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



Although such ferruginous pools are very characteristic of Ceylon, they 

 are not essentially tropical, and are found quite frequently in temperate 

 regions. I have observed a similar reddish-brown deposit accumulating 

 since several years in one of the small ditches on Sheen Common, Pdchmond 

 (Surrey), and very much the same flora occurs associated with it as in the 

 Ceylon pools. Spirogyra, CEdogonium, Zygnema, Ulothrix, Conferva, Pleuro- 

 tcenium, Closterium, Netrium, Pinnularia, Navicida, Surirella, and Synedra 

 lunaris are the more important forms I have observed in the course of 

 casual visits during the last three years. The species are apparently in 

 many cases not identical with those found in the tropical pools, but that 

 alters little in the astonishing similarity. During the period of observation, 

 the granular deposit has steadily increased in quantity, while the Algae have 

 slightly decreased in numbers. Latterly, blue-green forms are putting in an 

 appearance, which tallies with what I observed in Ceylon. The vegetation 

 of these ferruginous pools thus constitutes one of the most striking cases of 

 parallel between temperate and tropical algal formations that I have noticed, 

 and probably stands isolated in this respect. 



There is one more phenomenon noticed mainly in the pieces of water under 

 discussion (also in the padi-fields), that may be briefly described. As above 

 mentioned, pools without a noticeable Conjugate element are generally 

 inhabited by Cyanophyceous growth, — often Oscillaria. A visit to such 

 a pool in the early morning shows the muddy bottom covered with a more 

 or less continuous film of the Alga, as is characteristic of the species of this 

 genus. The film consists of a thin layer of algal filaments with a certain 

 amount of mud between them. Soon after sunrise gas-bubbles make their 

 appearance studded all over the surface of the films, whilst at many points 

 the latter show signs of inflation indicating a similar development of gas- 

 bubbles on the lower side. Soon after midday floating patches of Oscillaria, 

 still with numerous adhering gas-bubbles, are found on the surface of the 

 water, and a little patience enables one to follow the detachment and rising 

 of such a patch to the surface. This state of affairs continues for the rest of 

 the day, but early on the next morning there are again no floating patches of 

 Oscillaria, which merely forms a film on the bottom. In the course of the 

 day the same process repeats itself. This phenomenon is observed in waters 

 which are shallow (and almost unshaded) and in which the midday tempera- 

 ture is very considerable (viz., 35° C). The rise of patches of the films to the 

 surface is, of course, due to the expansion of the gas-bubbles formed during the 

 assimilation and respiration of the Alga, whilst the cooling of the water after 

 nightfall involves a contraction and perhaps partial solution of these bubbles 

 (and probably their consequent detachment), and this results in the sinking 



