2 JO 
Fritsch . — The Subaerial and Freshwater 
whether they attain an adequate development in tropical freshwaters, but 
there are slight indications to the contrary. 
(xi) The genus Spirogyra is even more abundant in the tropics than 
it is with us. The species so far recorded are mainly broad forms with two 
or many chloroplasts in their cells. Forms with infolded end-walls are 
strikingly rare. The other genera of Zygnemaceae are not very abundantly 
represented. 
(xii) There is a marked filamentous tendency amongst the Desmids of 
tropical waters (especially in certain kinds of stagnant water), and it is 
suggested that this may again be due to the poor aeration of the water. 
(xiii) The genus Oedogoniuni is very abundant in the tropics, but is 
mainly represented by species with narrow filaments. 
(xiv) Freshwater Florideae appear to be not at all uncommon in the 
tropics. 
In conclusion, I should like to mention that I am fully aware of the 
speculative basis of a great part of the matter contained in this paper ; pre- 
liminary experiments are, however, already in progress with a view to testing 
the accuracy of many of the suggestions. The investigation is in too early 
a stage at present to admit of any communication on the subject. I should 
also state that I am indebted to the Government Grant Committee of the 
Royal Society for the grant which made it possible to investigate the fresh- 
water algal flora of Ceylon— -an investigation which has been the chief 
stimulus for the present paper. 
University College, London. 
February 18, 1907. 
Bibliography. 1 
1. Askenasy (’89) : E. Askenasy, Algen, mit Unterstiitzung der Herrn E. Bornet, A. Grunow, 
P. Hariot, M. Mobius, O. Nordstedt, in Die Forschungsreise S.M.S. ‘Gazelle’ in den 
Jahren 1874 bis 1876. IV. Teil : Botanik. Berlin, 1889, pp. 1-58 (only partly tropical). 
2. Boergesen (’90) : F. Boergesen, Desmidieae, in * Symbolae ad floram Brasiliae centralis 
cognoscendara Vid. Meddelels. fra d. Nat. Foren. i. Kj^benhavn, 1890, pp. 24-53. 
1 I have already pointed out that this bibliography, though great pains have been taken to 
include all the important papers dealing with the tropical freshwater and subaerial algal flora, is not 
exhaustive. Very few of the older papers are included, and of recent ones many minor contributions, 
not bearing on the main points of the present paper, have been omitted. I have also not taken into 
account the records in De Toni’s ‘ Sylloge/ since a considerable number would have been duplicates 
of those already given. Nor have any exsiccata been considered for the same reasons. Messrs. 
Wittrock, Nordstedt and Lagerheim’s ‘Algae aquae dulcis exsiccatae’ do indeed contain a certain 
number of records of tropical forms, which, as far as I am aware, are not published elsewhere, but 
reference to the recent general index (published, Lund, 1903) will show them at a glance. 
Certain papers marked with a star were not considered in all points sufficiently distinct or 
precise to be taken into account in all of the lists compiled in the course of my paper ; omission of 
data from them in certain cases must not therefore be attributed to negligence on my part. 
