The Pedras Nci:^ras of Pmiin^o Aiidoiigo in A/ii:;o/a 



35 



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 spring. 



In conclusion, I have to observe that the wonderful growth of 

 Scytonemas in such immense quantities, is not confined to the 

 mountain ridge of the Presidium. It also extends eastwards, with 

 more or less interruption, effecting a most important and bene- 



F"iG. 2. — Pedras Ncgras after the rainy season. 



ficial influence on the maintenance and existence of many other 

 small herbs. In proof of this fact, I may refer to the damp sandy 

 upper valley of the Cuanza River, where a species of Scytonema is 

 abundant, frequently extending across the wide meadows, closely 

 spread, like a net over the soil, intergrown with 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 



