320 



MyxophyceoB 



dark brown in colour, of considerable toughness, and with a very 

 uneven exterior. The filaments are branched, and the branches 

 arise by a growth from one of the cells of the main filament. The 

 filaments increase in length by the division of the cells towards 

 the apices. The heterocysts are intercalary, being scattered at 

 intervals between the vegetative cells. When more than one row 

 of cells exists in a filament the heterocysts are situated in a lateral 

 position. 



The robust filaments of most of the members of this family, with 

 their true branches and somewhat irregular outlines, are easily 

 distinguished from the more uniform and thinner filaments of the 

 Scytonemaceae. The presence of more than one row of cells within 

 the sheath, and the frequent irregular disposition of these cells, 

 are also distinctive features. 



There are only two British genera, Stigonema and Hapalosiphon. 

 In the former the reproduction normally takes place by the 

 development of hormogones from the extremities of the branches, 

 whereas in the latter the normal method of reproduction is by spores. 



Genus Stigonema Ag., 1824. [Sirosiphon Ktitz., 1843.] The 

 filaments are free-floating or aggregated to form soft, felt-like 



- 



; .c.j^ioi. 



B 



D 



C 



Fig. 146. A and B, Stigoncma minutum Hass., from Slieve Donard, Down, Ireland; 

 3 A, xlOO; B, x440. C E, St. dcellatum (Dillw.) Thur., from Llyn Teyrn, 

 Snowdon, N. Wales; C, xlOO; D and E, x 440. h, heterocyst. 



masses. The cells are always rounded, and are disposed either in 

 two or more rows, or more rarely in a single row. The heterocysts 

 are commonly lateral, or less frequently intercalary. The sheaths 

 are thick, lamellose, with an uneven exterior, and they are usually 



