FAMILY STIGONEMATACEAE 



In this family the cells are arranged in uniseriate or multiseriate 

 branched filaments. The trichomes have definite sheaths which may 

 be close and firm or wide, lamellate, and mucilaginous. Whereas 

 cells may be quadrate or cyhndrical in some genera, they are globose 

 or transversely elhptic in others, and in some species intercellular 

 connections are conspicuous. The heterocyst in this family is ex- 

 tremely variable in shape. In some genera it is characteristically 

 quadrate or cylindrical, with truncate apices, but in others it is 

 globose or ellipsoidal. It may be intercalary or lateral, in which 

 case it develops by longitudinal division of a vegetative cell. The 

 habit of branching in this family clearly differentiates it from the 

 Scytonemataceae. The branches (also sheathed) arise by cell divi- 

 sion in a plane at right angles to the longitudinal axis of the prin- 

 cipal trichome. In some genera the lateral proliferations are more 

 slender than the main filament, and special honnogonia may occur 

 in the apices of the branches. 



Key to the Genera 



1. Filaments uniseriate, rarely biseriate in part; heterocysts intercalary; 



sheaths ( in our specimens ) relatively thin, firm, and close Hapalosiphon 



1. Filaments composed of 2 to several series of cells (rarely uniseriate); 

 heterocysts lateral and intercalary 2 



2. Branches narrow, erect, mostly unilateral, with hormogonia 



formed in their apices -— — - Fischerella 



2. Branches nearly or quite as wide as the main filament, and arising 

 from all sides; not regularly producing hormogonia in the apices 

 of the branches Stigonema 



HAPALOSIPHON Naegeh 1849, p. 894; emend. Borzi 1917, p. 90 

 Creeping, branched, uniseriate trichomes inclosed in a wide and 

 lamellated, or a close, firm sheath; branches usually at right angles 

 to and about the same diameter as the main filament but with 

 thinner sheaths; cells cylindrical in most species, sometimes elliptical 

 and much constricted at the cross walls; heterocysts oblong or 

 quadrate-cylindrical, the contents homogeneous. 



The uniseriate trichome, bearing true branches which seldom 

 rebranch and which are about the same diameter as in the main 

 filament characterize this genus. The branches are usually unilateral, 

 but there is considerable variation in this, and one must be careful 

 in using keys to species which employ the arrangement of branches 

 as a point for differentiation. 



After studying species in this genus and after examining her- 

 barium material, including several type species, it immediately 



[543] 



