The Pcdras Ncgras of Piingo Andongo in Angola 23 



bare walls of our houses marked by broad stripes or spots of 

 a yellomsh or dark green colour. These are due to the presence 

 of oscillatoraae, or other microscopic algae.* In many stagnant 

 waters and slow-running streamlets, even in the reservoirs of the 

 ornamental waters of our gardens, we see appear, in the course of 

 a few days, a velvet-like covering of greyish green, or yellow 

 colour, which, on minute examination, is found to consist of an 

 accumulation of algae, so small that one drop of water not seldom 

 contains several thousand individuals.t 



In still more conspicuous and varied phases does this pheno- 

 menon force itself on the attention of the wanderer in the damp 

 ravines of Highland valleys and the Alps, where huge rock walls 

 are coloured sometimes sulphur-yellow by the mysterious Leprarise, 

 or again are tinted in blood-red circles by the Haematococcus ; 

 and it is seen in its grandest scale in the so-called red and green 

 snow, which, particularly in polar countries, and in the regions of 

 the Alps, occasionally cover miles with a rose-red or emerald-green 

 hue, an appearance due, as is well known, to the multiplication of 

 a few species of Protococcus. J 



Not only the mainland and its fresh water shew us such pheno- 

 mena, produced by algoid vegetation, but also salt waters, nay, 

 even the wide ocean itself becomes often their birthplace ; and in 

 such instances we see with amazement the otherwise dark blue 

 waves glistening with a purple hue, or tinted as with a crimson dye. 

 It is generally understood that the Red Sea owes its name partially 

 to the occasional appearance of immense quantities of a species of 

 Trichodesmium, a microscopic alga which in the early state of its 

 existence is of a deep red colour. § 



* In some of the quieter, less-populated streets of London, we observe even 

 larger filamentous alg^ like Lyngbya and others, covering the basements of 

 walls with a delicate green. 



•f Among the number of species of algse which produce such phenomena, it 

 will be sufficient to mention Anabcena flos-aquK, Kiitz ; A. chalybea, KUtz ; 

 Limnochlide flos-aqu?e, Kiitz ; Sphoerozyga floccos Agaand Sph. insigiiis, Kiitz ; 

 and various species of Desmidiacse which cover stagnant waters with a green 

 scum. 



X Protococcus nivalis, Ag. (Conf. Saussure Voyages dans lesAlpes, tomeii., 

 p. 44 ; Wrangel Acta Holm. 1823 ; Schuttleworth, in Biblioth. Univer. de 

 Geneve, Feb. 1840. 



§ Conf. Trichodesmium erythrinum Ehrenberg, in Poggendorf s Annal. 1830, 

 p. 506 ; and Montagne Memoire sur le Phenomene de la Coloris. de la Mer Rouge, 

 Comptes Rend. Acad. Sc, 15 Juillet 1844 ; also in Ann. des Sc, 3, ii., p. 332, 

 tab. 10. 



