1907.] Fresh-water Algal Flora of Ceylon. 233 



rare. The numerous records of species of (Edogonium from the tropics show 

 a very considerable preponderance of narrow forms*.* As already pointed 

 out above, the filamentous algal flora of the tropics is prevalently narrow, 

 Spirogyra and Pithophora being the only broad forms at all commonly met 

 with. This is no doubt due to the smaller percentage of dissolved oxygen 

 in the water, forms with narrow filaments being better suited to these con- 

 ditions than broader ones. The phenomenon finds its parallel in the extensive 

 subdivision of the foliage in many of the higher aquatics. Spirogyra and 

 Pithophora are probably able to exist as broader forms, owing to their thin 

 cell-walls admitting of more rapid diffusion of gases. A careful study of the 

 tropical habitats of broad species of (Edogonium will probably disclose some 

 special factor favouring their development. Possibly the rather limited 

 occurrence of Pithophora may also be due to the necessity of some special 

 condition. The Spirogyras, however, are too widely distributed to admit of 

 such an assumption. 



The ChgetophoraceaB do not constitute a very important element, but species 

 of Stigeoclonium were observed in some amount in a few cases {e.g., 

 Balaluwewa, Habaranewewa), whilst Aphanochmte is occasionally found as an 

 epiphyte on other filamentous Algae. Chmtophora was not seen in the tanks. 

 Coleochcete is an occasional epiphyte. 



The lake at Nuwara Eliya, the only large piece of upland water studied, 

 could, unfortunately, only be subjected to a very cursory examination. There 

 is no doubt, however, that the flora is in no way dominated by the blue- 

 green element. At the time of my visit there was very little algal growth. 



(ii) Algce of Roadside Ditches and Pools in the Lowlands (excluding rock- 

 pools, see Section iv). — Although the tanks are the most important pieces of 

 inland fresh-water in Ceylon, a considerable bulk of algal growth is also 

 found in the numerous small pools and roadside drains occurring all over the 

 island. The general character of this vegetation is in most respects similar 

 to that of the tanks, but there are certain noteworthy differences which 

 require a brief discussion. The above-mentioned pools (p. 226, footnote) which 

 are found round about the tanks in the dry season, but constitute part of them 

 when the water rises, occupy an intermediate position between isolated pools 

 and the actual tanks as far as their algal flora is concerned. They generally, 

 however, approach more nearly to the latter, since their flora is (to a great 

 extent at least) dependent on that of the tanks, and is consequently dominated 

 by rather different conditions than apply to the pools, etc., which form the 

 chief subject of discussion in the present section. Most of the forms found 

 in the pools around the tanks are also met with in the main waters of the 

 * Cf. Fritsch, 'Annals of Botany,' April, 1907, p. 265 et seq. 



