1 92 1. No. 4. THE PHYTOPLANKTON OF SOME NORWEGIAN LAKES. 39 



Others will only grow in the Arctic and Alpine cold water, although all of 

 them have the peculiarity in common that they have no distinctive or clearly 

 defined geographical distribution. 



Thev are found where the physical conditions are suitable, in other 

 words where the right conditions of life are present. The Desmids, 

 on the other hand, have a typical geographical distribution, like the 

 higher plants. — This is owing to the fact that they soon perish, when 

 drj'ing up in vegetative state, and that they also so rarely produce zygotes. 

 Even partial drying up has an instantaneous fatal effect upon them, save 

 some few kinds of the lower species, specially the Saccoderm ones. 



In this connection it may be of interest to call to mind the fact pointed out 

 already by Schmidle that it generally is the species having the most complex 

 external surface that have the most distinctive geographical distribution. 

 One ma}' perhaps think that this is owing to the fact that their more complex 

 outer surface tends to effect a quicker drying up when they are transported, 

 but it should also be borne in mind that the most complex external sur- 

 faces very often are found in the newest and most advanced species which 

 no doubt have developed independently of each other on Continents that 

 have been separated from one another during the later geological ages 

 whereas the more simple kinds are the more ubiquitary original types. 



Generally speaking, a more careful investigation of the geographical 

 distribution of the Desmidiaceae might throw a light on the distribution of 

 land and sea during the later geological ages. 



We are already now able to differentiate between a number of geo- 

 graphical communities of Desmids, a West-European (might perhaps 

 properly be called the Occidental), the distribution of which to some extent 

 corresponds with the distribution of the Caledonian phytoplankton, i. e. the 

 Western Europe and large parts of the Eastern North America. A great 

 number of species is confined to the uttermost West of Europe and to the 

 East of North America. 



Other distinctive communities are the Indo-Malayan, which extends 

 downwards rather far into Australia, and the Australian proper. Likewise 

 there are a number of Desmidiaceæ having a characteristic circumpolar, 

 Arctic distribution. — (How many of these Arctic species haxe a geogra- 

 phical distribution in the above-mentioned stricter sense it is hard to tell 

 but in view of experiences gained by myself I am disposed to think that 

 it is the greater part). 



Barriers are probably raised between the different zones by the oceans 

 but e\en more by the large tracts of unfavourable localities for Desiiiid- 

 iacetr. (In his excellent treatise on the pelagic organisms in the Scottish 

 lakes Mr. James Murray also calls attention to this point). 



Tlie Desmidiaceæ that have the most (listi)ictive geographical distribittion 

 are chiefly those that make the greatest claims on the physical resources of 

 the groicii/gplaces. 



