44 THE FLORA OF NEBRASKA. 



Stanrastrnm crenulatum (Naeg.) Delp. Ex Wolle Desm. U. S. 126. 

 Phycastrum crenulatum Naeg. Einzell. Alg. 129. 1819. 



Half cells in front view elliptical or oval, end view with 4 to 6 angles each 

 produced into a somewhat tapering ray; rays tipped with short spines 

 which are often rudimentary; diameter 30-38 //. 

 Minden. PL VH. ; Fig. 20, a, b, c. 



Staurastrum pseudopachyrhyncliiiim Wolle Desm. U. S. 125. 1884. 



Small, smooth, or imperfectly punctate, slightly longer than broad, deeply 

 constricted, sinus wide; half cells subcuneate, widening to a broad sub- 

 truncate end, angles rather broadly rounded, with a slight constriction 

 near the apex, end view triangular or quadrangular, sides sinuate, isth- 

 mus about one-fourth the diameter of the cell; diameter 20-24 /i, length 

 22-25 n. 



Two forms, PI. VI., Fig. 8, a and b, were found at South Bend in Septem- 

 ber, 1893. 



Family— DIATOMACEAE. 



Characterized in the synopsis, supra. The Diatomaceae are a group of such size and 

 importance that it has been thought best to reserve them for a separate part. 



Family.-ZYGNEMACEAE. 



Aquatic or rarely growing on damp ground; composed of cylindrical cells united 

 into simple filaments, often pale green, rarely yellow-green or reddish-brown, usually 

 surrounded by a more or less mucous stratum; chloroplasts, two, axillary and star- 

 shaped, a central lamina, or one or more parietal bands usually spirally coiled, rarely 

 straight; propagation by cell division or by the rounding up of the cell contents into a 

 spore (azygospore); reproduction by the union of 2 cell contents, either from the same 

 or different filaments; the zygospore formed either in one of the filaments or midway 

 between them. 



Sub-fam. I.— Mesocarpeae.— Chloroplast forming an axillary lamina, 

 pyrenoids many, membrane thin, slightly or not at all involved in 

 a gelatinous integument Mougeot'a 



Sub-fam. IT.— Zygnemeae.— Chloroplasts in one or more parietal bands, straight 

 or spirally coiled, or axillary and star-shaped; membrane often broad, 

 involved in a mucous integument. 



Chloroplasts axillary Zygnema 



Chloroplasts in parietal bands Spirogyra 



Sub-fam.— Mesocarpeae.— Chloroplasts axillary, forming a lamina; pyrenoids 

 many; membrane thin, scarcely involved in a mucous sheath; reproduc- 

 tion by the union of 2 similar cells, the protoplasm of the two cells not 

 uniting at once, but at first separating into 2-4 cells which finally dis- 

 appear. 



1. MOUGEOTIA Aa. Syst. 83. 1824. 



Cells cylindrical; chlorophyll in a single axial layer; pyrenoids many; zygo- 

 spore arising from scalariform copulation; oval, globose, or cylindrical, 

 filaments bent towards each other at the point of copulation or straight. 

 Etymology: dedicated to Mougeot, a French botanist. 



