i54 SCIENCE PROGRESS 



the eruption of 1883, found the ground covered with a thin, 

 gelatinous, hygroscopic layer of blue-green algae of which the 

 genera To/ypot/irix, Anabcvna, Symploca, and Lyngbya were 

 the first to appear on the bare rock. These growing on 

 the volcanic ash and pumice, of which the whole island was 

 composed, gradually formed a soil on which higher plants 

 could grow. 1 



Welwitsch describes a similar growth of algae on the " Black 

 Rocks" of Pungo Andongo, in Angola. These black rocks owe 

 their colour to the abundant growth of a sub-aerial alga, 

 Scytonema myochrous van chorographicum, which generates and 

 multiplies so rapidly during the rainy season that the upper 

 portions of the mountains are covered with it in a very short 

 while. Soon after the hot season has set in, at the end of May, 

 the black plantlets begin to be discoloured by the intense heat. 

 They gradually become dry and brittle, until they peel off 

 entirely by-and-bye, after which the rocks lose their sombre 

 aspect and reappear in their natural grey-brown colour. 2 Bohlin 

 has described four algae in the Azores living on the volcanic 

 rocks in a similar way. 3 



The blue-green algae are, however, semi-aquatic and they 

 can only live in moist places ; when, however, they are joined 

 with a fungus symbiotically to form a lichen, the weft of the 

 mycelium of the fungus protects them sufficiently from desicca- 

 tion and the blue-green algae are rendered practically in- 

 dependent of moisture. Welwitsch describes how in the 

 sandy valley of Cuanza River, in Angola, a blue-green alga, 

 Porphyrosiphon iwtarisii, extends over wide meadows. By 

 reason of its hygroscopic nature, it absorbs the atmospheric 

 moisture during the dewy nights, affording by this means a 

 refreshing protection to the roots of the larger plants during 

 the glowing heat of the day. Boodle has described a more 



1 M. Treub, Notice sur la nouvelle Flore de Krakatoa, Ann. Jard. bot. 

 Buitenzorg, vol. vii. 1888, p. 213 ; see also Penzig, he. tit. vol. xviii. 1902, p. 92. 



* F. Welwitsch, /our/?, of Travel and Nat. Hist. vol. i. 1868 ; see also 

 Apontamentos Phyto-Geographicos solve a flora da provincia de Angola, etc., 

 Annaes do Conselho Ultramarine, Parte nao off. Ser. i. Dez. 1858, p. 533; also 

 E. Tenzl, Bericht iiber einige der wichtigsten Ergebnisse der Bereisung der 

 •Portugiesischen Kolonie von Angola in den Jahren 1850-1860 durch Dr. F. 

 Welwitsch, Vienna, 1864. 



3 F. Bohlin, Etude sur la Flore algologique d'eau douce des Azores, Bib. K. 

 Svenska vet. Ak. Handl. vol. xxvii. Afd. iii. No. 1, p. 12. 



