Oscillatoriacece 



333 



B. 



ttf Filaments free and simple, free-floating or 



forming a matted stratum Lyngbya. 



** Filaments simple ; sheaths thin, always hyaline, mucous, and 

 more or less diffluent, in some species absent; apices of 

 trichomes commonly curved, 

 t Trichomes more or less agglutinated by their 

 mucous sheaths ; cells of trichome often 

 slightly separated by a thin mucous layer... Phormidium. 

 +t Trichomes destitute of sheaths, free, straight, 



or with curved extremities Oscillatoria. 



ttt Trichomes destitute of sheaths, twisted into 



a regular spiral Arthrospira. 



Trichomes consisting of one cell, twisted into a regular 



spiral Spirulina. 



Genus Plectonema Thuret, 

 1875. The filaments are free- 

 floating, forming csespitose masses, 

 or they are intricately matted 

 amongst damp mosses and on wet 

 rocks, forming felt-like or mat- 

 like expansions. The sheaths are 

 firm, hyaline or. lamellose, and 

 rarely of a yellow-brown colour. 

 The trichomes fit closely within 

 their sheaths, and there is often 

 a slight constriction between the 

 cells. The filaments give origin 

 to false branches exactly similar 

 to those of Scytonema ; in fact, 

 Plectonema only differs from Scy- 

 tonema in the absence of hetero- 

 cysts and in the more irregular 

 manner of its false branching. 



P. Tomasinianum (Kiitz.) Born, is 

 the largest and most frequent British 

 species, occurring as greenish-brown 

 felt-like masses on wet rocks. Thick- 

 ness of filaments 16 27 /i; thickness 

 of trichomes 12-522 /* ; length of 

 cells 3 9 p.. Thick-walled resting 

 cells, probably of the nature of spores, 

 have been observed in this species. 



Fig. 152. A, Microcoleus delicatulus 

 W. & G. S. West, from Glen Tummel, 

 Perthshire ( x 350). B and C, Symploca 

 muralis Kiitz., from Frizinghall, W. 

 Yorks. (B, nat. size ; C, x 350). 



