338 Myxophycece 



new trichomes from some of the inferior cells of the original 

 trichome, the new growth often occurring immediately above an 

 intercalary heterocyst. It is in Dichothrix that the false branching 

 and fusion of the sheaths are most marked. 



Asexual reproduction takes place by the formation of hormo- 

 gones, the hair-like apical portion of the trichome being lost. 

 Asexual spores commonly arise in Gloeotrichia (and also in Calothrix) 

 in close apposition to the basal heterocysts. 



There are five British freshwater genera, which can be distinguished as 

 follows : 



A. Without heterocysts. Filaments free, very narrow, 



forming a thin stratum Amphithrix. 



B. With heterocysts. 



* Filaments free, simple or forming a dichotomous thallus. 

 t Filaments simple or falsely branched; branches 



distinct and free Calothrix. 



ft Filaments branched ; branches several (2 6) 



within a common sheath Dichothri.r. 



** Filaments forming a hemispherical or globular thallus, closely 

 united by mucus. 



t Filaments radiately disposed in a tough, 

 globose or hemispherical, attached thallus. 



Spores unknown Rivularia. 



tt Filaments radiately disposed in a soft, globose, 

 free-floating thallus. Spores regularly pro- 

 duced Gloeotrichia. 



Genus Amphithrix Kiitz., 1843; em. Born. & Flah., 188C. 

 The filaments form a thin expanded stratum of a purple or violet 

 colour, which consists of two layers. The inferior layer is com- 

 posed of densely intricate filaments, or of minute radiately disposed 

 series of cells ; the superior layer consists of simple, erect filaments, 

 closely packed together and attenuated to fine points. The sheaths 

 are thin, close and continuous, and there are no heterocysts. 



A. janthina (Mont.) Born. & Flah. occurs on wet rocks and is known from 

 W. Yorkshire; thickness of filaments 1'5 2'25 p. 



Genus Calothrix Ag., 1824. [Mastigonema Schwa'be, 1837 ; 

 Mastigothrix Kiitz., 1843; Schizosiphon Kiitz., 1843 (in part); 

 Sympkyosiphon Kiitz., 1843 (in part).] The filaments are simple 

 or slightly branched, forming penicillate tufts or soft velvety 

 expansions, which are generally attached to submerged rocks and 

 stones. The heterocysts are basal or intercalary, and in a few 

 species they are absent. The plants often exhibit a slight 



