FAMILY RIVULARIACEAE 



The outstanding characteristic of plants belonging to this family 

 is the pronounced tapering of the trichomes, which exhibit a distinct 

 basal-distal differentiation. The sheath is firm, at least in the basal 

 portion, and may be lamellate or homogeneous. There may be one 

 or more trichomes within the sheath. In most forms there is a basal 

 heterocyst or a short series of them (and rarely intercalary hetero- 

 cysts as well), whereas in 2 genera some species are without hetero- 

 cysts. The trichomes may be solitary or they may form colonial 

 aggregates of spherical or hemispherical shape, or the thalli may be 

 amorphous. In most forms there is an evident false branching which 

 occurs immediately below an intercalary heterocyst. In at least one 

 genus the branches lie semiparallel with the original trichome in 

 the same sheath for some distance before they emerge. Branches may 

 arise also by in situ proliferation of hormogonia. The presence or 

 absence of gonidia is of taxonomic interest. These, if present, are 

 adjacent to the heterocysts. 



Key to the Genera 



1. Filaments numerous, closely arranged in radiate or parallel series 

 within copious mucilage to form globular or hemispherical, free- 

 floating or attached colonies 2 



1. Filaments arranged otherwise 3 



2. Mucilage firm, often hard and lime-encrusted; akinetes not present; 

 trichomes often parallel, compacted Rivularia 



2. Mucilage soft (especially when the plants are mature); akinetes 

 present; trichomes radiately arranged ...- Gloeotrichia (in part) 



3. Trichomes contained in amorphous gelatinous mucilage; attached 



or free-floating ____ ____ 5 



3. Trichomes not contained in amorphous mucilage; more or less 

 solitary, or forming plant masses of definite shape 4 



4. Colonial mass small, saccate and torn, with the investing mucilage 

 lamellate and much folded Sacconema 



4. Colonial mass otherwise, large and expanded, at first attached, later 

 free-floating, mucilage not lamellate or folded Gloeotrichia ( in part ) 



5. Akinetes absent 6 



5. Akinetes present __. 7 



6. Trichomes many, arising from a basal pseudoparenchymatous 

 portion of the thallus. Amphithrix 



6. Trichomes few together, not arising from a basal pseudoparenchy- 

 matous thallus Calothrix (in part) 



7. Filaments freely branched, the branches inclosed within the sheath 

 of the primary filament, forming dichotomously branched tufts; 



not epiphytic Dichothrix 



7. Filaments seldom branched; solitary or gregarious, sometimes form- 

 ing stellate tufts; plants epiphytic on walls of other algae or inclosed 

 in colonial mucilage; forming encrustations in 

 aerial habitats Calothrix (in part) 



[550] 



