253 
Algal Flora of the Tropics . 
was absolutely stagnant 1 . In addition to its thinner walls and often 
narrower filaments, however, Pithophora differs in another respect from 
the other members of the Order. It is capable of forming specialized 
resting-spores (akinetes), which may be a feature of great advantage to 
it in view of the frequency of rather sudden desiccation in tropical waters, 
especially in the smaller ones, in which Pithophora was almost alone 
observed in Ceylon. It is just possible that this may be a second factor 
contributing towards the scarcity of Cladophora and Rhizoclonium , and that 
their means of existence during times of exposure and desiccation may not 
be suited to the exigencies of the tropics. This factor is, however, certainly 
in no way as important as the aeration of the water. 
Pithophora is certainly a prevalently tropical genus — in fact, all the 
European records are subject to the suspicion of the forms found there 
being introduced 2 . 
With the great scarcity of Cladophora and Rhizoclonium in tropical 
freshwaters goes hand in hand the lack of one of the most important 
substrata for aquatic algal epiphytes. Every one is probably familiar 
with the dense covering of characteristic Diatoms ( Cocconeis , Gomphonema , 
Synedra splendens , &c.) found in temperate regions on the filaments of the 
two genera in question, often completely enshrouding them in an hetero- 
geneous brown mass. Scarcely any other filamentous Alga (saving perhaps 
Vattc heria , which is also very scarce in tropical waters, cf. below) approaches 
the Cladophoraceae in this respect, and in tropical waters, where these 
genera are not represented, epiphytic algal growth is rarely at all well 
developed. There are, however, other reasons to account for this. A large 
number of the Diatoms generally seem to prefer cold to warm water, 
and with us mostly attain their maximum in the cold winter months, and 
steadily decrease in amount as the water becomes warmer in spring. 
There are, however, also a number of Diatoms which exist in great 
numbers in much warmer waters, as is shown by the numerous records 
of their occurrence in hot springs. These latter, besides including cosmo- 
politan species, consist mainly of a few probably specialized forms 
(filamentous Diatoms seem to be predominant). In waters of intermediate 
temperature, such as are afforded by those of the tropics, the conditions 
seem unfavourable both for the colder forms and for the hot-spring forms, 
and the Diatom-flora is generally poorly developed. In Ceylon I only 
met with Diatoms in striking abundance in river-waters, as epiphytes on 
Cladophoraceae in wells, and in the hot springs (Kannia near Trin com alee). 
1 That Pithophora , however, also has a taste for aerated water is shown by the fact that it was 
found commonly in the water of wells examined in Ceylon. 
2 There are no data as yet forthcoming as to the distribution of Schmidle’s new genus Chaetonella 
with C. Goetzei (Schmidle, ’02 b, p. 253). Further observation may show it not sufficiently 
distinct to warrant generic separation. The habitat is in accord with the general remarks made 
above (‘ Pfiitze in der Hahe der Brandung *). 
