50 MINNESOTA BOTANICAL STUDIES. 



BIBLIOGRAPHY. 



Greville, R. K. Scottish Cryptogam ic Flora, 222. 1823-28. 

 Berkeley, M. J. Gleanings of British Algae, 23: pi. 7. f. 2. 1833. 

 Harvey, W. H. Nereis Boreali Americana, 3: 99. pi. 48 A. 1858. 

 Wood, H. C. A contribution to the history of the fresh-water algae of 



North America, 66, 67. 1872. 

 Wolle, F. Fresh-water algae of the United States, I : 267. 1887. 



2: pi. i88.f. is, 16. 1887. 

 Bornet and Flahault. Revision des Nostocacees heterocystees. Ann. 



Sci. Nat. Bot. VII. 5: no. 1887. 

 Kirchner, 0., in Engler and Prantl, Die natiirlichen Pflanzenfamilien, 



I : (i Abt. a) 79. f. 57 c. 1900. 



EXPLANATION OF PLATE XIII. 



Figure i. Apex of a filament showing striae of the sheath, or the 

 " mouths" of the younger sheaths projecting from the older ones, #, 

 trichome, x 154. 



2. Same view as Fig. i, but with different focus showing the con- 

 centrically arranged striae with the central portion. #, trichome, 

 X 154. 



3. Apex of a filament with well-developed trichome about to slip 

 out from end of sheath, x 154. 



4. A portion of a normal filament showing heterocysts and hormo- 

 gone with a very dense central folding of the sheath about the trichome, 

 X 154. 



5. A portion of an older filament showing a branch given off at the 

 heterocyst, x 154. 



6. A portion of a filament showing four branches not in the neigh- 

 borhood of a heterocyst, x 154. 



7. A trichome showing active division removed from sheath, x 154. 



8. 9. Stained trichomes removed from sheath. 8. Mature pseudo- 

 cysts. 9. Young pseudocysts, x 154. 



