226 Dr. F. E. Fritsch. On the Subaerial and [Jan. 2, 



regions, we meet with serious difficulties, for our existing tropical algal floras, 

 though doubtless of considerable systematic and phytogeographical value, 

 are practically useless from this point of view. I have nowhere found a 

 particular comment on the dominance of the blue-green element, although 

 Messrs. West and West* remark on the presence of well-developed 

 individuals of Clathrocystis in tropical Plankton. However, I do not hesitate 

 to suggest that in other parts of the tropics the blue-green element plays 

 the same important part that it does in Ceylon, in some cases to a less, in 

 others, perhaps, even to a more marked extent, according as the external 

 conditions vary. I shall return to this subject elsewhere,t and need here only 

 point out that in view of the very distinct aspect of the fresh-water algal 

 vegetation in the tropics it is certainly astonishing that its general character 

 has not previously been subjected to a critical analysis. 



I will now endeavour to give a brief sketch of the essential character of 

 the blue-green macrophytic Algre (excl. Plankton), and of their mode of 

 occurrence in the different tanks of Ceylon.J The blue-green vegetation 

 forms either (a) a dense felt or tufted growth or adhesive film on submerged 

 rocks, tree-stumps, etc. ; or (b) floating masses, the latter in particular often 

 attaining a very prominent development. The more important and charac- 

 teristic floating forms belong to the genera Oscillaria, Lyngbya, Tolypothrix, 

 Scytonema, Hcvpalosiphon, and Rivularia. A member of the second genus 

 (L. majuscula (Dillw.) Harv.) is exceedingly common in many of the 

 tanks (e.g., JSTalandewewa, Tissawewa, Balankulam, Madawachyawewa, 

 Habaranewewa, etc.), forming large dark-green floating masses of densely 

 interwoven coarse filaments. At Nalande and Anuradhapoora it completely 

 dominates the vegetation of all the waters fed by the respective tanks ; at 

 the latter place many of the smaller tanks threaten to become completely 

 silted up by the growth of this Alga and by the mud which collects amid 

 the tangle of its filaments. Other (smaller) species of Lyngbya also occur in 

 the tanks (e.g., in Villamkulam, Andankulam), but play no very important 

 part (see, however, the next page). 



Another very important constituent of the algal vegetation of the tanks 

 are species of Rivularia (Gloeotrichia). They absolutely dominate the algal flora 

 in Basawakkulam (Glceotrichia Rabenhorstii, Bornet), and also play a great part 



* West and "West, "Observations on the Conjugatse," 'Annals of Botany,' vol. 12, 

 No. LXV, 1898, p. 36. 



t ' Annals of Botany,' April, 1907, p. 243 et seq. ' 



I When the water of the tanks falls in the dry season, a number of pools are usually 

 left behind, which, of course, again constitute a part of the tank in the wet season. The 

 vegetation of these pools is taken into account as a constituent part of that of the tank in 

 the following remarks, wherever it is deemed necessary. 



