1907.] Fresh-water Algal Flora of Ceylon. 241 



of the floating patches. The phenomenon is certainly no rare one in the case 

 of Algse capable of forming compact discs like Oscillaria (cf. also p. 227). 

 Analogous, though not absolutely similar, is the rising of tangles of filaments, 

 occasionally also to be seen in Oscillaria, though more frequent in green 

 filamentous genera such as Conjugates. CEdogonium, Cladophora, etc.* The 

 facts mentioned in this paragraph are also, of course, commonly to be noticed 

 in temperate regions, but I am not aware that a daily rise and fall of such 

 constancy ever occurs in our parts. 



It seems probable that the Algse in this case are merely the passive objects 

 of a useless physical phenomenon, and that the daily rise and fall is of no 

 service to them. But it seems well to point out that the rising of the 

 oscillariate patch to the surface may be advantageous from the point of view 

 of respiration and of assimilation (the latter owing to the considerable 

 percentage of mud included in these oscillariate films). In any case the Alga 

 only avails itself of a physical process, which would take place whether it was 

 of service to it or not. 



(iii) Algce of Marshes and Padi- -fields {i.e., of ground permanently or 



periodically saturated with water which does not form definite pools). — There 



is very little difference on the whole between the algal flora of the padi-fields 



and that of the lowland pools and ditches, but the biological conditions are a 



little different, and it is a desire to emphasise these that leads me to hold the 



two distinct. The rice-fields are fully exposed to the strong sunlight, and the 



only (very scanty) source of protection is the frequent muddiness of the water. 



Desiccation is a regular periodic phenomenon, and periodicity would probably 



be found to be better marked in this flora than in the algal vegetation of any 



other Ceylon fresh- water. In their exposed habitat the rice-fields resemble 



the tanks, whilst the desiccation risks are analogous to those in the small pools 



and ditches. In correspondence with this the blue-green element is rather 



more pronounced than in the latter, although the Conjugates still very often 



have matters their own way. The latter are sometimes almost the only forms 



present in a padi-field, but just as often there is practically nothing but 



Cyanophyceous growth. CEdogonium is not so commonly met with as in the 



pools and ditches, but Bulbochwte is again quite a common form. The Desmid- 



flora is often very rich, whilst in one or two cases Diatoms (e.g., Gomphonema 



attached to water-weeds) were present in exceptional numbers. A very 



* Cf. Oltmanns, ' Morphologie und Biologie der Algen,' vol. 2, Allgemeiner Teil, Jena, 

 1905, p. 142 ; also H. Devaux, " Du Mecanisme des Echanges gazeux chez les plantes 

 aquatiques submergees," 'Ann. Sci. Nat. Bot.,' 7th ser., vol. 9, 1889, p. 144. In the case 

 of filamentous green Algae, etc., the bubbles are retained between the tangle of filaments 

 and, by their alternate expansion and contraction, cause the rising and falling of the 

 tangles. 



