332 Myxophycece 



G. S. West, which is another very small species (thickness of trichomes 

 0*5 0'7 p.), the branches of the filaments are twisted round each other like 

 the strands of a rope. 



Genus Dasygloea Thwaites, 1848. This genus differs principally 

 from Schizothrix in the possession of relatively wider sheaths, which 

 enclose fewer and more remote trichomes. 



D. amorpha Berk, occurs in permanent bogs ; thickness of trichomes 

 4 6 p. A figure is given of a small form of the species (fig. 151 C). 



Genus Microcoleus Desmazieres, 1823. [Cthonoblastus Kiitz., 

 1843.] The filaments are simple, terrestrial or aquatic in habit, 

 and are furnished with a conspicuous hyaline sheath. This sheath 

 is more or less cylindrical, not in any way lamellose, and its apex 

 is generally diffluent. The trichomes are numerous, closely aggre- 

 gated within the central part of the sheath, and often spirally 

 interwoven. The apical cells are acute or acutely conical. 



Plants of this genus are very rare in Britain. M. subtorulosus (Breb.) 

 Gom. (thickness of trichomes 4 5 p.) and M. delicatulus W. & G. S. West 

 (thickness of trichomes 1*5 2 p ; fig. 152 A) are known from Scotland. 



Sub-family II. LYNGBYE^. 



In this sub-family of the Oscillatoriacese there is never more' 

 than one trichome within a sheath, and the latter, although firm 

 and lamellose in some genera, is indefinite or wanting in others. 

 The trichomes vary much in thickness and in the relative length 

 of their cells. As a rule, the thicker the trichome, the shorter the 

 cells. In Arthrospira, Spirulina, and some species of Lyngbya, 

 the trichomes are regularly and spirally twisted. In Spirulina 

 the trichome is unsegmented, thus consisting of a single cell. This 

 genus, therefore, approaches certain genera of Bacteria, such as 

 Spirillum and Spirochcete. The extremities of the trichomes may 

 be obtuse, capitate, acuminate, or even greatly attenuated. Move- 

 merits of a gliding or rotatory nature are exhibited in the genera 

 Oscillatoria, Phormidium, Arthrospira and Spirulina. 



. A. Trichomes consisting of many cells. 



* Filaments simple, or falsely branched ; sheaths firm ; apices of 



trichomes straight, 

 t Filaments free or forming felt-like masses, 



branched ; pseudo-branches often geminate Plectonema. 

 ft Filaments forming erect tufts, often branched ; 



pseudo-branches solitary Symploca. 



