KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 57. N:0 7. 59 



must thus be an adaption to the hydrography of that Arctic water. Now tliere is a period 

 of the year, when the Scandinavian coastal waters offer exact the same ice-cold and pro- 

 perly dilute water, to which the Sira-formation is accustomed. It develops and propa- 

 gates luxuriously, thanks to a good supply of alimentary stuffs, but it is by no means 

 sure of finding afterwards the same conditions of rest, nor the same chances for getting 

 the spores preserved and brought into favourable germinating conditions, as in its Arctic 

 home. Put it that a transition to spores is only partial, while other cells keep unchanged; 

 then tliere will always be a predominance of the lighter, ordinary cells kept floating near 

 the upper surface after the general mixing through in November. The spores must then 

 necessarily be handicapped as germs for a new generation to develop in the undermost 

 part of the subsequently outbursting Baltic Current. There must in other words be a 

 selection, attempting to diminish the disposition to spore-formation. 



Out from this reasoning, I have looked for data about the relative frequency of 

 spore-formation in the Skager Rak and in the Arctic Sea. From the latter, exact infor- 

 mation is unluckily not to be had as yet. In his report on the Campagne arctique of the 

 Duke of Orleans, Meunier (1910) states most of the Thalassiosirae to occur with spores 

 in formation or ready. By Th. Nordenskiöldii they were, however, very rare, by Th. 

 gravida on the contrary common, as well as by Lauderia glacialis. These samples were 

 fished round Nowaja Semlja in July — August 1907. In the Skager Rak, Gran in February 

 1912 seems to have found only one species readily changing into spores, viz. Lauderia 

 glacialis, the season being apparently too little advanced yet for this phenomenon. In 

 my material from February 1914, spores are found in Chaetoceras contortum very rarely; 

 in Thalassiosira gravida and Th. fallax Meun. less rarely, and in Thalassiosira Norden- 

 skiöldii only once, in a bottom-sample from S. Extra II, where they attained 2 °/ on ly- 

 At this station, spores were as a whole more common than elsewhere, amounting to 

 10 — 20 % of the total number of cells for Th. gravida and fattax. This seems to be the 

 maximum percentage for our waters, as far as yet known, and there is hardly any binding 

 reason for looking just to this minority for the preserving elements of the race. 



Also the Leptocylindrus-associ&tion passes into spores before resting, at least as re- 

 gards some more or less common species of Chaetoceras. I have observed spores in Ch. 

 contortum, Ch. debile and Ch. constrictum of this formation, and further in Ch. sociale and 

 Ch. diadema, that have to be separated from the former association. Only in these last 

 species, the changing into spores is fairly complete, which is of a so much greater interest, 

 as no species could aff ord a better support for the pelagic spore-hypothese, if we may call 

 it so, than does this minute Chaetoceras sociale from the nature of the spores themselves. 

 In regard to their small size (diam. c. 5 \>.), they must necessarily sink only very slowly 

 and get whirled up by the slightest movement in the water, should they succeed in getting 

 to rest at the bottom. Moreover, they keep several together, which must increase the 

 floating power. As in other species of Chaetoceras, this feature is caused by the old and 

 empty cell-walls holding together by the setae long after the spores are ready-formed. 



If the persistance of chains after a complete passing to the sporal stage is a feature 

 of pelagic adaption, this may also apply to another feature, viz. the prominences, such 

 as branches and hairs, so common in spores. An adornment like that of the spores in 



