238 
Fritsch . — The Subaerial and Freshwater 
where the abundance of the subaerial algal covering is truly striking. 
I have no doubt that, wherever in other parts of the tropics the two 
necessary factors (temperature and moisture) combine, there exists a similar 
abundance of subaerial Algae. The literature indeed affords very few data 
to go upon. The few really extensive, and to some extent apparently 
complete, tropical algal floras do indeed enumerate a considerable number 
of subaerial Algae (see especially De Wildeman, ’00 ; Gomont, ’02 ; West 
and West, ’93 and ’97 a), but, as above mentioned, they give no data as 
to extent of occurrence. The only positive statement that I know of is 
as follows : Welwitsch 1 describes the prolific growth of a blue-green Alga, 
Scytonemachorographicum , Welw. (— S. Myochrous (Dillw.), Ag. var. choro- 
graphicum , W. and G. S. West), on the mountains of Pungo Andongo in 
Angola ; this Alga causes a characteristic feature of the country, since it 
gives rise to a black colouration on these mountains (hence locally known 
as ‘pedras negras ’ or black rocks) (cf. W. and G. S. West, ’97 a, p. 303) 2 . 
Another ‘ Scytonema ’ (= Porphyrosiphon Notarisii , Kiitz.) plays an equally 
important part, according to Welwitsch, in the damp sands of the upper 
valley of the Cuanza River, where it * frequently extends across the wide 
meadows, closely spread like a net over the soil, intergrown with the other 
herbs and smaller shrubs.’ A perusal of Gomont’s account of the Myxo- 
phyceae hormogoneae collected by the Danish expedition to the Gulf of 
Siam (Gomont, ’01, p. 202 et seq.) also tends to show that there is a con- 
siderable subaerial algal element in these regions. The same impression 
is gained if we look over the records of Cyanophyceae in De Wildeman’s 
algal flora of Buitenzorg (De Wildeman, ’ 00 ). Other data in support of 
this view are given in the table on p. 244. As will be seen, there is little 
direct evidence to go upon, but it is sufficient to show that in all probability 
all damp tropical regions harbour a rich subaerial algal vegetation 3 . 
An examination of the abundant algal covering in the damp lowlands 
of Ceylon showed that it was practically entirely constituted by members 
of the Cyanophyceae (Myxophyceae). I have elsewhere 4 dealt with the 
many interesting biological peculiarities of this Cyanophyceous growth, 
and we are here only concerned with the systematic and phytogeographical 
sides of the question. There is practically no green element in the sub- 
aerial algal flora of the Ceylon lowlands (except Trentepohlia , see below, 
p. 242), the only marked exception observed being the growth on the 
subaerial portions of the stilt-roots of the mangroves at Kalutara (a form 
1 Welwitsch, Journal of Travel and Natural History, vol. i, 1868, pp. 22 et seq. 
2 Cf. also Warming, Okologische Pflanzengeographie. German Edit., Berlin, 1896, p. 215. 
3 It is of course not impossible that under exceptional conditions even damp tropical regions 
may be poor in subaerial algal growth. Lemmermann (’05, p. 609) found very few aerophilous 
forms in his material from the Sandwich Islands, but it is open to question as to whether this is not 
due to imperfect collecting. 
4 Proc. Roy. Soc., Ser. B, vol. lxxix, 1907. Section a of this paper deals with the subaerial 
Algae (cf. also a forthcoming paper in the Geographical Journal, vol. xxix, 1907). 
