Occurrence and distribution 31 



ground and among wet stones, in the form of innumerable, small, brownish- 

 purple, gelatinous patches, which are much lobed and curled 1 . 



The majority of the Hormogonese occur aggregated to form a stratum. 

 This may be thin and papyraceous, but is more often slimy ; or it may 

 be tough and leathery, or even of a cartilaginous consistency. Sometimes 

 the stratum is felt-like, and in Scytonema Myochrous it usually takes the 

 form of a coarse mat. This felt may sometimes cover extensive areas of 

 the ground, as in Phormidium autumtiale and Porphyrosiphon Notarisii. 

 The former is world- wide in distribution, but is most prolific in damp 

 temperate areas, whereas the latter is mostly tropical and subtropical. 

 In 1868 Welwitsch called attention to extensive sheets of a Blue-green 

 Alga (since shown to be Porphyrosiphon Notarisii} in the damp sandy valley 

 of the Cuanza River in Angola. The sheets were closely spread like a net 

 over the soil, intergrown with small herbaceous plants and shrubs. The 

 mat-like sheets of the Alga eagerly absorbed the atmospheric moisture 

 during dewy nights, affording by this means a refreshing protection to 

 the roots of many other and larger plants during the glowing heat pf the 

 following day. Welwitsch stated that the growth arid thriving of the 

 numerous small spermatophytes in these places was conditional on the co- 

 presence of the Alga (W. & G. S. W., '97, p. 303) 2 . Some species of 

 Phormidium, such as Ph. purpurascens, often cover the vertical faces 

 of wet rocks with a brightly coloured stratum many square feet in extent. 



One of the most frequent habitats of the Myxophycese is amongst Mosses 

 and Hepatics in the deep gullies and glens of mountainous regions, and 

 among similar luxuriant growths of Bryophytes on the tree-trunks of 

 damp forests, more especially in the tropics. In these situations species 

 of Chroococcus, Glceocapsa, Scytonema, Stigonema, and Nostoc, often occur in 

 great quantity, together with various members of the Oscillatoriacese. It 

 has been shown that where the climate is sufficiently moist Blue-green Alga3 

 are often the first colonists of newly-bared rocks, just as they were the first 

 colonists on the pumice and ash after the terrific volcanic outburst at 

 Krakatau in the Straits of Sunda. 



Quite a number of the Myxophycea3 have adapted themselves to a life in 

 the freshwater plankton, and many of these forms have developed those 

 dark-coloured pseudovacuoles which are regarded by some authors as special 

 bodies for increasing the floating capacity of the Algae which possess them. 

 Species of Anab&na are among the most abundant of the plankton- 

 Myxophycese, but the two species of Oscillator ia, 0. Agardhii and 0. rubescens, 

 often occur in quantity, and a number of rather narrow species of Lyngbya 



1 These gelatinous masses are known to the local inhabitants as ' Mountain Dulse,' and in 

 past times they were rubbed into a pulp and used for purging calves (Lightfoot, 1777). 



2 Mats of Zijgnema ericetorum sometimes fulfil a similar function. 



