248 Frit sch . — The Subaerial and Freshwater 
taceae and Stigonemataceae (especially Scytonema , Tolypothrix , Hapalo - 
siphon , and Stigonema ), and to the large number of genera of Oscillatoriaceae 
represented. 
4. The Systematic Composition of the Green Algal 
Element in the Freshwaters of the Tropics. 
There are certain marked peculiarities of the green algal element as 
found in tropical freshwaters, which are very manifest in Ceylon, and, 
judging by the data obtainable from existing floras, equally so in other 
parts of the tropics. Some of them are so striking that it is astonishing 
that there has been no previous comment upon them. Although the 
excess of Cyanophyceae is much the most prominent feature, the peculiar 
composition of the green algal flora supplements the particular stamp 
which the former already give tropical freshwater vegetation. The more 
important features presented by the green element are as follows : — 
(a) The relative scarcity of Cladophora and Rhizoclonium , and the 
consequent scarcity of a number of characteristic epiphytes, which in our 
waters find one of their main substrata on these filamentous forms (e. g. 
Cocconeis Placentula , Synedra splendens). 
( b ) The replacement of these two genera by Pithophora . 
(c) The great scarcity of Vaucheria (both aquatic and terrestrial 
species) and Botrydium . 
(d) The relative scarcity of Ulotrichales and Confervales. 
(e) The important part played by the genus Spirogyra combined with 
the relative scarcity of other Zygnemaceae, and the special systematic 
composition of the Spirogyra-e lement. 
(/) The relative abundance of filamentous Desmids. 
(g) The relative scarcity of broad species of Oedogonium (cf. above, 
p. 246). 
These features have been dealt with briefly elsewhere and a number 
of explanations to account for them are there suggested. It was, however, 
not possible to give full data in support of these suggestions, and I propose 
to do that here. 
(i) The Freshwater Cladophoraceae of the Tropics. 
Every algologist knows how commonly Cladophora or Rhizoclonium 
are to be met with in temperate freshwaters, and although no doubt in our 
parts conditions are often unfavourable for their development in certain 
waters, no considerable area can be surveyed without finding them well 
represented in the aquatic vegetation. In Ceylon matters are very 
different, and one can search for a long time before finding freshwater 
1 Fritsch, Proc. Roy. Soc., Ser. B, vol. Ixxix, 1907. 
