252 Fritsch. — The Subaerial and Freshwater 
illustration of this statement we may take the following data from Forel 1 
dealing with the relative amounts of dissolved atmospheric gases (per litre) 
as found in waters (Lake of Geneva) of different temperatures : — 
Temperature. 0 . N. C 0 2 . 
5°C. 7*3 cc. 13-6 cc. o-6 cc. 
20 0 C. 5*7 cc. 10-7 cc. 03 cc. 
The lowest temperature attained by the lowland waters of Ceylon 
I found to be 25 0 C., so that the amount of dissolved oxygen will probably 
be less than 5 cc. Both Cladophora and Rhizoclonium have thick coarse 
walls, which are not suitable for a ready diffusion of gases from the 
surrounding water, and even in temperate regions these two genera seem 
to flourish best in waters which are not absolutely stagnant. Moreover, 
their period of maximum abundance generally falls into the cold part of 
the year, and although not completely absent in the summer they are 
much reduced in quantity. From this point of view, therefore, we need 
not be surprised to find the two genera in question very scantily repre- 
sented in tropical freshwaters, and practically confined to such as are in 
some way or other well aerated. It is of course not impossible that certain 
species of Cladophora or Rhizoclonium can occasionally frequent stagnant 
water in the tropics, but up to the present there is no conclusive evidence 
on this point. 
I found the order Cladophoraceae in Ceylon mainly represented by 
species of Pithophora. They cannot be said to be really common forms, 
although frequent enough to be a sensible constituent of the algal flora. 
As regards their habitat, they appear to favour mainly the smaller pieces 
of water, and were not met with in the larger tanks. Reference to the 
literature shows a remarkably small number of records of species of this 
genus 2 , which does not at all tally with my observations in Ceylon. Can 
it be that some of the records of Cladophora above enumerated really 
concern species of Pithophora ? I do not think it very likely, but it is well 
to keep the possibility in view. It does not, however, seem at all probable 
that Ceylon is exceptional in the relative abundance of Pithophora as 
compared with the other Cladophoraceae, and we must await a careful 
investigation of the algal flora in some other part of the tropics before 
this point can be absolutely settled. In its relatively thin walls Pithophora 
certainly seems better suited than the other Cladophoraceae for life in 
tropical waters, and in Ceylon I frequently observed it in water which 
1 Allgemeine Biologie eines Siisswassersees, in Zacharias, Die Tier- tind Pflanzenwelt des Siiss- 
wassers. Leipzig, 1891, p. 15. 
2 The more important species are : Pithophora aequalis , Wittr. ; P. affinis, Nordst. ; P. clavi- 
fera , Schmidle ; P. Cleveana, Wittr.; P. oedogonia (Mont), Wittr.; P. pachy derma, Schmidle ; 
P. poiymorpha , Wittr. ; P. radians , W. and G. S. West ; P. Reineckei , Schmidle ; P. Roettleri 
(Roth.), Wittr. ; P. sumatrana (Mart.), Wittr. ; P, variabile, Schmidle. 
