1907.] Fresh-water Algal Flora of Ceylon. 229 



(c) The important part played by the genus Spirogyra to the practical 

 exclusion of other filamentous Conjugates, and the special systematic 

 composition of this Spirogyra-element. 



(d) The usually marked absence of broad filamentous forms (excepting 

 JSpirogyra). 



The genus Cladophora and its ally Rhizoclonium are exceedingly rare in 

 €eylon,* and the explanation of their absence seems no very difficult one. 

 In our parts they generally frequent water which is well aerated, and in 

 larger pieces of water, for instance, are in great part confined to near the 

 banks, where there is plenty of motion owing to the wind.f The practical 

 absence of these two genera in the fresh-waters of Ceylon is probably due 

 to an insufficiency of dissolved oxygen. The thick coarse walls and broad 

 filaments of these genera are not well suited for a ready diffusion of 

 igases. The absence of Vaucheria also may be due to its broad filaments, 

 although (as far as the tanks are concerned) this genus is not a very 

 common constituent of lake- vegetation, even in our parts. Vaucheria, 

 however, has a decidedly different assimilatory process to that of most other 

 :green Algae, and it is possible that this process does not go on readily 

 amid tropical conditions. This view may receive some measure of support 

 in the fact that the entire group of the Heterokontae (Confervales, Borzi), 

 which probably have a similar assimilatory process,^ is very rare in the 

 lowlands of Ceylon.§ Ulotrichaceae are certainly also§ rare in Ceylon, and 

 their scarcity is not easy to understand. Some species of Ulothrix are 

 known to favour aerated water (although this does not apply to all of them), 

 •and possibly the small percentage of dissolved oxygen may have something 

 to do with their absence. We have practically no data regarding the 

 influence of the amount of dissolved oxygen on the different genera of 

 Algae, but it seems very likely that some of them may be far more susceptible 



* The following remarks (to last paragraph on p. 231) apply to the fresh-waters of the 

 •Ceylon lowlands as a whole. 



t Cf. Chodat, "Algues vertes de la Suisse," ' Pleurococcoi'des — Chroolepoides,' Berne, 

 1902, p. 77 ; also p. 90. 



o 



J Eegarding this assimilatory process, see especially K. Bohlin, " Studier ofver nagra 

 ;slagten af Alggruppen Confervales, ' Borzi," <Bih. K. Svenska Vet.-Ak. Handl.," vol. 23, 

 Afd. Ill, No. 23, German Kesume, pp. 52 — 54. The chloroplasts contain a relatively 

 large amount of xanthophyll, and the first product of assimilation appears to be a glucose, 

 which subsequently becomes; transformed into oil. The assimilation is thus decidedly 

 different to that of the remaining green Algae, and in the conditions of illumination, 

 temperature, etc., prevalent in the tropics, there may be some factor or factors unfavour- 

 able to the process of photosynthesis in this case. 



§ Messrs. West and West (" Fresh- water Algae of Ceylon," ' Linn. Soc. Trans.,' vol. 6, 

 1902, p. 124) have already commented on this subject. 



