1907.] Fresh-water Algal Flora of Ceylon. 251 



observed in the well at Matale, where it occurred as an epiphyte on the 

 Pithophora. 



The only other forms of any importance are Diatoms, which were present 

 in enormous numbers .in two of the wells ; in the third (at Ambalangodda) 

 there were practically none, which is due, perhaps, to the main mass of algal 

 growth found here being above water-level and consequently exposed to a 

 higher temperature than in the other two cases. In the wells at Nalande 

 and Matale the Diatoms occurred mainly as epiphytes on the Pithophora ; in 

 the former case there were epiphytic Gomphonemas and Synedras and loose 

 floating tangles of Fragilaria (with intermingled blue-green forms and 

 Synedra), whilst in the latter, except for a few Synedras, all the Diatoms were 

 epiphytic (Achnanthes, Gomphonema, and Synedra). Nowhere else in the 

 lowland fresh- waters was such a striking abundance of Diatoms observed, and 

 I think this feature is here rather a result of the colder water than of the 

 good conditions, of aeration. Fragilaria is, however, a genus which prospers 

 quite well in warm water (cf. below), and the conditions influencing its 

 presence or absence are very unclear. 



The vegetation of these wells is thus characterised by a Cladophoraceous 

 and Diatomaceous element and shows a very uniform composition. The 

 conditions are, of course, exceptionally favourable for Cladophoraceee, for not 

 only is the aeration adequate, but the lower temperature of the water involves 

 a greater power of dissolving the necessary gases. The wells are just the 

 kind of habitat in which we should expect to find Cladophoracese flourishing 

 in the tropics (c/. p. 230), if anywhere. And their predominance in such 

 waters furnishes, to my thinking, a striking confirmation of the causes 

 suggested above for the scarcity of this order in other tropical fresh- 

 waters. 



A few words may be added on the vegetation of the hot springs at Kannia, 

 near Trincomalie, although they will be dealt with more fully elsewhere. In 

 hot springs we have aerated water at a high temperature, and this is the 

 marked distinction between them and the wells. The springs at Kannia are 

 roughly of two temperatures, viz., 37° C, and 40° to 41° C, and these two 

 different temperatures appear to call forth an entirely distinct algal vegetation 

 in the respective springs. Those at the higher temperature have the stone 

 sides above and below water-level covered with a continuous tangled or filmy 

 growth of blue-green Algse (mainly Lyngbya and Scytonema), whilst the less 

 hot springs (37° C.) have a brown, fluffy covering of Fragilaria on their sides. 

 This difference is very well marked, especially as regards the Fragilaria, 

 which scarcely appears at all in the hotter springs. Since there is no other 

 obvious factor than the difference of temperature to explain this diversity, we 



VOL. LXXIX. — B. U 



