230 Dr. R E. Fritsch. On the Subaerial and [Jan. 2, 



in this respect than we imagine, so that tropical conditions may even 

 exclude forms which favour stagnant water in our parts. 



We will now briefly notice the occurrence of representatives of the 

 groups and genera just discussed in Ceylon waters. I have practically only^ 

 met with fresh-water species of Gladophora in two wells (at Matale and 

 Ambalangodda, see p. 250), i.e., in well aerated water. Rhizoclonium, a 

 genus which is exceedingly common in our temperate waters, was scarcely 

 met with except in the estuaries and lagoons. On the other hand, the 

 most important representative of the Cladophoraceae in Ceylon is quite 

 undoubtedly the genus Pithophora, which, although neither a very common 

 form, was met with sufficiently frequently to make it a sensible constituent 

 of the algal flora. It was never found in the actual tanks except in those 

 of small size, but generally occurred in the side-pools above mentioned 

 (p. 226, footnote). The species of Pithophora have, on the whole, thinner 

 cell walls* than either Cladophora or Rhizoclonium, so that they may be in 

 a better position to withstand the increased difficulty of respiration in the 

 tropics. Pithophora may, however, in its spores also have a far better 

 adaptation to the vicissitudes of the tropics (desiccation) than either of the 

 two other genera ;f this is a point which should not be very difficult to 

 settle experimentally. If we examine into the existing records of the 

 occurrence of species of Cladophora and Rhizoclonium in the tropics, we find 

 that in most cases (where the habitat is at all adequately described !) these 

 forms were growing in flowing or otherwise well aerated water.J The- 

 remaining cases do not prove anything to the contrary until it has been 

 definitely ascertained that the water in which these forms were growing was 

 actually stagnant. I think it may be safely stated that both Cladophora 

 and Rhizoclonium are rare forms in tropical fresh- waters, and that they 

 normally only appear in a well aerated habitat. They are replaced by the- 

 genus Pithophora, which apparently flourishes quite well in stagnant waters. 



I have met with no species of the genus Vaucheria in the lowlands of 

 Ceylon (terrestrial and aquatic species in the uplands ! cf. pp. 217, 254), 



* See especially Wittrock, " On the Development and Systematic Arrangement of the 

 Pithophoracese," 'Nov. Act. Eeg. Soc. Sc. Upsala,' Ser. 3, 1877, p. 8: "The vegetative 

 cells .... have all a thin membrane of cellulose without layers. In Cladophoreae, 

 particularly the cells belonging to the lower part of the thallus have often a thick 

 membrane in distinct layers." See also the plates illustrating the paper cited. 



t It is interesting in this connection to mention that Miss F. Kich, working jointly 

 with me on the biology and ecology of British fresh -water algal growth, has come to the 

 conclusion that Cladophora deserts those ponds which are subject to drying up in 

 summer. 



| The records of tropical Algse found in the existing literature will be considered more 

 fully elsewhere ('Annals of Botany,' vol. 21, No. LXXXII, April, 1907, p. 248 et seq.). 



