1907.] Fresh-water Algal Flora of Ceylon. 221 



considerable, and not far behind that of the air {e.g., in Villamkulam, water 

 from 25° to 30° C. ; air from 24° to 30 a 5 C. in the shade) at least, as far as 

 the surface-layers of the water are concerned. These observations, which, of 

 course, refer to the temperature round the margins of the tanks only, and 

 apply also to all the different kinds of inland fresh- waters, show that a very 

 high temperature is frequently attained during the day, and that there 

 is a considerable, though gradual, cooling over night, which, however, never 

 results in a temperature which would be considered anything but warm 

 in our parts. It seems probable from what is known that a considerable 

 number of fresh-water Algse prefer rather cold water,* and this may be 

 one of the causes of the practical absence of a number of green forms from 

 the fresh-waters of the tropics (cf. below). On the other hand, most of the 

 Cyanophycese seem to nourish best in warm waters, and hence their pre- 

 ponderance in the aquatic flora of Ceylon. The relatively high temperature 

 of tropical waters, however, also involves the solution of a proportionally 

 smaller amount of oxygen and carbon dioxide. Especially the smaller 

 percentage of oxygen, and the consequent greater difficulty of respiration, 

 seems likely to be an important feature, although it remains to determine 

 its influence by exact experiment. I incline to asume that this is one of 

 the chief causes of the practical absence of certain temperate genera from the. 

 waters of the tropics, and that it also accounts for certain peculiarities 

 of tropical fresh-water algal growth.f 



The temperatures of the upland waters are, of course, in no way so high ; 

 thus lake at Kandy (10.30 a.m.) = 25° C. ; lake at Nuwara Eliya (8 a.m.) 

 = 16° C, the latter being almost temperate, although on hot sunny days the 

 water probably becomes rather warmer than it ever does in our parts. The 

 range of daily temperature is certain to be very considerable here (cf. 

 introduction). 



The third condition to which the algal vegetation of the tanks is subjected is 

 a frequent change of water-level, which probably takes place with a rapidity 

 unknown in our parts. During the dry period some of the smaller tanks 

 decrease to more than half their normal size, and are then surrounded by very 

 poor meadow-land with stunted grassy growth, the external portions of which 

 are often occupied by the dead trees above referred to ; the limit of the tank in 

 the wet season is well marked by the fringe of dense jungle, which begins some 



* Cf. Oltmanns, loc. cit., vol. 2, 1905, pp. 186 and 187. 



t Vf. Warming, ' Okologische Pflanzengeographie,' Germ. Ed., Berlin, 1896, p. 121. 

 " Dass das Absorptionsvermogen des Wassers fur Gase mit steigender Temperatur 

 abnimmt, ist vielleicht der wesentlichste Grand, weshalb gewisse Wasserpflanzen im 

 Sommer . . . verschwinden " ; cf. also loc. cit., p. 124. 



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