Desmidiacece 



163 



produced mostly by pathological conditions. Zygospores of few 

 species are known; they are globose and smooth. 



Fig. 59. Tetmemorus granulatus 

 (Breb) Kalfs, from Lough Anna, 

 Donegal, Ireland. A, front view of 

 cell ; B, zygospore ( x 315). 



Fig. 58. A, Pleurotcenium coronatum 

 (Breb.) Rabenh., from Helvellyn, West- 

 moreland ( x 236). B, zygospore of PI. 

 Ehrenbergii (Breb.) De Bary, from 

 Thursley Common, Surrey ( x 315). 



Species of this genus are much more abundant in the tropics than in 

 temperate regions, and the tropical species frequently assume a secondary 

 filamentous condition. Only nine species are known as British, of which 

 PL maximum (Reinsch) Lund, is the largest, reaching a length of 852 /* and 

 a diameter of 54 p,. The most abundant species is PL Ehrenbergii (Breb.) 

 De Bary; length 240400^; breadth 18 24/*; fig. 58 B. PL Trabecula 

 (Ehrenb.) Nag. and PL truncatum (Breb.) Nag. are each widely distributed. 

 PL nodosum (Bail.) Lund, is the most striking species and is confined to the 

 western districts of Wales, Ireland and Scotland. 



Genus Tetmemorus Ralfs, 1844. This is a well-marked genus 

 with straight cells of moderate length, slightly constricted in the 



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