34 Myxophycex 



the plants survive and resume activity as soon as circumstances are suffi- 

 ciently favourable (W. & G. S. W., '11 ; and Fritsch, 12). 



The genus Gomontiella exhibits a unique adaptation against desiccation. 

 The plant is a member of the Oscillatoriacea? in which the filaments are 

 greatly flattened and at the same time rolled to form a capillary tube (fig. 20). 

 The plants occur in shallow water-holes which are liable to become 

 temporarily dry, but little evaporation can take place from the ends of the 

 tube-like plant, and thus the filament retains its own water supply and very 

 rarely becomes completely desiccated. (Teodoresco, '01.) 



Some of the Myxophycea? have become adapted to a life in hot water, and 

 they constitute the principal vegetation of hot-springs. The part played by 

 certain of these Alga? in the formation of rock-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. All 

 shades of yellow, orange-red, pink, blue, and blue-green occur, and are caused 

 by the brilliantly-coloured Alga? included within the deposit. In the case of 

 travertine deposits the deposition of the calcium carbonate is due largely to 

 the extraction by the Alga? of the carbon dioxide dissolved in the water. 

 That Alga? do actually cause the elimination of carbonate of lime from 

 water in which it is contained in solution was first shown by Cohn ('62), 

 and Weed ('88) has given a very able account of the assistance of the 

 Myxophycea? in the formation of the travertine and sinter deposits of the 

 Yellowstone National Park in the United States. Dichothrix gypsophila, 



one of the Rivulariacea?, is one of the most active 

 species in building up calcareous deposits, and such 

 deposits generally exhibit a well-marked zoning (fig. 

 21 ). The Alga? of hot-springs often grow as gelatinous 

 masses in which a glassy form of silica gradually 

 appears, and ultimately all except the peripheral 

 portion becomes firmly silicified. Weed found that 



Fig. 21. Photograph of a the thickness of travertine formed in three days 

 section through a frag- -in ~K 



ment of a calcareous varied from 1*25 to 1*5 mm. Me also round that the 



nodule built up by Dicho- character and colour of the deposit depended very 



tnnx gypsophila (Kutz.) 



Born. & Flah. Natural largely upon the temperature of the water and the 



size. Note the well- -, c ,, mi i u 



marked zoning. situation of the spring or geyser. The highest 



temperature at which Myxophycea? have been found 



is 87*5 C., at which temperature Phormidium laminosum has been stated to 

 occur. In the spray of a small geyser at Hveravellir in Iceland, Phormidium 

 angustissimum, Ph. tenue, and Mastigocladus laminosus were thriving (G. S. W., 

 '02). The temperature of this spray was 85 C., but it is highly probable that 

 it would be so rapidly reduced that the wet stratum of the Alga? would be at 



