1907.] Fresh-water Algal Flora of Ceylon. 207 



Cyanophyceae in exposed situations, is the genus Trentepohlia ; the latter, as 

 is well known, is characterised by the presence of haematochrome in its cells, 

 and this probably also has a protective function against too intense illumina- 

 tion. Such an interpretation is suggested by the fact that the colour of the 

 filaments varies according to the intensity of the light, being more or less 

 green where shaded, and yellow to yellowish-red in the full light.* Species 

 of this genus are particularly common on the trunks of the cocoanut trees in 

 the plantations along the sea shore and on embankments by the roadside.f 



The subaerial Cyanophyceae in Ceylon exhibit a number of different 

 modes of growth, which are, no doubt, of the nature of adaptations to special 

 external conditions. My stay in the island was not sufficiently prolonged to 

 make it possible to analyse these conditions satisfactorily, but I am able to 

 put forward a number of probable suggestions regarding the factors favouring 

 the one or other mode of growth. The simplest and one of the commonest 

 habits assumed by the subaerial blue-green element may be styled the 

 " adhesive " mode of growth, the Alga being firmly adpressed to the 

 substratum ; this occurs in all the gelatinous^ and encrusting unicellular forms 

 (Aphanocapsa, Aphanothece, Gloeocapsa, Chroococcus, etc., fig. 1, B and D), and also 

 in certain of the filamentous genera {Nostoc, a few species of Calothrix, Oscil- 

 laria, Hypheothrix, Lyngbya, especially the section Phormidium). On the whole 

 this type of growth rarely attains considerable thickness, a result probably of 

 the difficulties of interchange of gases in the inner layers of the stratum. 

 The only well-marked exception is the genus Phormidium, but here the inner 

 portions of the often thick lamellae merely consist of empty sheaths, while 

 only the outer parts contain living filaments. The forms exhibiting the 

 adhesive mode of growth are the first successful colonists of new ground, and 

 are especially adapted to grow on smooth surfaces, where they often form a 

 very thin, slippery film. 



A second type of growth may be described as the " tangled " one ; this (like 

 the remaining two) is confined to the filamentous forms, and is due to the fact 

 that the filaments are bound together in bundles, which are irregularly 

 interwoven with one another, much as in many of the submerged filamentous 

 Algae. The denseness of the tangles varies very considerably. In the majority 

 of cases, this growth was found to be relatively thin (although generally 



* See also the statements on p. 248 of Oltmanns' 'Morph. u. Biol. d. Algen,' vol. ], 

 Jena, 1904. 



t Species of Trentepohlia may become epiphyllous, like some of the Cyanophyceae. I 

 did not observe any such cases in Ceylon, although that is probably due to the small 

 amount of time at my disposal. Other Chroolepideee are permanent leaf-epiphytes. 



\ It should be pointed out, however, that these gelatinous forms are rather rare in the 

 lowlands, and far commoner in the moist uplands (cf. below, pp. 215, 216). 



