50 minnesota botanical studies. 



Bibliography. 



Greville, R. K. Scottish Cryptogamic Flora, 222. 1S23-28. 

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

 Harvey, W. H. Nereis Boreali Americana, 3: 99. pi. 48 A. 1S5S. 

 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. 188. f. is, 16. 18S7. 

 Bornet and Flahault. Revision des Nostocacees heterocyst^es. Ann. 



Sci. Nat. Bot. VII. 5: no. 18S7. 

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



I : (1 Abt. a) 79. f. J7 c. 1900. 



Explanation of Plate XIII. 



Figure 1. Apex of a filament showing striae of the sheath, or the 

 " mouths" of the younger sheaths projecting from the older ones. a y 

 trichome, X 154. 



2. Same view as Fig. 1, but with different focus showing the con- 

 centrically arranged striae with the central portion. a, 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. 



