WELWITSCh's AFRICAN FRESHWATER ALG-?5. 803 



Myxophyce^ : — 



XV. Rivulariacefe 



XVI. Sirosiphoniaceffi 



XVII. Scytonemacefe 



XVIII. Nostoceffi . 



XIX. Camptotricbefe 



XX. Vaginariese . 



XXI. Lyngbj'eae . 



XXII. Cliamoesiphouene 



XXIII. Cystogoneae 



XXIV. CiiroococcacesB 



77 300 



P.S. — Mr. Hiern has kindly called our attention to a paper by Wel- 

 witsch in the Journal of Travel and Natural History, vol. i. pp. 22-3G 

 (1808), entitled " The Pedras Negras of Pungo Andongo in Angola." 



In this paper Welwitsch shows that the prolific growth of a 

 subaiirial alga is the cause of one of the main features of the 

 country, as it gives such a peculiar aspect to the mountains that 

 they are known as the "black rocks." He writes that it is 

 "probably a non-described species of the prolific genus Scijtonema, 

 which in that situation during the rainy season generates and 

 multiplies so rapidly that the upper portions of the mountains are 



covered with it in a very short time Soon after the hot 



season has set in, at the end of May, when the horizon above the 

 Presidium is generally clear and bright, the black plantlets begin 

 to discolour with the intense heat. They gradually become dry and 

 brittle until they peel off altogether by and bye, after which the 

 rocks lose their sombre black aspect, and reappear in their natural 

 grey or grey-brownish colour before the succeeding spriug." Wel- 

 witsch named this plant Scijtonema chorojraphicuni, and gave a figure 

 which is hardly characteristic. We have described it above as Seyto- 

 nema Myochrous var. choroyraphicum. Mixed with it is a quantity of 

 Dichothrix yypsophila. He states that " the wonderful growth of 

 Seytonema in such immense quantities is not confined to the moun- 

 tain ridge of the Presidium In proof of this fact, I may 



refer to the damp sandy upper valley of the Cuanza River, where a 

 species of Seytonema is abundant, frequently extending across the 

 wide meadows, closely spread like a net over the soil, intergrown 

 with the other herbs and smaller shrubs. Through its hygroscopic 

 nature it eagerly absorbs the atmospheric moisture during the 

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

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

 the following day. The growth and thriving of the numerous 

 small phanerogamous plants in these places is conditional on the 

 co-presence of the prolific .S'c(/ton(?Hw." This second "Seytonema'' 

 alluded to by Welwitsch is Porphyrosiphon Notarisii, one of the 

 VayinariecB. 



A parallel to this phenomenon is found on the sandy heaths of 

 the south and some parts of the north of England. At the drier 

 and hotter periods of the year, thickly-matted sheets of Zyynema 



