204 Dr. F. E. Fritsch. On the Subaerial and [Jan. 2, 



(Myxophyceae) and, owing to this characteristic composition, the algal 

 covering is for the most part of a much more sombre hue than the bright 

 green one of our parts. This dark colour is only relieved here and there 

 by an occasional brighter patch, due either to a growth of Mosses, which, 

 however, do not apparently prosper very well in unsheltered situations, or to 

 the thin bright blue -green coating of an Oscillaria, or of some member of the 

 Chroococcacese (sensu Kirchner), which are often able to obtain a foothold on 

 very smooth walls, etc., which are unsuitable substrata for other forms. 

 Trentepohlia, with its reddish-yellow tufts, also forms a relatively frequent 

 and agreeable interruption to the dark Cyanophyceous growth. 



Subsequent considerations (cf. p. 224, et seq.) will show that the blue-green 

 Algse play a far more important part in the tropics than they do with us, and 

 a tropical fresh- water or subaerial* algal flora obtains its stamp from the 

 prevalence of this group. In order to realise the extent of development of 

 the subaerial Cyanophycese in the tropics, it is necessary to pay a visit to a 

 damp hot-house,f or to some hot spring ,J where conditions of a similar nature 

 prevail. Here also we find the green Algae crowded out and superseded by 

 the blue-green forms. 



Most of the Cyanophyceae require two factors for abundant development ; 

 the first is plenty of moisture, the second a sufficiently high temperature. 

 These two factors determine the distribution and character of the blue-green 

 subaerial growth.' The importance of the former factor is, of course, perfectly 

 obvious without much comment ; it is also manifest in comparing the moist 



* The only reference I am aware of that speaks of an abundant occurrence of subaerial 

 Cyanophyceous growth in tropical regions is one by Welwitsch (' Journ. Travel and Nat. 

 Hist.,' vol. 1, 1868, pp. 22 — 36), on " The Pedras Negras of Pungo Andongo in Angola." 

 The author describes the prolific growth of a species of Scytonema on the upper portions of 

 the mountains, and also refers to another species (Porphyrosiphon Notarisii) as covering 

 the sandy soil in the valley of the Cuanza Eiver for considerable stretches (cf. also West 

 and West, in ' Journal of Botany,' vol. 35, 1897, p. 303 ; and Warming, ' Okol. 

 Pflanzengeogr.,' German Edit., Berlin, 1896, p. 215). 



t Cf Fritsch, " Algol. Notes, IV. — Eemarks on the Periodical Development of the Algae 

 in the Artificial Waters at Kew," ' Annals of Botany,' vol. 17, No. LXV, 1903, p. 274 ; also 

 " Algae," in "The Wild Fauna and Flora of the Eoyal Botanic Gardens, Kew," 'Bull. 

 Miscell. Inform. Eoy. Bot. Gards., Kew,' Addit. ISeries V, 1906, p. 187 et seq. In the 

 Nepenthes-house at Kew the blue-green Algae even settle down on the leaves of the 

 cultivated plants (cf. also Schmidle, "Epiphylle Algen," etc., 'Flora,' vol. 83, 1897, p. 323 

 et seq.). 



J See G. S. West, "On some Algae from Hot Springs," 'Journal of Botany,' vol. 40, 

 1902, pp. 241 — 248 ; W. H. Weed, " Formation of Travertine and Siliceous Sinter by the 

 Vegetation of Hot Springs," 9th Ann. Eep. U.S. Geol. Survey, 1887—88, p. 619 et seq. ; 

 and F. Cohn, " Ueber die Algen des Karlsbader Sprudels," etc., ' Abh. Schles. Ges. Vaterl. 

 Cultur,' 1862, p. 35 et seq. The vegetation in such springs is almost entirely composed of 

 Cyanophyceee and Diatoms (see also Sect, b (v), of the present paper, p. 251). 



