousin vesti- 

 gation of the water-system of Stockholm, I have come to similar results and found the 

 plankton exceedingly sensitive as to small changes in salinity. Here, three different 

 associations are localised, one to Lake Mälaren, a second to the big, slightly brackish Bay 

 of Värtan just outside the city, and a third to the Baltic Skärgård outside of Vaxholm. 

 None of tbem is able to live within the boundaries of the other ones as an intact forma- 

 tion, and there are exclusive species for each of them, though other forms may stånd the 

 differences between two of the water -kinds. 



3. Persistence and recruiting of the formations. Röle of the sporification in 



plankton-diatoms. 



Alreadythe present material bas furnished convincing evidence as to the mode of 

 persistence and regeneration of our common endogenetic forms from one flowering sea- 

 son to another. It could be stated, that species such as LepU>ct/Hndrus and Thal&ssiosira 

 Nordenskiöld ii are present in the Skager Rak all the year round, in more or less spread 

 specimens, that may be supposcd capable of division again, as soon as allowed by t be 

 pbysico-chemical conditions. Noav a large number of the frequent neritic diatoms change, 

 as is well known, into so-called resting spores 1 at the end of their maximum development . 

 and it bas been suggested, especially by Gran, that such spores are sinking to the bottenn 

 and will germinate in a coming season of vegetation, after baving »oversummered» there. 

 As far as the Skager Rak is concerned, I have, bowever, been led to a somewhat different 

 opinion, whieh does not of course mean, that Gran cannot be right in other, special cases, 

 where the nature of the coast and the movements of the waters are different. For the 

 following reasons, an intermediate sporal stage does not seem to signif y f or the tif e-cyele 

 of pelagic diatoms strictly what Gran means. Firstly, many common and important 

 species do not form resting spores. Such are for instance Sceletonema, the Thalassio- 

 thrix and the Nitz.schiae, Biddulphia aurita, Cerataulina Bergonii, many Rhizosoleniae 

 among the neritic diatoms, not to speak of Guinardia and other more oceanic forms. 

 In this case, there is but one possibility of regeneration open, viz. by division of a few erring 

 germs of ordinary cells, that have managed to keep alive through an often long dead season. 

 If this now proves to be possiblc for a number of species, the same mode of persisting 

 cannot be denied for the other one-; witbout binding arguments, so much the less, since 

 ordinary cells of sporogene diatoms, as Thalassiosira, are really observed to occur regularly 





It is not kuowu, for liow long a tinie tliej- rest, and even whether they can be said to rest at all. 



