CYANOPHYCEJE 257 



Since the Reproduction by hormogonia has already 

 been described under the Heterocystecc, there is no 

 distinctive character under this heading. The ab- 

 sence of spores has also been noted above. 



The Geographical Distribution of Nostocaccm is world- 

 wide in fresh-waters and damp situations, on coasts 

 and in mid-ocean. They are of rare occurrence 

 in cold situations, and the littoral marine forms are 

 generally to be found near high-water mark. The 

 calcareous forms are more characteristic of fresh- 

 waters than of the sea, and are especially abundant 

 in hot springs. When they live in waters strongly 

 charged with lime, the precipitation of carbonate 

 of lime is caused by the absorption of carbonic 

 acid. However, this precipitation occurs only 

 where the Alga3 do not find carbonic acid in sufficient 

 quantity to meet the needs of vegetation. When it 

 is abundant, as in certain mineral waters, the salt 

 is precipitated only in very small quantities. The 

 deposition may be round the individual filaments 

 or so abundantly distributed as to envelop the whole 

 thallus in a calcareous concretion. The genera Masti- 

 gocolcus, Plcctcncma, and Phormidium, contain species 

 which perforate shells and other calcareous bodies in 

 the sea. The pelagic Oscillariem occur in enormous 

 abundance in the warm waters of tropical seas. 

 From the periodical occurrence of Trichodcsmium cry- 

 thrcvum in great banks the Red Sea has obtained its 

 name, and the same species and others allied to it have 

 often been recorded from tropical seas in extraordin- 

 ary floating masses. Most of the genera of Nostocacece 

 which occur in the sea are known in British seas. 



