Oscillatoriacece 335 



Genus Phormidium Klitz., 1843. [Hyphceothrix Kiitz., 1843 

 (in part); Leptothrix Klitz., 1843 (in part).] This genus is inter- 

 mediate in character between Lyngbya and Oscillatoria, and 

 undoubtedly serves a useful purpose for the reception of species 

 which cannot be strictly referred to either of those genera. The 

 filaments are simple and invested by delicate hyaline sheaths, 

 which frequently become confluent or altogether diffluent. The 

 filaments are often agglutinated to form an expanded stratum, 

 either on damp earth, wet rocks, or entirely submerged. Some- 

 times this stratum becomes hard and leathery, but it is more 

 often soft and slimy. The trichomes are cylindrical, arid there is 

 frequently a constriction between the cells, a feature which is so 

 prominent in some species as to give the trichome a moniliform or 

 torulose appearance. The apices of the trichomes may be straight 

 or curved, and the apical cells are attenuated, capitate, or even 

 thickened at the extremity. 



Plants of this genus are amongst the commonest of the blue-green Algae, 

 occurring in all kinds of damp and wet situations, and often giving a decided 

 tint to large areas of damp ground, vertical limestone rocks, or those rocks 

 and stones subject to the spray of waterfalls. There are some 13 British 

 species occurring in damp situations or in fresh water. Ph. autumnale (Ag.) 

 Gom. (diam. trich. 4 7 /t) is abundant on damp earth, and Ph. tenue 

 (Menegh.) Gom. (diam. trich. 1 2/*; fig. 153 E and F) is frequent among 

 other Algse in ponds, ditches, arid rivers. Ph. purpurascens (Kiitz.) Gom. 

 forms reddish-purple patches on the vertical faces of wet limestone rocks 

 (diam. trich. 1'5 2/t). 



[Note : Clonothrix gracillima W. & G. S. West is one of the Schizomy- 

 cetes of the genus Cladothrix.'] 



Genus Oscillatoria Vaucher, 1803. [Oscillaria Bosc, 1800 1 .] 

 The trichomes are free and cylindrical, without a sheath, or with 

 one so thin as to be almost imperceptible. Faint constrictions are 

 sometimes evident between the cells, but more often the edges of 

 the filaments present a continuous and unbroken line. The cells 

 vary much in relative length, but in the larger species they are 

 always much shorter than their diameter. The extremity of the 

 trichome may be straight or curved, and it is often attenuated. 

 The apical cell is sometimes much attenuated, and may be capitate, 

 being frequently furnished with a terminal thickening of the cell- 

 wall known as a calyptra. The genus is found in great profusion 



1 For reasons for adopting the generic name "Oscillatoria" consult Gomont in 

 Journ. Botan. Morot, v, 1891, p. 273. 



