220 Dr. F. E. Fritsch. On the Subaerial and [Jan. 2, 



{Myriophyllum indicum, Trapa hispinosa, Nymphmas, Neptunia oleracea, 

 Aponogeton monostachyon, Limnophytum obtusifolium, Cham, etc.), and 

 beneath these numerous algal forms generally find shelter. In some tanks 

 the muddiness of the water, in others an abundant Plankton (notably 

 Clathrocystis /) serve as a screen to the remaining life in the water. As 

 a general rule (which is, however, by no means without exceptions, cf. table 

 on p. 224), a tank with a poor Plankton is not rich in any kind of algal 

 growth {e.g., Kurunegalawewa, Nalandewewa), whilst one with a fairly 

 abundant Plankton generally contains a good deal of other algal vegetation 

 {e.g., Kelawewa, Lakes Mineri and Kantelai). On the other hand, it seems 

 that, in some cases, the Plankton may become so extremely abundant that 

 it does not allow of the passage of sufficient light for any other noticeable 

 growth; this was the case in the tanks at Dambulla {Clathrocystis!) and 

 Habarane {Clathrocystis and Pediastrum !). The Plankton is, of course, 

 in no way the only source of shading in the tanks, and where there is 

 a well-marked growth of water-weeds, for example, these may shelter 

 abundant Algse, although the Plankton is poor.* The light to which 

 the aquatic Algse of the tropics are subjected, however, always remains 

 very strong, and it is, therefore, not surprising to find the Cyanophycese 

 again predominating (probably owing to their protective pigment, cf. p. 206), 

 although, perhaps, not to such a marked extent as in the subaerial vegetation 

 We have already, in the preceding section, learnt to associate the 

 Cyanophyceous element with a high temperature, and this is, of course, 

 the second condition to which the flora of tropical waters is subjected. 

 Leaving the lakes at Kandy and Nuwara Eliya out of consideration, the 

 approximate mean daily temperature of the water near the edge of the 

 tanks is 28°*5 C. ; the lowest temperature observed (6 a.m.) was 25° C, 

 which is only attained on hot summer days in our parts ; the highest 

 temperatures (from 10 a.m. to 2 p.m.) range from 30° to 35° C.,f the latter 

 in shallow tanks like most of the smaller ones, as well as Kantelai and 

 Sigiri ; in the deeper tanks {e.g., Villamkulam, Dambulla tank) the tem- 

 perature does not seem to rise much above 30° C. In the shallow tanks 

 the range of daily temperature is very considerable (viz., from 25° to 

 34°* 5 C. in Lake Kantelai, for example) and, in the small tanks, such a wide 

 range is probably the rule. Even in the deeper tanks the range is apparently 



* Attention may be drawn to the fact that in the absence of water-weeds, the green 

 macrophytic algal growth of the tanks is almost invariably attached ; floating green 

 patches only occur in those tanks in which the abundant growth of large aquatics 

 furnishes a considerable shelter against the strong light. 



t The temperature of a warm bath is about 36° C. 



