opment in some forms, whereas in others the branching occurs 

 between the heterocysts. The branch arises when hormogonia in 

 the primary filament prohferate and produce series of cells that 

 push out laterally through the sheath, often continuing to form 

 successive branches. Occasionally branches arise in pairs when 

 adjacent ends of two hormogonia proliferate. 



The sheath is usually firm and definite and sometimes lamellate. 

 It may be colorless or ochraceous. In general, the cells are quadrate 

 or short-cylindric, although in some species they are somewhat 

 barrel-shaped and constricted at the cross walls. 



Key to the Genera 

 1. Heterocysts wanting Plectonema 



1. Heterocysts present — 2 



2. Heterocysts basal (with intercalary heterocysts sometimes also 

 present;' branching rare Microchaete 



2. Heterocysts all intercalary; false branching frequent — - 3 



3. Branches arising in unilateral pairs, about midway between two 

 heterocysts (sometimes branches at the heterocysts also); sheath 

 usually lamellate Scytonema 



3. Branches arising singly, just below a heterocyst or a series of them 4 



4. Filaments aggregated, somewhat radially arranged in a mucilaginous 



layer; branching regularly dichotomous; heterocysts sohtary Diplonema 



4. Filaments solitary or aggregated, sometimes forming free-floating, 

 cottony tufts; branching irregular; heterocysts sohtary or 2-4 in 

 a series Tolypothrix 



SCYTONEMA C. A. Agardh 1824, p. xxii 

 A falsely branched, usually thick- sheathed, filament, the false 

 branches ordinarily developing in pairs (rarely singly) between the 

 heterocysts ( and sometimes also at the heterocysts ) ; forming wooly 

 mats or tangled clots. Trichomes solitary within the sheath, forming 

 hormogonia in the branches. Cells quadrate or short-cylindric. 

 Heterocysts subglobose or quadrangular-globose. Gonidia rare; glo- 

 bose or ovate; about the same size as the vegetative cells. Sheaths 

 sometimes homogeneous but in most species definitely lamellated, 

 with the layers either parallel or diverging; hyaline or, especially in 

 the main filament, ochraceous. 



This genus should be compared with Tolypothrix. Scytonema 

 mirabile, for example, seldom forms branches in pairs but singly, as 

 in Tolypothrix. 



Key to the Species 



1. Sheaths very wide, forming lateral, wing-like expansions-— S. alatum 



1. Sheaths not forming wing-like expansions 2 



[533] 



