1919 J Setchell-Gardner : Myxophyceae 51 



Order 2. IIORMOGONALES atkixson 



Thallus pluricellular, filamentous, with or without a sheath, cylin- 

 drical, or tapering to a hair, unbranched, or with false or true branch- 

 ing; filaments single, or several within a common sheath, or united 

 into a more or less gelatinous mass; all of the cells of the filaments 

 similar, or with occasional specialized cells called heterocysts; multi- 

 plication by the filament breaking into segments a few cells long, 

 called hormogonia, through the death of certain cells, and by the 

 formation of resting spores. 



Atkinson, A college text book of botany, ed. 2, 1905, p. 163. 



Hoi'mogoneae Thuret, Essai Class. Nost., 1875, p. 377. 



As previously stated under Coccogonales, Thuret was the first to 

 divide the Myxophyceae into two groups and it seems best to consider 

 these as orders. The Ilormogonales differ from the Coccogonales in 

 being uniformly filamentous, either with or without branches, and 

 reproducing vegetatively by short pieces of two, to three, to many 

 cells instead of separating single cells for this purpose. The hormo- 

 gonia are formed by the definite splitting off of these small lengths 

 of the filaments, usually in a series, the hormogonia being separated 

 from one another by the death of intermediate cells. Spore formation, 

 when present, is by means of cells changing their shape and size to a 

 greater or less extent and forming a thick, smooth, or at times variously 

 roughened, outside coat. Gonidia formation is described for certain 

 of the Hormogonales, but needs further observation and study before 

 being accepted as equivalent to that of the Chamaesiphonaceae. 



After careful consideration of the various views, it has seemed best, 

 for reasons given below, to divide the Hormogonales into two sub- 

 orders, Homocystineae and Heterocystineae. 



Key to Suborders and Families. 



1. Cells alike, neither differentiated into hair cells nor heterocysts 



(Suborder 1. Homocystineae) Family 3. Oscillatoriaceae (p 53) 



1. Cells of two sorts, vegetative and either hair cells or heterocysts, or Ixith 



(Suborder 2. Heterocystineae) 2 



2. Trichomes alike at both ends, branching wanting 



Family 4. Nostocaceae (p 89) 



2. Trichomes unlike at opposite ends 3 



3. Branching false, hairs usually present, abundant Family .5. Rivulariaceae (p 93) 



3. Branching true, hairs (in our genera) present, occasional or al)uii(l,iiit 



Family G. Stigonemataceae (p 109) 



