CYANOPHYCE.Z 257 
Since the Reproduction by hormogonia has already 
been described under the Heterocystew, there is no 
distinctive character under this heading. The ab- 
sence of spores has also been noted above. 
The Geographical Distribution of Nostocacece 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 Algee 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, Plectonema, and Phormidiwm, contain species 
which perforate shells and other calcareous bodies in 
the sea. The pelagic Oscillariew occur in enormous 
abundance in the warm waters of tropical seas. 
From the periodical occurrence of Trichodesmium ery- 
threum 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 Nostocacew 
which occur in the sea are known in British seas. 
S 
