Chroococcus] xxiv. chroo \< i b (West). 381 
mountains ore covered with Li in a very short tim< 
after the hoi season has ael in, .-it the end of May, when the 
horizon above the Presidium is generally clear and bright, the 
black plantlets begin bo discolour with the intense heal 
gradually become dry and brittle until they peel off altogethei 
n\ and-by, after which the rocks lose their sombre black 
and reappear in their natural grej or grej brownisl [ova before 
the succeeding spring." Welwitsch named this planl Scytonema 
chorographicum, and gave a figure which is hardly charact 
We bave described i1 n&Scytonema Myochroua vox. chorographicum 
(Journ. Bot. 1897, p. 265) ; see ante, p. 371. Mixed with it is a 
quantity of Dichothrix gypsophila. \\>- states that " the wonderful 
growth of Scytonema in such i lense quantities is no! confined 
to the mountain ridge of the Presidium. ... In proof of this 
fact, 1 may refer to the damp sandy upper valley of the Cuanza 
Eliver, where a species of Scytonema is abundant, frequently ex- 
tending across the wide meadows, closely spread like a nel over 
the soil, iiitt'iiirnwii with (lit' other heihs ami sm.-i ll#-i- shrubfi 
Through its hygroscopic nature it eagerly absorbs the atmo- 
spheric moisture during the dewy nights, affording by this means 
eshing 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 Porphyrosiphon Notarisii, one of the Vaginariea ; 
see ante, p. 375. 
A parallel to this phenomenon is found on the sandy heaths <>!' 
the south and some parts of the north of England. A.1 the drier 
and hotter periods of the year, thickly matted sheets of Z 
ria torn in arc t<> be found extending over wide patches of almost 
bare sand, round such plants as Brosera, r,,,-;,-.-, etc. These 
interlacing masses have grea! absorptive capacity, gi lily taking 
up water ; it is also highly probable thai they protecl and render 
the growth of other plants more possible by regulating the moisture 
of the surface soil. It is interesting to note that Zygnema 
ericetorwm is found intermixed in small quantity with the Por 
ohyronphon Notarisii which covers such extensive areas in Angola. 
' K\ Journ. Bot 1897, pp. 303, 304.) 
