206 



The British Freshwater Phytoplankton. 



in the Desmid-community of the plankton quite as much as in the general 

 Desmid-community of the surrounding country. In fact, they appear to 

 be well marked.* 



Even with the meagreuess of our present knowledge we can recognise 

 three distinct plankton-communities of Desmids, which can at once be 

 distinguished from each other, and which form a most interesting com- 

 parison. These are (1) the Desmids of the British (and to a certain extent 

 of the Scandinavian) plankton, (2) the Desmids of the plankton of Victoria 

 Nyanza, and (3) the Desmids of the Victorian plankton (as exemplified by 

 the Yan Yean Eeservoir). There are doubtless several other distinct 

 plankton-communities of Desmids, notably in the South American and the 

 Indo-Malayan regions. There are marked geographical peculiarities in 

 the general Desmid-community of these regions, and should any of the 

 lakes be found on investigation to possess a Desmid-plankton, it is highly 

 probable that many of the species will possess their proper geographical 

 character. 



* In considering this question it should be borne in mind that Desmids in the vegetative 

 condition cannot be blown about by the wind, as even partial desiccation is almost 

 invariably fatal. Also, that in most species zygospores are very rarely formed, and that 

 no zygospore has yet been observed of any of the typical plankton-species. The only 

 Desmids which appear to survive a partial desiccation are certain species of Cylindrocystits 

 and Penium, and possibly of Mesotcenium. 



