1907.] Fresh-water Algal Flora of Ceylon. 235 



Thus the Spirogyras may be better suited to meet the emergencies of desiccation 

 than most of the aquatic Cyanophycese (which generally have a rather poorly 

 developed sheath as compared with the subaerial forms) owing to the ease 

 with which they form resting spores. Azygospore-f ormation is not uncommon 

 in my material, and although we are quite ignorant as to the rapidity with 

 which such azygospores are formed, it seems reasonable to conclude that it is 

 a much more rapid process than the sexual development of zygospores. In 

 case of approaching desiccation, which in the tropics will often take place 

 very rapidly, there will be a sudden concentration of the water, and this may 

 well act as a check on ordinary conjugation and lead to the formation of 

 azygospores.* The prevalence of Spirogyra may, however, also be due to the 

 frequent strong shadingf of these pieces of water, or to the success of 

 Spirogyra in perfectly stagnant waters such as these, both conditions which 

 though, perhaps, not unfavourable to the blue-green element, may still enable 

 the Conjugates to flourish and crowd out the Cyanophycese. In comparison 

 with pools and ditches in temperate regions, however, the blue-green Algae 

 still play a relatively important part, whilst Conjugates are much more 

 strikingly abundant. Without there being much difference in generic 

 constitution the facies is quite another one. 



In a number of the pools and ditches examined, Spirogyra was practically 

 the only form present, but in other cases it is accompanied by a subsidiary 

 vegetation composed of CEdogonium, Microspora, Zygnema, Mougeotia, BulbocliwU, 

 Ulothrix, Stigeoclonium, and various Cyanophycese^ {Anabama, Zyngbya, 

 Oscillaria, Scytonema, etc.) ; in a few cases one or other of these forms (nearly 

 always (Edogonium) was dominant, while the Spirogyra takes a second place. 



* Cf. Klebs, ' Die Bedingungen der Fortpflanzung bei einigen Algen und Pilzen,' Jena, 

 1896, p. 246 et seq. In Spirogyra Weberi a 4-per-cent. solution of cane-sugar induced the 

 formation of a large number of azygospores, whereas in S. inflata a 6-per-cent. solution 

 was necessary to interfere with ordinary conjugation and produce parthenogenesis ; in 

 this latter species a 4-per-cent. solution induces ordinary conjugation, so that a sudden 

 increase in concentration from 4 to 6 per cent, leads to parthenogenesis. 



t Messrs. West and West (" Observations on the Conjugate," ' Annals of Botany, 

 vol. 12, No. XLV, 1898, pp. 35, 36) point out that the Conjugates can stand prolonged 

 exposure to fairly strong light in their natural habitat, which is not quite in agreement 

 with the statement that the effect of such exposure is to induce the formation of 

 zygospores {cf. also Ewart, " The Action of Cold and of Sunlight upon Aquatic Plants,''' 

 ' Annals of Botany,' vol. 12, 1898, p. 379 et seq.). It would seem as though strong 

 light stimulated the reproductive functions which probably means cessation of the 

 vegetative activities. On the other hand, Conjugates appear to be rather less sensitive to 

 illumination than many of the other filamentous green genera, and it is possible that the 

 tropical members of this order are adapted to a fairly strong light. 



\ Except for occasional floating forms, the Cyanophycese occur most commonly attached 

 to water- weeds and branches and twigs lying loosely in the water. 



VOL. LXXIX. — B. T 



