228 Dr. F. E. Fritsch. On the Suba'erial and [Jan. 2, 



are ultimately raised to the surface by gas -bubble development (cf below, 

 p. 240). Similar discs are sometimes to be observed in temperate regions, but 

 their development has, as far as I am aware, not been studied. 



Scytonema (S. tolypotrichoides, Kutz) was a dominant form in only two of 

 the tanks, viz., Madokotaikulam near Vavoniya and Karambewawewa, near 

 Anuradhapoora. In both cases it formed spherical aBgagropilous masses floating 

 round the margin and consisting of large quantities of knotted, more or less 

 radially arranged filaments, frequently of a yellowish colour on the surface, 

 although deep blue-green in the interior of the spherical structure. The 

 tangle of filaments, no doubt, includes air, by virtue of which the masses, 

 which are often about double the size of a cherry, float. The mode of 

 development of these masses has not been studied, but is probably analogous 

 to that of the aagagropilous Cladophoras* depending on successive movement 

 of the growing structure into various positions by the currents in the water. 



There is much greater specific diversity to be found in the attached 

 Cyanophyceous growth than among the floating forms, which, as the above 

 remarks will have shown, are referable to a small number of species ; the most 

 important attached forms belong to the genera Oscillaria, Zyngbya, and 

 Tolypothrix. In comparison to our parts, the practical absence of unicellular 

 species in the attached blue-green flora is noteworthy. The dense pilose tufted 

 growth found on submerged rocks (especially Lake Kantelai), tree-stumps, 

 etc., is usually not due to a single species, but is composite (both blue-green 

 and green with intermingled Diatoms, viz., Synedra, Navicula, Surirella). 

 There is probably no essential difference between the attached growth in the 

 majority of the tanks, and that found in many of the small pools (cf below) ; 

 in both cases we are dealing with algal consortia.! 



We will now turn our attention to the composition of the green algal 

 element in the flora of the tanks. It exhibits a number of characteristic 

 features (common in part to the algal flora of all the inland waters), which are, 

 on the whole, just as striking as the dominance of the Cyanophyceae, viz. : — 



(a) The practical absence or great scarcity of certain genera, such as 

 Cladophora, Bhizoclonium, Vaucheria, Ulothrix, and Conferva, and the conse- 

 quent scarcity of a number of epiphytic Diatoms so characteristic of the fresh- 

 water algal flora of our parts (e.g., Cocconeis Placentula, Synedra splendens). 



(b) The replacement of Cladophora by the genus Pithophora. 



* Cf. especially Brand, " Die Cladophora-JEgagroipileii des Siisswassers," ' Hedwigia,' 

 vol. 41, 1902, p. 34 et seq. ; and Wesenberg-Lund, " Sur les JZgagropila Sauteri du lac 

 de Sorb," 'Bull. Ac. B. d. Sci. et d. Lettres Danemark,' 1903, No. 2, p. 167 et seq. 



t A few further facts regarding the attached growth on rock-surfaces in the tanks are 

 mentioned in the section dealing with the algal flora of the rock-pools (p. 248). 



