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



The pilose growth (fig. 5), so often found coating rocky surfaces in the 

 tropical fresh-waters, is almost entirely blue-green, and presents features 

 analogous to those noticed above in connection with the subaerial flora. The 

 pilse consist of a single or of several different species, and in the latter case 

 we are again dealing with a consortium in which the interrelation could be 

 determined to some extent. In one of the rock -pools at Dambulla the whole 

 submerged surface of the rock was thickly covered with such pilse. When 

 some of these were carefully scraped off, a dark dot was seen with the naked 

 eye at the base of each of them. Microscopic examination showed that this 

 dot was due to little radiating colonies of Rivularia (fig. 5, B and D), whilst 

 the remainder of the pila was constituted by a species of Hypheothrix, the 

 bundles of filaments of the latter arising from in between the threads of the 

 basal Rivularia. The branched bundles of the Hypheothrix are held together 

 to form the tuft by a very narrow species of Oscillaria ramifying in all 

 directions between the bundles of Hypheothrix-fil&ments. In the outer 

 portion of each pila there was a considerable amount of mud and decomposing 

 algal substance, which afforded shelter to a number of green unicellular 

 forms (e.g., Scenedesmus, Tetraedron). The consortium here thus consists of a 

 Rivularia, a Hypheothrix, and an Oscillaria with a few unicellular forms. It 

 seems as though the Rivularia prepares the ground for the Hypheothrix which 

 follows. A very similar pilose covering was found on the rocks in a small 

 pond near the tank at Habarane, but the pilae in this case were considerably 

 longer. The basal Rivularia showed up very distinctly and, with a lens, its 

 filaments could be seen radiating out in all directions (fig. 5, A). The species 

 of Hypheothrix differed from the one observed at Dambulla, the filaments 

 forming thick unbranched bundles, each constituting a single pila ; as a 

 result, there was no binding Oscillaria in this case. The slight differences, 

 however, do not detract from the interest of the very considerable analogy. 

 Rivularia plays a very important part in all the rock-pools, and in some of 

 them was found growing on every conceivable object (including loose 

 sediment). 



As above mentioned (p. 228) the rocks round the edge of Lake Kantelai were 

 covered by a thick pilose blue-green growth. This, again, consists of a species 

 of Hypheothrix, the bundles of which are bound together by a delicate 

 Oscillaria, but there is no basal Rivularia in this case. At the base of many 

 of the pilse a dark dot is indeed to be seen (fig. 5, C), but microscopic examina- 



p. 157. Every piece of water contains forms, which are in part at least dependent one on 

 the other, be it from the point of view of substratum, of shading, of transference to a 

 suitable position in the water, or of protection from animal attacks : and all such associa- 

 tions of species occur together as a result of necessity. 



