culms of rushes. At first the plant mass is attached and spreading 

 over the substrate, but soon becomes free-floating, forming muci- 

 laginous flakes. 

 Mich., Wis. 



Cylindrospermum stagnale var. angustum G. M. Smith 1916, p. 481 



PI. 123, Fig. 1 

 A foi-m with gonidia smaller than in the typical plant; vegetative 

 cells 4-4.5ix in diameter, 8-10/a long; heterocysts 5.5-5.6/x in diam- 

 eter, 7-1 1/i, long; gonidia 7-9/x in diameter, 18-25/^ long. 

 Benthic and tychoplanktonic. Wis, 



AULOSIRA Kirchner 1878, p. 238 

 Trichomes solitary or loosely clustered in small bundles, inclosed 

 in a thin but definite sheath which is closed at the ends. Trichomes 

 the same diameter throughout or narrowed in the midregion and 

 larger toward the apices. Vegetative cells rectangular to short- 

 cylindric, or somewhat barrel-shaped. Heterocysts intercalary; 

 round, ovate, or subcylindric. Gonidia 1 to several in a series, some- 

 times adjacent to the heterocysts. 



Aulosira laxa Kirchner 1878, p. 238 

 PL 123, Figs. 2, 3 



Filaments straight or slightly curved; 8/x wide; usually solitary 

 but sometimes in 2's or 3's, entangled and parallel, with thin, color- 

 less, and close sheaths which are somewhat diffluent and not lamel- 

 late. Cells short and discoid or as long as wide (rarely a httle 

 longer); 6-7.2^ in diameter, 3-8)u long; much constricted and often 

 not adjoined at the cross walls; sometimes forming a double series 

 for a short distance in the sheath. Heterocysts quadrate to angular- 

 globose, 7.5|u, in diameter. Gonidia solitary or in series of 2-3; cylin- 

 drical with rounded ends; 5-7.5ja in diameter, 16-19|U. long. 



Entangled with other algae, forming soft gelatinous expanses on 

 submerged sticks, etc. Wis. 



FAMILY SCYTONEMATACEAE 



The chief characteristic of the sheathed plants which comprise 

 this family is their habit of forming false branches. The filaments 

 are all uniseriate, but in Desmonema and Diplocoleon (not repre- 

 sented in our collections), more than one trichome is inclosed by a 

 single sheath. All the genera except 2 {Plectonema and Spelaeopo- 

 gon) have heterocysts. These determine the point of branch devel- 



[532] 



