1907.] Fresh-water Algal Flora of Ceylon. 231 



•although one may regard my collections as fairly representative of the algal 

 flora. A consideration of the published papers further brings out the 

 startling fact that very few determinable fresh- water species of this genus 

 have been recorded from the tropics, and there are only few records of 

 terrestrial species (chiefly V. sessilis (Yauch.), DC.).* In the course of my 

 travels I came across many likely localities for the occurrence of Botrydium, 

 but I never found it ; there are only three records from the tropics (from 

 Xoanda in Africa, Brazil, and Ecuador). We may, therefore, further charac- 

 terise the algal flora of the tropics by the practical absence of fresh-water 

 species of Vaucheria, and the extreme scarcity of the terrestrial Siphonese 

 { Vaucheria and Botrydium). 



I must here refer to another feature which is intimately connected with the 

 rarity of Cladophora, Rhizoclonium, and Vaucheria, viz., the scarcity and different 

 composition of the epiphytic growth, which in our waters finds one of its 

 main substrata on the filaments of these genera. The characteristic epiphytic 

 Diatoms (Cocconeis, species of Gomphonema, Synedra splendens, etc.) are very 

 rare in the Ceylon fresh- waters, which may, in part, be the result of the 

 lack of a suitable substratum. But even where I found Pithophora with a 

 well-marked epiphytic growth, it was not made up of the above-mentioned 

 .species of Diatoms, but usually consisted largely of species of Achnanthes. 



The entire group of the Confervales seems to be poorly represented 

 in the tropics ; only three genera (Characiopsis, Conferva, and Ophiocytium) 

 have been recorded and, except in the case of Ophiocytium, the records are 

 rather few in number, t In Ceylon I have only met with occasional filaments 

 of Conferva in fresh-water, although future more detailed examination of my 

 material may disclose small numbers of the unicellular forms. Whether 

 the peculiar type of assimilation in this group is really responsible for its 

 •scarcity in the tropics can only be settled by a careful investigation of those 

 tropical localities in which members of this group appear. 



The Conjugates are amongst the most important constituents of the green 

 •element in the algal flora of the tanks, and it will be best to consider the 

 Conjugate vegetation of these latter separately from that of the other pieces 

 of Ceylon fresh-water. The Conjugates are best developed in those tanks 

 which possess a rich growth of water-weeds ; where larger aquatics are 

 wanting, conditions are not favourable for the growth of any floating green 

 Alga, which is bound to rise to the surface during the hot hours of the day, 

 and so be fully exposed to the strong sunlight. The development of the 

 filamentous Conjugate element in the tanks takes place almost entirely at the 

 •expense of species of Spirogyra; Mougeotia is exceedingly rare (M.parvula, Hass., 

 * Cf. 'Annals of Botany/ loc. cit., p. 254. f Loc. cit., pp. 256, 257. 



