34 



liUinial of J'/aTi'/ and N'atiiral History 



Scytonema, wliicli in that situation during the rainy season 

 generates and multiphes so rapidly, that the upper portions 

 of the mountains are covered with it in a very short time.* The 

 rains faUing in the early part of the year, especially those in 

 March and April, are accompanied by heavy thunderstorms and 

 frequently also by thick fogs. The latter envelope the rock sum- 

 mits from dawn till noon, creating a surrounding damp warm 

 atmosphere, which naturally favours the rapid growth and propaga- 

 tion of this Alga, which, besides, is otherwise known to be of a 

 prolific kind. Hence arises the phenomenon that in very wet 



_^M^ 



!■ iG, I. — Ptdrns Negras before the rainy season. 



years, most of the upper rock portions are covered with the 

 black Scytonema towards the end of April ; whilst in years 

 widi scanty rainfill, only single rock-clefts and a few of the 

 cliffs appear more or less with the algoid covering (see figs, 

 r and 2). Soon after the hot season has set in, at the end 



* Supported by this observation, made in 1857, I stated already, in 1858, in 

 my printed Report to the Portuguese Government, that the black colouring of the 

 rocks in the Presidium, must be attributed to the massive gnnvth of a species 

 of Scvtonema. 



