No. IO.] FRESH-WATER ALG^E. 17 



SUB-ORDER II. PSILONEMATE/E. 



This sub-order contains the greater part of the Hormo- 

 gonese. The filaments, with or without a sheath, are cylindri- 

 cal, sometimes showing globular swellings. The sheath may 

 be very thin, hyaline and gelatinous, or tough and lamellose. 

 The apical cell, or sometimes that and the sub-apical cell, are 

 occasionally attenuated, or the filaments may end obtusely. 



Key to Families. 



i. Filaments showing true branching stigonemace.e 



Filaments showing false branching; heterocysts 



present scytonemace^e 



Filaments usually simple, without branching; where 

 they show false branching they are without hetero- 

 cysts 2 



2. Filaments nearly straight; heterocysts absent 



OSCILLATORIACE^ 



Filaments tortuous; heterocysts present 



NOSTOCACE^E 

 FAMILY I. OSCILLATORIACE^E. 



The distinguishing feature of this family, which is the 

 largest one of the Psilonematese, is the absence of heterocysts. 

 The trichomes are a single and regular row of cells, although 

 occasionally false branching is seen. Sometimes the cells are 

 so closely joined that the whole seems a perfectly homogeneous 

 cylinder ; but at other times there are constrictions at the ends 

 of the cells. Apical cell sometimes attenuated. The filaments 

 are nearly always in sheaths of various character, which some- 

 times enclose more than one filament. Some of the genera 

 show gliding or rotary motion. They occur in great profusion, 

 submerged in ponds and ditches, or form scums upon their 

 surface. 



There are two sub-families, as follows : 



Sub-family I. lyngbye^:. Only one trichome in a 

 sheath. 



Sub-family II. vaginariele. Several trichomes in one 

 sheath which is often branched. 



