branched in the apices; sheaths close, becoming diffluent; trichomes 

 numerous and twisted within the sheath, which is stained by chlor- 

 zinc-iodide reagent; trichomes not tapering toward the apices, 

 which are bluntly rounded. Cells 1.4r-2Afi in diameter, 1.4-3)a long; 

 constricted at the cross walls; cell contents pale blue-green. 

 Attached to submerged aquatics, Mich., Wis. 



Schizothrix vaginata ( Naeg. ) Gomont 1892, p. 302 

 Filaments forming a cushion-like mass, sometimes calcified and 

 gray-brown in color; filaments coalesced below but becoming forked 

 above; sheath thin, close, and slightly lamellate, narrowed toward 

 the apex and closed (sometimes expanded), coloring by chlor-zinc- 

 iodide reagent; trichomes single or few within the sheath, not 

 tapering toward the apices, which are broadly rounded; cells 2-3ja 

 in diameter, nearly quadrate or shorter than wide, not constricted at 

 the cross walls, which are granular; cell contents pale yellow-green. 

 On rocks and entangled among colonial, mucilaginous algae. Wis. 



Suborder Heterocystineae 



The chief characteristic of plants in this group is the heterocijst, 

 a differentiated cell in the trichome, although in a few rare cases it 

 is lacking. In such genera the habit of branching or some other 

 morphological feature of the trichome definitely relates them to 

 plants possessing a heterocyst. The trichomes may taper toward 

 the apex, but usually they are the same diameter throughout 

 ( Rivulariaceae an exception ) . Although many forms do not branch, 

 especially in the Nostocaceae, there are both true and false branch- 

 ings. The filaments may be uni- or multiseriate. (See remarks re- 

 ferring to the Order Hormogonales and the key to the families, p. 

 417 etseq.) 



FAMILY NOSTOCACEAE 



In this family the plants are uniseriate, unbranched filaments 

 which do not taper toward their apices ( or scarcely so ) . The sheath 

 is soft, diffluent and usually indistinct and nearly always without 

 structural conformity (as, for example, in Lyngbya of the Oscilla- 

 toriaceae, or Tolypothrix in the Scytonemataceae ) . Sometimes the 

 sheath is close, but not firm and rigid. In many forms there is a 

 copious secretion of mucilage with a multitude of trichomes enclosed 

 in a common gelatinous matrix in which individual sheaths are con- 

 fluent (e.g., Nostoc). All genera in this family have heterocysts, 

 which may be either terminal or scattered in the trichome, solitary 

 or (rarely) in series. Vegetative cells are globose, barrel-shaped, or 



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