1907.] Fresh-water Algal Flora of Ceylon. 243 



-with huge black masses of basalt, which, besides forming the substratum 

 of the local tanks, generally contain numerous small but deep pools, which 

 often harbour an abundant growth of Algae of a characteristic type, although 

 the neighbouring tanks are generally very poor in littoral algal vegetation 

 {often with a very rich Plankton, e.g., Dambullawewa, Habaranewewa).* 

 The temperature of the water in these pools is often high (31°'5 C. at 

 JDambulla at 4 p.m. ; 28°'5 C. at Hanwella at 8 a.m. ; particularly high in 

 the shallow pools at Kurunegala), but the daily range is certainly not very 

 ^considerable, especially where the pools are deep, as at Dambulla and 

 JNalande.f The pools are exposed to the full light, though the black rock- 

 surface is probably a good absorber. The deeper pools, in particular, give 

 one the impression of receiving relatively little light beyond the surface 

 layers. The decided prevalence of Cyanophyceae (especially in the shallower 

 pools), however, speaks for the shading being less perfect than in the case 

 of the pieces of water considered in Section (ii). The risk of desiccation 

 is small in the deep pools, and it seemed as though many of the shallower 

 ones owed their origin to springs. This would also mean good aeration, as 

 is indicated by the whole aspect of the flora in many of the rock -pools ; 

 .species characteristic of stagnant water are in most cases absent. The 

 .special conditions in these pools are thus: rocky substratum, high 

 temperature of no considerable daily range (especially pronounced in the 

 deeper pools), full exposure to light (except in case of deep pools), and 

 probably fairly good aeration. 



The algal vegetation in the rock-pools either takes the form of a close 

 pilose or fluffy covering on the rocky sides, or of a floating mass. More 

 than half the number (18) of typical pools examined contained a blue-green 

 flora with very few other forms ; this is what we should expect under the 

 dominant conditions. The chief representatives of the Cyanophyceae are 

 •attached species of Bivularia and Hypheothrix (see below) and floating forms 



* Similar masses of basalt are found round about other tanks {e.g., Punchikekirawa, 

 and between Kelawewa and Balaluwewa), and are no doubt covered by their waters in 

 the wet season, so that the pools in them then become part of the tank. During the 

 dry period, however, the pools are isolated and show a certain amount of individuality, 

 though strongly dominated by the flora of the tank. They are considered in the present 

 •section, however, owing to certain similarities to the true rock-pools, as also are some 

 pools observed in rock-quarries at Hanwella. The characteristic rock-pools (i.e., those 

 which are not in connection with a tank) are distinguished in the above description as the 

 " typical " ones. 



t It should be pointed out that the water of these pools is not only heated directly by 

 the rays of the sun, but indirectly through the medium of the surrounding rock. The 

 latter will retain the heat much more tenaciously, and this will serve to decrease the 

 daily range of temperature in the pools in the rock, especially in the deeper ones. 



