distal ends of the branches; 6-12/a in diameter, 4-14fx long. Hetero- 

 cysts quadrangular-globose, yellow-brown in color, the contents 

 yellow-green, sheaths thick and lamellated, with layers diverging; 

 brown in color. Filaments 18-36/i, wide. 



This species is recorded frequently from aerial habitats. 



On logs and stones; tychoplankter. Mich., Wis. 



Scytonema tolypothricoides Kuetzing 1849, p. 307 

 [S. mirahile var. tolypothricoides ( Kuetz. ) Lobik 1915, p. 42] 



PI. 123, Figs. 8, 9 

 Filaments forming dark blue-green or brown floating, cottony clots. 

 Trichomes more or less radiating from a common center; freely 

 branched, the branches mostly in pairs. Cells quadrate to subcylin- 

 dric in the older portions, compressed and much shorter than wide 

 in the branches; 6.8-12ju, in diameter. Heterocysts quadrate or ovate; 

 about 10/A in diameter. Sheaths wide, lamellate, at first colorless, be- 

 coming brown or orange colored. Filaments 10-15/x in diameter. 



Common in the plankton of a variety of lakes, mostly soft water. 

 Appearing as Tolypothrix-\ike clots floating among aquatic vegeta- 

 tion near shore. Mich., Wis. 



TOLYPOTHRIX Kuetzing 1843, p. 227 

 A sheathed, falsely branched trichome; solitary or, more usually, 

 forming cottony tufts and expansions; the false branches mostly long 

 and flexuous, arising from just below a heterocyst. Cells quadrate 

 to cylindric or barrel-shaped, constricted at the cross walls. Hetero- 

 cysts quadrangular, globose, or subglobose, single or in series. 

 Sheath firm and thin, or somewhat gelatinous and lamellated. 

 Gonidia ovate to elliptic, or subglobose; with a thin membrane; 

 often occurring in a series. 



Key to the Species 



1. Sheaths thick, about as wide, or wider than 



the diameter of the trichome ._ 2 



1. Sheaths thin, usually close, but lamellated in some; less than the 

 diameter of the trichome in width 3 



2. Trichomes 5.5-9/^ in diameter; cells barrel-shaped; 



sheaths not sticky T. limbata 



2. Trichomes S-lO/u in diameter; cells quadrate, not at all or but very 



little constricted at the cross walls; sheaths sticky T. conglutinata 



3. Cells very short, disc-like, 9-12^ in diameter T. distorta 



3. Cells longer than wide, or quadrate 4 



4. Cells 8-10/1 in diameter; heterocysts 7fi in diameter, quadrate or 

 cylindrical; filaments up to 18fi wide T. lanata 



4. Cells smaller, 5— 8u in diameter; heterocysts globose, subglobose 



to subcylindric; filaments up to 10m wide T. tenuis 



[536] 



