1907.] Fresh-water Algal Flora of Ceylon. 223 



their bearing upon algal vegetation in the waters of Ceylon, but I saw 

 enough to satisfy myself that a careful study would probably reveal somewhat 

 analogous algal associations in the tropics to those occurring in our parts of 

 the world. 



I will now briefly consider the minor factors, and will begin with the little 

 I can say regarding the chemical composition of the water. Some of the 

 tanks must certainly be rich in dissolved organic substance owing to the 

 large amounts of water- weeds they contain ;* in other cases the faeces of the 

 abundant water-fowl must contribute to the organic contents. The frequent 

 presence of Characeae probably means dissolved carbonate of lime. 



Most of the smaller tanks are sheltered from the ordinary winds, and the 

 water, even at the edge, was usually quite undisturbed at the time of my visit. 

 In the larger tanks, on the other hand, a relatively large surface is exposed 

 to the wind, so that there are usually numerous small waves at the edge ; the 

 littoral flora is therefore growing in water, which is frequently disturbed and 

 well aerated (especially noticeable in Nuwarawewa, Tissawewa, and Lake 

 Mineri). 



The substratum in the tanks is muddy or clayey except where the tank 

 occupies an excavation in the rock (cf. p. 219). In Balaluwewa and 

 Villamkulam part of the substratum seems to be sandy. The dams are 

 often mere clay embankments, but in a number of the larger tanks are made 

 up of blocks of rock, and these rocks, of course, afford a suitable base for 

 lithophytes when under water. 



In many of the smaller tanks the suspended particles of mud are of an 

 exceedingly finely divided nature, giving the water a yellow colouration ; 

 these particles are so fine that they often take several days to settle com- 

 pletely. The water of these tanks (e.g., Punchikekirawa, Mancadawawewa) 

 is generally, but not always (e.g., tank between milestones 101 and 102 on 

 road from Vavoniya to Madawachya), poor in organic life, and this latter 

 consists mainly of animals. 



Briefly summarised, the conditions of life of the macrophytic Algae in the 

 Ceylon tanks are thus as follows : — Strong illumination ; high temperature of 

 considerable daily range (varying from 6° to 10° C); small amount of 



* Thus, especially in Tank Basawakkulam and Lake Kantelai, the edge was fringed by 

 an accumulation of partly bleached and partly putrid remains of Chara, Myriophyllum, 

 etc., many yards in extent, and I have little doubt that such a mass of decomposing water- 

 weeds forms the bottom of the entire lake. Bound about the edge of Lake Mineri I also 

 found Chara washed up in large amount. It is a well-known fact that especially 

 Cyanophycese prosper very well in a medium containing dissolved organic substance 

 (cf. p. 227, and also Kirchner, in Engler u. Prantl, ' Die Natiirl. Pflanzenfam.,' I Teil., 

 Abt. la, 1900, p. 63). 



