264 Fritsch . — The Subaerial and Freshwater 
But examination of certain types of tropical waters (in Ceylon, especially 
small stagnant ditches and rock-pools) shows a wealth of such forms, which, 
although not very great in number of species, makes up for it by the large 
number of individuals. In such waters we also find many forms making 
attempts at filament-formation, which ordinarily exist as independent 
individuals ( Micrasterias foliacea , Bail., species of Pleurotaenium, Triplo- 
ceras , j Euastrum^ &c.). The relative excess of filamentous Desmids in 
tropical waters comes out very plainly in comparing pools with a rich 
Desmid-flora from the lowlands and uplands (from around Nuwara Elija, 
alt. 6, coo feet and more) of Ceylon. The upland pools (in which the 
conditions of existence are semi-temperate) were found to be just as poor 
in filamentous forms as the waters of temperate regions. I have elsewhere 1 
suggested that we may again find an explanation for this phenomenon in the 
relative aeration of the water, the presence of little dissolved oxygen 
appearing to encourage filament-formation. This is a suggestion 2 which 
must be left to experiment for verification, but it would certainly explain 
the relative abundance of filamentous Desmids in tropical waters. The same 
theory also accounts for the abundance of filamentous Diatoms in hot 
springs (cf. p. 353). 
In turning our attention to the literature with reference to the point just 
discussed, I know of no more striking example to illustrate it than the 
records in Borge’s paper on Australian freshwater Chlorophyceae (Borge, 
’98). The Algae enumerated in this paper are partly tropical and partly 
extra-tropical ; of the sixteen species of filamentous Desmids recorded, 
thirteen are from tropical habitats, six only from extra-tropical localities. 
In further illustration of the same feature we may notice the number of 
species of filamentous Desmids in some of the more complete tropical algal 
floras, viz. Borge, ’03 (sixteen species) ; De Wildeman, ’00 (eleven 
species and numerous varieties) ; Joshua, ’86 (thirteen species) ; Nordstedt, ’80 
(seven species and numerous varieties) ; Raciborski ’95 (twelve species) ; 
Schmidle, ’03 A (seven species) ; Turner, ’92 (twenty-six species) ; West 
and West, ’02 B (twenty-one species). It is, of course, possible that there 
are regions of the tropics in which filamentous Desmids are not as 
important as they no doubt are in certain waters in Ceylon, but I think 
it more probable that the lack of records in many of the floras is due to the 
material not having been collected from habitats in which these Desmids 
flourish. Small collections of stagnant water are apparently the most 
favourable ; this is also shown by the large number of filamentous forms 
found amongst the Desmids, which live in the bladders of tropical species 
of Utricularia (cf. especially Raciborski, ’95, p. 30 ; Nordstedt, ’80). 
1 Proc. Roy. Soc., Ser. B, vol. lxxix, 1907. 
2 This theory is based on Senn’s observations on colonial Protococcales (Bot. Zeitung, Bd. Ivii, 
1899, p. 97). 
