260 Frit sch . — 77 ^ Sub aerial and Freshwater 
generally (p. 239). Draparnaldia has as yet only been recorded from 
the Sandwich Islands and Ecuador ; it was not observed in Ceylon. 
Although a considerable number of different species and genera of 
Ulotrichales are thus known to occur in the tropics, the number of actual 
records is not very great, and I am inclined to think that this group does 
after all take rather a back place in these regions. Possibly this may again 
be a result of the difficulties of respiration in the tropics, although there 
is nothing to show that these forms favour specially well-aerated tropical 
waters. Some species of XJlothrix , even in our parts of the world, are 
known to flourish best in flowing water, and it seems very possible that 
forms, which can get on satisfactorily in stagnant temperate waters, may 
be unsuccessful in similar tropical habitats. After all, we know as good as 
nothing at present as to the influence of the amount of dissolved oxygen 
in the water on the various freshwater algal genera. 
(v) The Zygnemaceae in the Tropics. 
Probably no feature of the freshwater algal flora of the tropics has 
played such a part in producing the universal impression of similarity 
with that of temperate regions as the excessive abundance of Spirogyra . 
This genus is indeed even more abundant than it is with us, and yet careful 
examination of the available data shows that the tropical Spirogyras have 
certain characteristic features, which are sufficiently striking. Spirogyra is of 
course an essentially stagnant-water form, and this, I think, is one reason 
for its immense success in the tropics. As pointed out above (p. 246) 
Spirogyra and Pithophora (together with a certain number of species of 
Oedogonium) are the only forms with really broad filaments found in the 
ordinary tropical freshwaters, and I am inclined to associate this with the 
evident small demand for aerated water made by Spirogyra. Moreover, 
the narrower species of this genus are even rather scarce in the tropics, and 
the broad ones are dominant (see the measurements given in the enumera- 
tion below). In relation to this point we have also to notice that species 
of Spirogyra with a single spiral in their cells are scarce (twenty records), 
and that the majority of forms have two, three, or more such chloroplasts 
(sixty-two records). Lastly, only five (viz. .S. injiata , 6*. quadrata> S . 
tenuis sim a, S. Grevilleana , and 6*. insignis ) of the forms hitherto recorded 
(six records) have infolded transverse walls, the latter almost invariably 
being simple. 1 The same observations were made on the abundant material 
of Spirogyra collected in Ceylon ; broad forms with many spirals were 
preponderant, and no case of infolded end- walls has been met with up to the 
present. 
1 Amongst the Spirogyras enumerated by De Toni (’ 89 , p. 743 et seq.) there are thirty species 
with a single chloroplast and forty-three with two or more in their cells ; amongst the species known 
from the tropics there are nine with a single spiral, and twenty-nine with two or more spirals. The 
difference in number is obvious. De Toni has twenty- two species with infolded walls as compared 
with fifty-one with simple end-walls. In the tropical records the relation is 5 : 33. 
