242 
Fritsch . — The Sub aerial and Freshwater 
In other respects also the Cyanophyceae show indications of being more 
favourably situated there. The heterocysts, in particular, point to such a 
conclusion, for in the greater portion of my material from Ceylon they show 
well-developed contents, often even coloured a deeper blue than the contents 
of the ordinary vegetative cells (e. g. in species of Rivularia 1 ). The tropics 
no doubt afford a much more suitable field for the investigation of the blue- 
green group than do our parts of the world, and many of the problems 
connected with this group may find an adequate solution in the investiga- 
tion of tropical material. It further does not seem at all unlikely that 
many of the Cyanophyceous forms of the tropics may pass through a more 
complete life-cycle than in the less favourable conditions of temperate 
regions. As an outcome of these considerations we may state that the 
Cyanophyceae are probably a group originally adapted to tropical con- 
ditions, and that there is some slight evidence for the view that they 
are the descendants of primitive algal forms, which flourished in earlier 
epochs of the earth’s history under conditions somewhat similar to those 
obtaining in the damp tropics at the present day. 
In talking of the minimal development of green forms in the subaerial 
algal flora of the tropics, the genus Trentepohlia was stated to be an 
exception. Species of this genus indeed play a great part in subaerial 
tropical vegetation 2 , a statement based not only on my own observations 
in Ceylon, but also on the abundant records in the literature (cf. especially 
Karsten, ’91 ; De Wildeman, ’91, ’94, ’97, and ’00 ; West and West, ’02 B ; 
Zeller, ’73 ; Hariot, ’89 and ’92). The yellowish-red tufts or tresses of 
Trentepohlia are very abundant amidst the sombre Cyanophyceous growth 
in Ceylon, and form an agreeable interruption to its monotony. There can 
be little doubt that it is the screening action of the haematochrome in the 
cells of this genus that makes it alone successful amongst all the green 
forms, and this also points to intensity of illumination as being one of the 
most important factors excluding green forms from tropical subaerial 
habitats 3 . A very large number (about 40 ) of species of Trentepohlia have 
been described from the tropics, and in most cases the number of records 
is quite considerable (see also table on p. 244 ). Although so successful in 
the tropics, there seems no reason at present for looking upon this genus 
as essentially tropical ; quite a large number of species are found in 
temperate regions, generally in well-illuminated habitats (especially abun- 
dant in alpine regions). 
The abundant moisture in the damp tropics not only leads to the 
development of a very rich subaerial algal growth on tree-trunks, rocks? 
1 Cf. Fritsch, Studies on Cyanophyceae II. Beih. Bot. Centralbl., Bd. xviii, Abt. i, 1905, 
p. 207, footnote 2. 
2 Lemmermann (’ 05 , p. 609) states that Trentepohlias are wanting in the subaerial flora of 
the Sandwich Islands. 
3 Cf, also Fritsch, Proc. Roy. Soc., Ser. B, vol. lxxix, 1907. 
