io8 



FRESH-WATER BIOLOGY 



33 (34> 35) Filaments more or less confluent by their mucous sheaths. 



Phormidium Kiitzing. 



Filaments many-celled, straight or bent; en- 

 closed in hyaline sheaths which frequently become 

 adherent to form an expanded stratum on wet 

 rocks or moist earth, or entirely submerged. 

 Usually this stratum is soft and slimy, but it 

 sometimes becomes hard and leathery. A genus 

 intermediate in character between Lyngbya and 

 Oscillatoria. 







FIG. 44- 



Phormidium subfuscum Kiitzing. X 575- 

 (After Kirchner.) 



34 (33, 35) Filaments generally without conspicuous sheaths; free, straight, or 

 with curved extremities. . Oscillatoria Vaucher. 



Filaments composed of numerous 

 short cylindrical cells, the end cell some- 

 times much attenuated; without a 

 sheath or with an almost imperceptible 

 one; generally showing lively creeping 

 and oscillating movements. Found in 

 great profusion in all kinds of wet situ- 

 ations; sometimes free-floating at the 

 surface of lakes and ponds, or forming 

 filmy growths on wet soil or rocks. 

 0. limosa is extremely abundant on the 

 soil, etc., in greenhouses, while O. pro- 

 lifica occurs in the plankton of some lakes in such quantities as to impart 

 a reddish or purplish color to the water and occasionally to form a "water- 

 bloom." The latter species has been found in some instances to persist even 

 into the winter and to color the ice a reddish or pinkish color. 



FIG. 45. A, Oscillatoria prolifica Gomont. B, Oscillatoria 

 limosa Agardh. X 465. (Original.) 



35 (33 > 34) Filaments without sheaths, twisted into a regular spiral. 



Arthrospira Stizenberger. 



Filaments commonly without a sheath, differing from 

 Oscillatoria in being regularly spirally coiled, and from 

 Spirtilina in being many-celled. Living singly or form- 

 ing dark-green slimy strata in stagnant water. 



FIG. 46. Arthrospira jenneri Stizenberger. 

 Gomont.) 



X 500. (After 



36 (32) Filaments not showing oscillating movements; sheaths firm. . 37 



37 (38) Filaments free and simple, free-floating or forming an expanded 



stratum Lyngbya C. Agardh. 



Filaments many-celled, straight or bent, 

 each enclosed in a firm, generally hyaline, 

 sometimes lamellose, membrane. Free-float- 

 ing, or forming densely intricate, floccose 

 masses, or an expanded stratum. Frequently 

 abundant in plankton. 



FIG. 47. Lyngbya major Meneghini. X 465. 

 (Original.) 



