Chroococciis] xxiv. chroococcace^ (West). 381 



mountains are covered with it in a veiy short time. . . . Soon 

 after the hot season has set in, at the end of May, when the 

 liorizon above the Pra^sidium 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-by, after which the rocks lose their sombre black aspect, 

 and reappear in their natux'al grey or grey-brownish colour before 

 the succeeding spring." Welwitsch named this plant' Scytonema 

 chorogrcqjhicum, and gave a figure which is hardly characteristic. 

 We have described it as Scytonema Myochrous var. clwrogra'pldcunii 

 (Journ. Bot. 1897, p. 265) ; see ante^ p. 371. Mixed with it is a 

 (quantity of Dichothrix cjypsophila. He states that '* the wonderful 

 growth of Scyto')iema in such immense quantities is not confined 

 to the mountain ridge of the Prtesidium. ... 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 ex- 

 tending 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 atmo- 

 spheric 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 Scytonema." This second '■^Scytonema" alluded to by 

 Welwitsch is Pwphyrosiphon Notarisii, one of the Vaginarieoi ; 

 see ante, p. 375. 



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 Zygnema 

 ericetorum are to be fovmd extending over wide patches of almost 

 bare sand, round such plants as Drosera, Carices, etc. These 

 interlacing masses have great absorptive capacity, greedily taking 

 up water ; it is also highly probable that they protect and render 

 the growth of other plants more possible by regulating the moisture 

 of the surface soil. It is interesting to note that Zygnema 

 encetorum is found intermixed in small quantity with the Por- 

 phyrosiphon Notarisii which covers such extensive areas in Angola. 

 (Ex Journ. Bot. 1897, pp. 303, 304.) 



