1907.] Fresh-water Algal Flora of Ceylon, 227 



in Lake Kantelai (Glceotrichia natans (Hedw.), Eabenh.). These two tanks, it 

 is interesting to notice, are both characterised by a very rich growth of water- 

 weeds, so that their waters certainly contain a very considerable percentage 

 of organic substance (cf. above, p. 223, footnote) ; further, they are both practi- 

 cally stagnant. Whether these factors are directly related to the abundant 

 occurrence of Bivularia in these tanks must for the present remain an open 

 question. Whereas in Basawakkulam the Glceotrichia is the only common 

 Alga, this cannot be said of Lake Kantelai, where another blue-green form 

 (Lyngbya cerugineo-ccerulea (Klitz), Gom.) is exceedingly common, and shares 

 the dominance of the former. The Lyngbya (like the Glceotrichia) is attached 

 to the segments of the abundant water-weeds, covering them with its thin 

 flocculent films for considerable stretches : in places this growth is so profuse 

 that it forms thick yellowish-green masses immediately below the surface of 

 the water. Exactly the same species occurs in abundance attached to the 

 submerged aquatics (Myriophylhim and Cham) in the open waters of Lake 

 Mineri (together with small quantities of Glceotrichia), but here it forms very 

 delicate films, tending to break up on being touched. In both Mineri and 

 Kantelai, however, the films do not consist of pure Lyngbya, but give shelter 

 to a considerable number of unicellular forms (various blue-green species ; 

 Diatoms, notably Epithemia and Pinnularia ; Desmids, e.g., species of 

 Cosmarium), the whole constituting a very characteristic assemblage of algal 

 species. The Diatoms and Desmids occurring in these films are in part quite 

 identical for the two tanks, and the films are thus a good example of the 

 " consortia " discussed below.* It is only open to surmise as to whether 

 there is anything more than a kind of " space-parasitism " in the case of these 

 composite films/f but there seem possibilities of more important (nutritive ?) 

 interrelations which we are not yet in a position to understand. 



Oscillaria, though rarely of great importance in the floating flora of the 

 tanks, was a prominent feature in Mahakekirawa ; here 0. tennis, Agardh. 

 var. natans (Kutz),»Gom. occurred as small floating discs (2 to 6 inches in 

 diameter, and often almost perfectly round) on the surface of the water. The 

 filaments are arranged in a radiate manner in these discs, which also include 

 a considerable quantity of yellowish mud, serving to increase their compact- 

 ness. No doubt the discs originate on the mud at the bottom of the tank, and 



* See p. 247 and cf. Fritsch, "Problems in Aquatic Biology," etc., 'New Phytologist,' 

 vol. 5, 1906, p. 157. 



t Inasmuch as Lakes Mineri and Kantelai are the only large tanks I have had the 

 opportunity of studying from a boat, I consider it very probable that such Lyngbya- 

 films may occur in the open water of a number of the other tanks, especially as the films 

 are not at all commonly found near the banks, but attain their greatest development over 

 the deeper portions of the tank. 



