1907.] Fresh-water Algal Flora of Ceylon. 219 



ancient date ; they occur all over the island, but are especially common 

 in the dry northern half. They have partly been made by the damming up 

 of broad river valleys, and partly by the collection of rain-water in specially 

 prepared hollows. In a few cases they occupy natural hollows in some 

 of the large basaltic masses which occur at many points {e.g., Nalande 

 and Dambulla). Large numbers of small shallow tanks are found near 

 every village in the drier parts of the island. Tanks of medium size, and 

 apparently of some depth, are Madawachyawewa,* Kekunadurewewa, 

 Yillamkulam,* Habaranewewa, Periyakulam, Kurunegalawewa, and we 

 may also include here the lakes at Kandy, Colombo, and Nuwara Eliya. 

 A few of the tanks are of very large dimensions ; of these I was able to 

 examine the following (approximately in order of size, beginning with the 

 largest) : Lake Mineri, Kelawewa and Balaluwewa, Nuwarawewa, Tissawewa, 

 Basawakkulam, Lake Kantelai, the last named being shallow throughout.! 



The algal flora is well developed in most of the tanks, and, as far as 

 the present state of affairs is concerned, their conditions of life are practically 

 natural. The special conditions may be briefly considered first. The illu- 

 mination is, of course, very intense, for the tanks are, as a rule, exposed to 

 the full glare of the sunlight.J Trees and bushes only rarely come down to 

 the water's edge, and the immediate neighbourhood of the bank tends 

 to be inhabited either by dead trees, whose bleached trunks and branches 

 look like spectres in the distance, or, at the best, only by a stunted growth, 

 owing to the frequent changes of water-level. The edges of the tanks 

 on the water-side are generally occupied by a rich growth of water-weeds 



* The termination wewa is the Singhalese name for tank, Iculam is the Tamil 

 designation. 



t It is almost impossible to obtain a boat on any of the tanks, and the presence of 

 crocodiles generally makes wading impossible. On the whole, therefore, my observations 

 are confined to the vegetation within a few feet of the water's edge. It would have been 

 interesting to examine a few of the smaller tanks a little more completely, but the littoral 

 region is, after all, generally the most abundantly populated, and most likely to afford 

 important results from our point of view. In all cases I made a point of walking round 

 more than half the embankment of each tank, so as to obtain a good general idea of the 

 type of growth it contained. I was able to investigate Lakes Mineri (by the courtesy of 

 Mr. McPhail) and Kantelai more completely with the help of a boat, and also the lakes at 

 Colombo (by the kindness of the Colombo Kowing Club), and Kandy (by the courtesy of 

 Mr. Baird). 



% Oltmanns has recently pointed out (' Morph. u. Biol. d. Algen,' vol. 2, 1905, p. 190), 

 that an important consideration with regard to the conditions of illumination influencing 

 aquatic algal growth is the quantity of light which gets reflected at the surface of the 

 water. Not only is the intensity of the light much greater in the tropics, but, owing to 

 the more or less vertical position of the sun, a far smaller percentage of the light is 

 reflected at the surface. I was only rarely inconvenienced by the glare of light from the 

 surface of the tanks (c/. also Ewart, in ' Annals of Botany, 5 vol. 12, 1898, p. 379). 

 VOL. LXXIX. — B. S 



