Occurrence and distribution 



33 



Many Myxophycese are epiphytic in habit, such as the genera Chatn.-t- 

 siphon, Pleurocapsa, Dermocarpa and Oncobyrsa; also certain species of 

 Calothrix, Rivularia, Lyngbya, etc. 



The Blue-green Alga? possess a remarkable capacity for withstanding 

 adverse conditions of environment, and 

 especially long periods of drought. It is 

 not merely in moist climates that these 

 plants occur : they are not uncommon in 

 semi-arid districts in which the rainfall 

 is relatively low. It is especially those 

 members of the Oscillatoriacese which form 

 slimy felt-like masses that are thus capable 

 of withstanding considerable desiccation 

 without the formation of resting-spores 1 . 

 In Angola the prolific growth of Scytonetna 

 Myochrous var. chorographicum during the 

 rainy season gives such a feature to the 

 mountains of the Presidium that they are 

 known as the pedras negras or ' black 

 rocks.' At the end of May the Algre begin 

 to be discoloured with the intense heat, and 

 the large patches begin to dry and peel off. 

 Soon the rocks reappear in their natural 

 grey or brownish-grey colour, and remain 

 thus through the hot season until the 

 beginning of the next rains (Welwitsch, 

 1868; W. & G. S. W., '97). In cold 

 latitudes and at high altitudes Myxophycese 

 of all kinds can withstand prolonged and 

 repeated freezing. In the Antarctic in the 

 vicinity of Ross Island and South Victoria 

 Land (lat. 7732'S. to lat. 7745'S.) Blue- 

 green Algas form the dominant vegetation 

 of the pools and lakes, which is remark- 

 able considering the severity of the 

 climate. The growing season only lasts 

 for about four weeks, and during the re- 

 maining part of the year the Algae are 

 completely frozen. Sometimes they pass 

 several years in this frozen state, without being once thawed, and yet 



1 According to Phillips the spores of Oscillatoria will germinate after a resting-period of a 

 year and a half. 



W. A. 3 



Fig. 20. (ronionticUa (sitbtubulosrt Teo- 

 doresco. A, ventral view of filament ; 

 B, transverse section ; C, optical 

 longitudinal section ; D, ventral view 

 of a longer and more twisted filament. 

 All x 580 (after Teodoresco). 



