1907.] Fresh-ivater Algal Flora of Ceylon. 203 



algal vegetation unless it has been studied, at least, throughout the course of 

 an entire year. So much seems probable, however, that a large number of the 

 algal formations of the tropics are quite different to those of our parts, 

 although certain of them may possibly be parallel in their nature (cf. especially 

 p. 240). On the whole, Phanerogams seem likely to be of more importance as 

 character-plants in the aquatic formations of our parts than in the tropics, 

 where the Algae will, probably in most cases, prove more distinctive ; this 

 suggestion is, however, merely based on recollection of the aspect of the fresh- 

 water flora of the tropics, and careful investigation may quite disprove it. 



(a) Subaerial Algce. 



A consideration of the subaerial algal flora affords a good illustration 

 of the existence of important differences between tropical and temperate algal 

 vegetation. 



(i) The Lowland Vegetation. — The first impression of an algologist after 

 landing in Colombo, which combines tropical heat with a fairly considerable 

 rainfall (88 inches), is the abundant algal covering on the walls of the houses, 

 on rocks, on exposed tree-trunks, etc. A rich growth of Algae on surrounding 

 objects is characteristic of every really moist locality, but it is only when heat 

 and moisture combine that we get so luxuriant a growth as in the tropics ; 

 moreover, a few minutes' walk about Colombo is sufficient to convince one that 

 the composititon of this algal growth is one quite unlike that of temperate 

 regions. One sees practically nothing of the Pleurococcus, Hormidium, etc., so 

 characteristic as epiphytes in our climates, green Algae being very inconspicuous 

 in all exposed situations (excepting Trentepohlia, cf. below). Isolated colonies 

 of green unicellular forms are occasionally to be found in very subordinate 

 amount amongst unicellular Cyanophyceous (adhesive) growth, but pure green 

 coatings are completely wanting. There are very few records in the literature 

 of the occurrence of green subaerial Algae in the tropics* (chiefly Pleurococcus 

 crenulatus, Hansg., P. Kutzingii, West, and P. vulgaris, Menegh., Hormidium 

 murale, Kiitz),f and none of these records give us any idea of the quantity in 

 which these species occur ; nor is it possible in the absence of accurate 

 descriptions of the habitat, to estimate the special conditions under which these 

 forms appear in the tropics. 



The subaerial \ Algae of the tropics nearly all belong to the Cyanophyceae 



* Eegarding terrestrial forms of Vaucheria, see p. 217. 



t The existing data as to the composition of algal growth in the tropics are discussed 

 more fully elsewhere (' Annals of Botany,' vol. 21, No. LXXXII, April, 1907, p. 239 et seq.). 



% The term "subaerial" algal flora is used to denote all those forms not actually 

 growing in water. Epiphytic Algae constitute those growing on parts of other plants 

 (trunks of trees, leaves, etc.), lithophytes are those growing on rocky surfaces. 



VOL. LXXIX. — B. R 



