CHAPTER SIX 



THE GENUS PLEURODISCUS LAGERHEIM 1895 



This genus was established by Lagerheim to classify purple- 

 colored vegetative filaments, the cells of which had two distinct 

 disc-shaped chromatophores, one on either side of a central 

 nucleus (P. purpureus). Skuja (1932) questioned the validity of 

 the genus and suggested that the chromatophore is merely an 

 expanded form of the Zygogonium chromatophore due to envi- 

 ronmental conditions. In 1936 Tiffany described a species P. bor- 

 inquenae from Puerto Rico, the first and only specimens found 

 in a fruiting condition. The chromatophores certainly were disc- 

 shaped and oriented at various angles to each other. The proc- 

 esses of conjugation and spore formation resemble those of 

 Zygogonium. 



Specimens resembling Lagerheim's figure, collected near 

 Eaglesmere, Pennsylvania, and those collected by Randhawa in 

 India, were associated with smaller vegetative filaments of Zygo- 

 gonium; there were no intergradations. Environmental factors 

 do not seem to account for the differences in chromatophores in 

 these collections. When Zygogonium filaments are growing lux- 

 uriantly, the pillow-shaped chromatophores are larger and there 

 is a fringe of several stringlike or radial projections, very different 

 from the smooth-edged disc, or saucer-shaped, bodies of Pleuro- 

 discus. Further study of the algae growing in the drainage from 

 wet acid rocks and soil will probably uncover additional species 

 and clarify the status of this genus. 



Description of Species 

 Pleurodiscus borinquinae Tiffany 1936. Brittonia. 2, p. 169, 



Figs. 31-39- 

 Vegetative cells 18-26 /a x 16-65 /a, pectic sheath sometimes thick; 

 filaments either simple or branched and having rhizoids; zygospores 

 ovoid to ellipsoid, within a sporangium partly formed by the tube 

 papillae and partly by a collar between them; zygospores 22-32 /^ x 

 26-32 ju. with a scrobiculate spore wall; pits 3 to 5 /a in diameter. (PI. XII, 

 Figs. 12-15.) 



Puerto Rico, Palmar, January to March (Wille Coll.). 



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