192 1. No. 4. THE PHYTOPLANKTON OF SOME NORWEGIAN LAKES. 4 1 



Hchridanini, a typical plankton variety, and an acute observer will no doubt 

 find many similar species that are dexeloping plankton varieties. 



The Cosiiiariimi abhrcviatimi and its variety plmictouicuni are specimens 

 of more advanced Desmids. Here the variety has got quite constant features 

 and the slight difference only between the same and the principal torm 

 renders a specific differentiation impossible. 



Special plankton species may be restricted to rather limited areas which is 

 perhaps a sign that the evolution can go on relatively quickly, an assumption 

 which is not at all unreasonable, considering the short life time of the 

 generations. — Thus, the Micrastcrias Murray i occurs only in Scotland; 

 and the Micrastcrias //(r^rr/v/ that has developed from the Mahahiilcslra^arciisis 

 group has only been observed in the Yan-Yean reservoir in Australia (but 

 may very well have a further distribution in Australian territor}'.! 



But it should be borne in mind that this theory of evolution in the 

 plankton Desmids is still merely a hypothesis, though greatly supported by facts. 



An established fact is however that they belong to three groups to 

 be characterized as follows: i. Dcsmidiacccc belonging to the luicropliytic 

 benthos, washed off to the plankton by chance ivithotit propagating there. 

 This group plavs as a rule only a trifling part, although it may be of some 

 significance in peaty districts and in districts where ri\ers and rain may 

 wash them oft' their habitats. 



2. Desniidiacecc originating from the niicrophytic benthos, but propagating 

 under the pelagic conditions of life. This group of Desmids pla_\-s an im- 

 portant part in many lakes. It includes chiefly the species that Messrs. 

 West have marked with (p), i. e. species which are more abundant in the 

 plankton than elsewhere ; but also other species that are generally distributed 

 åoth in plankton and in benthos can be included in this group. 



3. Varieties or species only occurring i/i the plankton. 



These are the genuine plankton organisms and come under the designa- 

 tions (Pv) and (P) specified by Messrs. West. 



It mav however be that some kinds of the Desmidiaceae of the pre- 

 ceding group should be considered as genuine plankton organisms in those 

 countries where the}' occur pelagically even if in other parts of the World 

 they are also benthos types. — Stanrastnini Brasiliense var. Lnndellii, 

 Cosniariitin pachyderinuni var. Æthiopicuni, Gotiatozygo)i aculeatuni, Desniidiiini 

 Pseudostreptonenia and other Tropical and Occidental forms may be taken 

 as illustrations of this. 



The dogma of the ubiquity of the phytoplankton in lakes must now 

 be said to be obsolete as far as the Desmid plankton is concerned. — 

 However, at the present stage of science we can clearly define one plankton 

 formation onl}-, viz. the Caledonian. 



The Baltic and Central-European one has, it is true, been subjected 

 to most thorough investigations, but its geographical area has not been sharply 

 defined. It seems to consist mainly of ubiquitous species and to occur 



