Forming a blue-green algal film on the bottom; tychoplanktonic. 

 Wis. 



Oscillatoria tenuis var. tergestina (Kuetz. ) Rabenhorst 1865, p. 102 



PI 110, Figs. 12, 13 

 A variety differing from the typical by its smaller proportions, 

 5.5-6ja in diameter; apical cell convex or cone-shaped. 



Drouet ( 1938, p. 269 ) states that both this variety and var. natans 

 may be found in the same collection, suggesting that the differences 

 in size hardly justify the use of two varietal names. Gomont in his 

 monograph ( 1892, p. 221 ) recognizes the variety. 

 Euplanktonic and tychoplanktonic. Wis. 



Oscillatoria terebriformis C. A. Agardh 1827, p. 634 

 PI. 107, Figs. 25, 26 

 Trichomes forming a plant mass, dark steel-blue in color; spirally 

 twisted, especially at the apex, slightly tapering in the apical region. 

 Apical cell round or pointed, not capitate, and without a calyptra. 

 Cells 4-6.5/x in diameter, 2.5-6/x, long; not constricted at the cross 

 walls. 

 Mich. 



PHORMIDIUM Kuetzing 1843, p. 190 

 Plant mass consisting of simple, unbranched filaments within 

 agglutinated and diffluent sheaths which form mucilaginous layers 

 or penicillate tufts (streaming in flowing water). Trichomes cylin- 

 drical throughout, except for a slight and brief tapering in the apical 

 region; either parallel or entwined. Apical cell conical, blunt- 

 pointed or capitate, with or without a calyptra. Individual sheaths 

 usually indistinct and difficult of demonstration, although the 

 diffluent sheaths of the plant mass are clearly in evidence. Cells 

 shorter than wide, or quadrate; constricted at the cross walls in 

 some species. 



Under certain conditions of preservation the sheath structure of 

 species in this genus may become so difficult of demonstration that 

 it is possible to confuse them with some forms of Oscillatoria. It is 

 the sheath which fundamentally separates the two genera. In the 

 field and in macroscopic appearance they may be differentiated 

 usually on the basis of the compact, felt-like layer constructed by 

 Phormidium. Whereas masses of some species of Oscillatoria may 

 form somewhat similar mats, the trichomes dissociate easily when 

 the collection is made. In Phormidium, the mat when present is 

 compact and does not dissociate. 



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