40 KAARE MÜNSTER STRØM. M.-N. Kl. 



Deserts and dry tracts with unsuitable j^e^logir formations are restricting^ 

 factors of importance. 



These different Desmidiaceæ districts are also met with again in the 

 distribution of the special plankton species. 



Now the (|uestion is, on what is the occurrence of the Desmidiaceæ 

 as plankton organisms in the lakes dependent. — Based on the investiga- 

 tions now made I can only confirm Messrs. West's conclusions. Plankton 

 Desmidiaceæ only appear in lakes containing no free, dissolved lime worth 

 mentioning. They may appear also in chalky districts, but then onlv in 

 brown water rich on humic acids. (When speaking here of occurrence of 

 Desmidiaceæ I mean occurrence in really large quantities, not merely 

 individual specimens that may have been washed off from the shores.) The main 

 condition on which the occurrence of the Desmidiaceæ depends is therefore 

 absence of l'une. Water derived from peat-moors and bogs is no necessity; 

 even where there are no bogs in the drainage area Desmidiaceæ may 

 occur in plenty. 



I have found that a slight contamination of the water from surrounding 

 cultivated areas does not exercise any unfavourable influence on the exi- 

 stence of the plankton Desmids but only augments the plankton bulk. 



The same experience has been derived by Professor G. M. S.MrrH, 

 in the American lakes. A more decided contamination, as is the case in 

 a number of lakes in Central Europe and in several Baltic lakes, would 

 however destroy the Desmids of the plankton (excepting the tycho-limnetic 

 species Staiirastnnii i^racile and Staiirastnini poradoxuiii]^ . 



In entirely clear water (in places where rivers or currents do not carry 

 away bottom forms) the eulimnetic species of Desmidiaceæ will be predominant, 

 in water with surrounding bogs they would be intermixed with benthos 

 forms washed down from same. 



A great number of these benthos forms will not propagate in the open 

 lake but will soon perish, but quite a lot of them will continue to flourish 

 in the lake and form a tycho-limnetic flora. 



In that manner the replenishment of the Desmid plankton has without 

 doubt gone on. — Bottom forms have been washed off into the lakes and 

 those that have been the most fit for the pelagic life have propagated there. 

 In the cuorse of times some certain kinds of these plankton forms adapted 

 themselves to the pelagic life and are now described as plankton varieties 

 of existing species or have even developed into new species exclusivel}' 

 confined to the plankton. This development is without doubt going on to 

 this very day. 



As an example the Xaiifhidiitin aiitilopcrun generally found both in 

 benthos and plankton may be mentioned, which developes the variety 



' These Desmids are also among the very few plankton species, which may thrive well 

 in limestone districts. 



