Rivnlariacece 



339 



branching, caused by the close application of the base of one 

 filament to the side of another, the sheaths being continuous. 

 The upper and more attenuated parts of the filaments are, how- 

 ever, always free. Borzi 1 has observed the formation of basal, 

 seriate spores in one species. 



There are about six British freshwater species, C. parictina (Nag.) Thur. 

 being the most frequent (thickness of trichomes 6 12 /*; fig. 156 A and B). 

 C. fusca (Kiitz.) Born. & Flah. and C. epiphytica W. & G. S. West are epi- 

 phytes on other larger Algae, such as Vaucheria or Batrachospermum. 



Fig. 156. A and B, Calothrix parietina (Nag.) Thur., from Arncliffe, W. Yorks. 



C, Dichothrix interrupta W. & G. S. West, from Slieve Donard, Ireland. 



D, D. Orsiniana (Kiitz.) Born. & Flah., from Langdale, Westmoreland. (All 

 x420.) 



Genus Dichothrix Zanardini, 1858. In this genus the filaments 

 are more or less dichotomously branched, several trichomes with 

 their sheaths being enclosed within an outer common sheath. The 

 heterocysts are basal or intercalary, and in one species they are 

 absent. The plants generally occur in penicillate tufts on dripping 

 rocks. 



1 Borzi in Nuovo Giornale Botan. Ital. 1882, xiv, p. 374. 



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