307 



masses by the extraction of carbonate of lime or silica from the 

 water of hot-springs is considerable. The deposits formed around 

 the hot-springs in many parts of the world consist of brightly- 

 coloured basins or terraces of travertine and sinter. The colours 

 are very varied, being all shades of yellow, orange-red, pink, blue, 

 and blue-green, and are due to the presence of brilliantly-coloured 

 Alga? within the deposit. In the case of the travertine deposits 

 the deposition of the carbonate of lime is due very largely to the 

 extraction by the Algae of the carbon dioxide dissolved in the 

 water. 



That Alga? do actually cause the elimination of carbonate of 

 lime from the water was first shown by Cohn 1 , and Weed 2 has 

 given a very able account of the assistance of the Myxophycese in 

 the formation of the travertine and sinter deposits of the Yellow- 

 stone National Park, U.S.A. He states that from ^ to a -jL of an 

 inch of travertine is formed in three days. He found that the 

 character and colour of the deposit depended upon the temperature 

 of the water and the situation of the spring or geyser. The highest 

 temperature at which filamentous Myxophycese are known to exist 

 is 85 C. 3 , but unicellular Algse have been observed by Brewer in 

 California in water at a temperature of 94'5 C. 4 



The cell-wall, which is never absent, is composed partly of 

 cellulose, partly of pectose compounds, and very largely of chitin. 

 It presents a similarity to the cuticle of higher plants 5 , offering 

 considerable resistance to chemical reagents, and it sometimes 

 contains silica 6 . 



Many of the colonial forms are embedded in a somewhat 

 extensive mass of mucilage, the external surface of which is 

 covered with a thin cuticle, and in most of the filamentous forms 

 there is secreted either a thin mucous sheath or a tough chitinized 

 sheath, often lamellose in character. This sheath is undoubtedly 

 secreted by the enclosed cells, and in some genera (e.g. Lyngbya) 

 all stages are met with between a thin hyaline investment and a 

 hard, lamellose, sheathing tube. 



1 Cohn in Abhandl. der Schles. Gesellsch. Nat. 1802, p. 35. 



2 Weed, 'Formation of Travertine and Siliceous Sinter by the Vegetation of 

 Hot Springs.' Rep. U.S. Geol. Survey, 1887-8. 



3 G. S. West, ' Some Algas from Hot Springs,' Journ. Bot. July 1902, p. 241. 



4 W. H. Brewer in Amer. Journ. Science, ser. 2, xli. 



5 R. Hegler in Pringsh. Jahrb. fiir wissensch. Bot. 1901, xxxvi. 



Hyams & Richards in Technol. Quarterly, Boston, 1902, xv, pp. 308315. 



202 



