1907.] Fresh-water Algal Flora of Ceylon. 253 



adequately explained. It may be noticed in this connection that the Desmids 

 most involved are capable of slow movement with the help of the excreted 

 mucilage,* and they probably tend to collect in considerable numbers on the 

 surface of the water- weeds, attracted, perhaps, by the oxygen liberated during 

 their assimilation. The prolific division of the Desmids thus established will 

 soon lead to an accumulation of numerous individuals, whilst their mucilage- 

 investments will flow together and form a whole. The frequent inclusion of 

 Protococcales and of muddy particles in these films is not difficult to under- 

 stand on this assumption, nor the occasional presence of Diatoms. Whether 

 these composite films can be regarded as a true consortium must be left to 

 further investigation. 



Turning our attention now to the specific constitution of these films, we 

 have, in the first place, to notice that the blue-green element is practically 

 unrepresented ; isolated filaments of Oscillaria are sometimes present, and 

 in one case the films included a number of colonies of Chroococcus. As a 

 rule, however, Cyanophycese are sought for in vain, and from this point 

 of view alone these upland pools acquire an aspect rather distinct from 

 those of the lowlands. The Desmids are the most important forms (species 

 of Gosmarium, Euastrum, Closterium, Micr aster las, Penium, Xanthidium, 

 Pleurotcenium, etc.), but filamentous species of this group are exceptionally 

 rare (only Spondylosium). This latter point is not difficult to understand 

 in the case of the films, for here the absence of filamentous Desmids is 

 probably due to the immobile character of these forms which, on the above 

 assumption of the origin of the films, would be excluded from participation 

 in their formation. A more puzzling feature, however, is the great scarcity 

 of filamentous Desmids generally in the upland waters examined (cf. also 

 p. 238, footnote). This is a second striking point of difference between the 

 upland and lowland pools, for in the latter, where the conditions are 

 favourable, filamentous Desmids are quite a prominent feature. It may 

 be that the diversity is due to the different amounts of dissolved 

 oxygen in the water in the two cases. We have only the analogy of 

 the colonial Protococcales to go by, but in some of these (Scenedesmus, 

 Ccelastrum) Sennf found that colonies were formed when there was a 

 scarcity of oxygen, whereas an abundance of the latter led to the formation 

 of isolated cells. Possibly, then, the conditions prevalent in the low- 

 land pools are such as to favour the existence of filamentous Desmids, 

 while in the uplands the reverse is the case. The lowland pools certainly 

 afford striking examples of the filamentous tendency amongst the Desmids 



* B. Schroder, loc. cit., pp. 157—159, Plate VII, figs. 3, 4, and 10a. 

 t " Ueber einige koloniebildende einzellige Algen," ' Bot. Zeitung,' vol. 57, 1899, p. 97. 



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