MICROCYSTIS Kuetzing 1833a, p. 372^ 



A free-floating or sedentary colony of numerous spherical cells 

 closely and irregularly arranged within copious mucilage, forming 

 ovate, globose, or irregularly shaped masses which are often lacerate 

 or perforate; individual cell sheaths confluent with the colonial 

 mucilage; cell contents pale or bright blue-green, or appearing 

 black or purplish because of pseudovacuoles, present in most species, 

 which are large and conspicuous, or sometimes numerous and small. 



This genus should be compared with Aphanocapsa in which the 

 spherical cells are evenly and widely spaced within a definitely 

 shaped (usually spherical) colonial investment of mucilage. Unlike 

 most of the species of Microcystis, the cells of Aphanocapsa are 

 always without pseudovacuoles. 



The reader is referred to Elenkin (1924), to Drouet and Daily 

 ( 1939 ) , and to Teiling ( 1946 ) , for critical remarks on the synonymy 

 of species in this genus. In the following key those species which 

 have been reassigned by students of the genus are given in paren- 

 theses. 



Key to the Species 



1. Colonies saccate, lobed and clathrate, the colonial mucilage 



thick and refractive in young plants M. aeruginosa 



1. Colonies globular, or ovate, definite in shape, 



not perforate or clathrate 2 



2. Cells without pseudovacuoles or, if present, 



small and inconspicuous 4 



2. Cells with large and conspicuous pseudovacuoles 3 



3. Colonies simple, a large mass of much crowded cells inclosed 



by a transparent, mucilaginous envelope (M. -flos-aqtiae) 



M. aeruginosa 



3. Colonies compound, several groups of cells, each with a colonial 

 envelope, inclosed within a common mucilage.— (M. ichthyohlabe) 



M. aeruginosa 



4. Cells 1-2/ui in diameter, small and numerous but uniformly 



distributed within the colonial mucilage M. incerta 



4. Cells 2— 3/i in diameter, compactly arranged 



within the colonial mucilage (M. pulverea) 



M. incerta 



''See notes by Drouet and Daily (Daily, 1942, p. 638) in which they explain the use of the 

 generic name Polycystis Kuetz. for the species grouped under Microcystis Kuetz. I prefer to 

 retain the latter name because it is generally accepted and well understood. Although 

 Microcystis was used originally for a miscellany of organisms, and at one time only for 

 flagellates, those species to which it referred in earUer times have all been transferred to 

 their proper places in other genera. Since the name Microcystis no longer applies to any of 

 them, there is no danger of taxonomic confusion. In any case, the creation of Polycystis as 

 a genus by Kuetzing in 1849 was antedated by the use of that name for a genus of 

 Uredineae (Polycystis Leveille 1846, Annales Sci. Nat. Bot., 5 (Ser. .3), p. 269). This 

 precludes ^he revival of the name for the species now grouped under Microcystis. 



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