Key to the Species 



1. Filaments 5-7/x in diameter; cells elongate-cylindric; 



chromatophores 2-4, regularly arranged - 2 



1. Filaments larger, 7-16-(30)ai in diameter:; cells slightly inflated, 



with constrictions at the cross walls; chromatophores many 3 



2. Cells cylindrical, not at all constricted at the cross walls, up to 



*8 times the diameter in length; chromatophores 2-4 folded plates — T. affine 



2. Cells slightly inflated, 2-4 (rarely 6) times the diameter in length; 

 chromatophores 2-4 irregular discs T. minus 



3. Cells 10-17 fj. in diameter; wall thick, clearly showing 



the overlapping of the 2 wall sections.. -— T. utriculosum 



3. Cells 6-11m in diameter; wall thin, not showing 



overlapping of the 2 wall sections — - — T. bombijcinum 



Tribonema affine G. S. West 1904, p. 208 

 PI. 96, Figs. 7-9 

 Filaments straight and slender; cells long-cylindric with thin walls, 

 5-5.6/A in diameter, 35-40/x long; chromatophores 4 pale, yellow- 

 green parietal plates with smooth margins. 

 Forming gray-yellow clouds in ponds, ditches, and small lakes. Wis. 



Tribonema bombycinum (C. A. Ag.) Derbes & Solier 1856, p. 18 



PI. 96, Fig. 10 



Filaments much entangled, forming a grayish-yellow cloudy mass 

 in quiet water. Cells with thin walls; cylindrical or slightly con- 

 stricted at the cross walls; 6-11;li in diameter, 15-38/x long. Chromato- 

 phores 4-8 small, parietal, pale yellow-green discs, sometimes in 

 contact, giving the appearance of 1 or 2 large, irregularly shaped 

 plates. 



Pascher's dropping of this name seems unwarranted, although 

 there is great confusion as to which plants belong to this time- 

 honored name. In his Heterokonten ( Rabenhorst's Kryptogamen- 

 Floray 11, p. 975, 1939) Pascher has transferred T. bombycinum 

 Derbes & Solier to T. viride Pascher ( 1925, p. 106. ) 



This is a common species in cold water of springs, ditches, and 

 dark-colored lakes. In bodies of water rich in humic acid, such as 

 lakes receiving drainage from wooded swamps, this species is often 

 the predominating if not the sole component of the algal flora (at 

 least in shallow water of the margin). Mich., Wis. 



Tribonema bombycinum var. tenue Hazen 1902, p. 185 



PI. 96, Fig. 11 

 A variety difFering from the typical by its more slender filaments; 

 cells cylindrical, 3-6ja in diameter, 10-36/x long; chromatophores 

 numerous small discs. 



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