No. IO.] FRESH-WATER ALG^. 23 



thous (Dillw.) Ag. ; S.Gguratum Ag. ;S. ocellatum (Dillw.) 

 Thur.] 



Tolypothrix Kiitz. (Hassallia Berkeley). Filaments 

 branched, with a distinct sheath. The branches usually ap- 

 pear where heterocysts occur, the trichome breaking through 

 the sheath just below the heterocyst and continuing its growth. 

 The sheaths are thinner than in Scytonema. The heterocysts 

 are sometimes two, three, or four in a row. 



[T. lanata (Desv.) Wartmann.] 



FAMILY IV. STIGONEMACE.E. 



The cells of this family are arranged in a single row or in 

 several irregular rows, in a strong, thick sheath, which is 

 brown and very uneven. The filaments are branched, and 

 grow by repeated division of the cells near the apex. The 

 heterocysts are never terminal, and they are placed in a lateral 

 position when there is more than one filament in a sheath. 



Key to Genera. 



Normal reproduction by means of hormogones, de- 

 veloped on the extremities of the branches. .Stigonema* 

 Normal reproduction by spores Hapalosiphori 



Description of Genera. 



Stigonema Ag. (Sirosiphon Kiitz.). Cells of the fila- 

 ments in one, two, or many rows, owing to the lateral division ; 

 the older filaments often having as many as ten series, while 

 the younger have only one or two. The cells are surrounded 

 by a membrane which is always distinct, but especially so in 

 the older filaments. The sheath is large, irregular, and usually 

 brown or golden yellow. The generally short, thick branches 

 are irregularly disposed. Found mostly on damp or wet rocks, 

 but sometimes free-floating in lakes or ponds. 



[S. mamillosum Ag. ; S.minutum (Ag.) Hass. ; S. panni- 

 forme (Ag.) Born, and Flah.] 



Hapalosiphon Nag. Filaments attached or floating ; 

 olive-green, blue-green, or, when older, bright or dark brown. 

 The branches rise singly at right angles to the prostrate stem, 



