246 Frit sc h, — The Sub aerial and Freshwater 
It will be seen that the three floras from temperate regions show on the 
whole a considerably smaller percentage of blue-green forms than do the 
tropical floras ; most of the exceptional cases among the latter (viz. Mobius, 
’88; Turner, ’ 92 ; West and West, ’02 b) are confessedly unrepresentative 
collections. Moreover, we must guard against attributing too much value 
to these figures, for they mean only relative abundance of species, and we 
know nothing about individuals. But the table of tropical floras does 
quite unmistakably seem to point to the Cyanophyceae being a very 
important constituent of the algal flora in all parts of the tropics (cf. also, 
for instance, the relative numbers of blue-green forms as compared with 
the total number of species in the two tables). 
Not only is the blue-green element of great importance in the sub- 
merged macrophytic growth of tropical freshwaters, but it also plays 
a very important part in the Plankton. More than half of the large pieces 
of water in Ceylon, whose Plankton I examined, abounded in floating 
microscopic Cyanophyceae (especially Clatkrocystis), which in many cases 
indeed were the sole components of the Phytoplankton. Reference to the 
literature, again, shows that Ceylon is in no way exceptional in this respect 
(cf. Schmidle, ’03 A and ’04 A ; Lemmermann, ’ 06 , p. 147). I prefer, 
however, to defer further discussion on this subject, as I hope at an early 
date to deal with the Plankton contained in my collections from Ceylon. 
The Cyanophyceae are a group of narrow forms, the diameter of the 
filamentous species rarely exceeding 15 fi, and very frequently being con- 
siderably less. That is a feature which may be of some advantage in water 
which is as poorly aerated as the ordinary tropical freshwaters are (cf. p. 352). 
A narrow filament will be better situated as far as gaseous exchange by 
diffusion is concerned than a broad one, and we may consequently expect 
that narrow forms will be more successful in the tropics than broad ones. 
As a matter of fact the entire freshwater algal flora of the tropics shows 
a preponderance of narrow forms. This is especially striking in the case 
of Oedogoninni (cf. p. 2 65), a very important form in the green algal vegeta- 
tion. The general prevalence of narrow forms is also due to absence or 
great scarcity of characteristic genera with broad filaments ( Vaucheria , 
Cladophora ). There are, however, two genera [Spirogyra, Pithophora) which 
furnish a considerable broad-celled contingent to the tropical aquatic algal 
flora, and of these Spirogyra is even highly abundant. This genus apparently 
gets on well in very poorly aerated water, whilst most of the freshwater 
algal genera have their broader representatives excluded from ordinary 
tropical freshwaters, or (probably) only developed under special conditions. 
This subject is discussed more fully in the following paragraphs. Confirma- 
tion for these theories must, of course, be sought in experiments, which will 
be undertaken in the near future. 
Before concluding this section on the part played by the Cyanophyceae 
