1907.] Fresh-water Algal Flora of Ceylon. 239 



filamentous Algae in these pools, large amounts of the granular deposit form a 

 dense, reddish-brown flocculent investment all round the filaments (see, for 

 example, fig. 3, J), so that the latter are often completely obscured ; a 

 similar coating is generally also found on any water-weeds present. It 

 is hardly possible to account for this investment in any other way than by 

 the assumption that it has originated round about the algal growth and 

 water-weeds, as a result of their metabolic activities. The water of these 

 pools contains a small amount of dissolved ferric chloride,* and this is 

 probably transformed (oxydised) into an insoluble form by the vital 

 activities of the plants present. As the ferric hydroxide accumulates, it 

 sinks down to the bottom of the pool, and forms a granular layer there.f 

 The gradual increase in the amount of the deposit may produce less 

 favourable conditions (shading ?) for certain algal forms, which thereupon 

 disappear. In the diverse pools studied, many different cases suggestive of 

 the above theory were met with. Thus, in some of the pools, the granular 

 matter merely formed a flocculent covering to the contained vegetation, and 

 there was little or none on the bottom ; in others, there was a quantity of 

 the reddish-brown deposit on the bottom, as well as enveloping the plant- 

 growth present; whilst, in still others, there was only a dense granular 

 layer on the bottom, and practically no vegetation rising up into the water 

 above it. In the pools last mentioned the vegetation was generally scanty 

 and least satisfactory, from the point of view of uniformity with the others. 

 Such pools generally also contained a quantity of Leptothrix, and it seems as 

 though this genus finds favourable conditions as the deposit accumulates ; 

 possibly it is the direct cause of the disappearance of the main mass of algal 

 growth. In any case, the ultimate result of the accumulation of the red 

 deposit is the elimination of a considerable mass of the vegetation. What 

 this process leads to finally I am unable to say, but there are some indica- 

 tions of the initial state of affairs to be found in pools and ditches having 

 no trace of the characteristic deposit, but containing a flora very similar to 

 that above described. 



* I have to thank Professor E. C. C. Baly, of University College, London, for kindly 

 analysing this water. 



t It is not impossible that iron-bacteria may have a hand in this process. They are 

 well known to occur in such waters, and by their activity to give rise to extensive deposits 

 of hydroxide of iron (of. S. Winogradsky, " Ub. Eisenbacterien," ' Bot. Zeit.,' vol. 46, 1888, 

 pp. 261 — 270). They would probably tend to collect about the surface of the members of 

 the green vegetation, owing to the evolution of oxygen going on there ; so that the red 

 deposit would primarily appear round about the water-weeds and Algse present. I do 

 not, however, think that these deposits can be solely due to their agency, for in many 

 cases there were very few bacteria evident ; they seem to occur more commonly in later 

 stages. 



