Stigonemacece 319 



conspicuous attenuation of the trichome, either from base to apex 

 or from the central part towards each extremity. This character 

 is utilized to subdivide the order. 



Sub-order 1. Psilommatece. Trichomes cylindrical, sometimes 

 narrowed at the extremities. 



Sub-order 2. Trichophorece. Trichomes conspicuously attenuated 

 towards one or both extremities, which are generally piliferous. 



Sub-order 1. PSILONEMATE.E. 



The majority of the filamentous blue-green Algae belong to 

 this sub-order. The trichomes are cylindrical or torulose in 

 character, of a uniform thickness, and with or without a sheath. 

 The sheath may be hyaline and gelatinous, very thin, or of con- 

 siderable diameter ; in many it is tough and lamellose. In some 

 the sheaths contain no small proportion of cellulose, colouring blue 

 with chlor-zinc-iodine, but in others they consist entirely of mucus. 

 The apical cell of the trichome may be obtuse or more or less 

 conical and attenuated, and occasionally the apical and sub-apical 

 cells are suddenly attenuated. 



The sub-order includes the four following families : 



Family 1. Stigonemacece. Filaments usually stout and branched ; 

 sheaths thick, firm, and often irregular; trichomes with heterocysts, 

 and frequently consisting of more than one row of cells. 



Family 2. Scytonemacece. Filaments with a false branch-system ; 

 sheaths firm and tubular, of more or less equal thickness ; trichomes 

 consisting of a single row of cells, with heterocysts, but not of uniform 

 thickness. 



Family 3. Nostocacece. Trichomes commonly tortuose and intricate, 

 enveloped within a large gelatinous mass, consisting of a single row of 

 uniform cells (generally torulose), with heterocysts ; sheaths very deli- 

 cate, mostly confluent. 



Family 4. Osdllatoriacece. Trichomes consisting of a simple row 

 of cells, uniform along their entire length except for the apical cells, 

 which are sometimes attenuated ; hetei'ocysts absent ; sheaths variable, 

 more or less gelatinous, and sometimes enclosing more than one 

 trichome. 



Family 1. STIGONEMACEJE. 



In this family the cells are arranged either in a single series 

 or in several more or less irregular series within a strong sheath. 

 This sheath is rarely thin and regular, being more often thick, 



