Problems in Aquatic Biology. 153 
not always the same in the same woodland-formation, so that we 
cannot characterise the latter by a definite type of submerged 
vegetation in the pools which it includes. There are not sufficient 
data as yet fully to settle this point, but for the time being at least 
we must separate the submerged from the surrounding terrestrial 
vegetation as a distinct formation. The conditions involving the 
existence of aquatics as a special biological group are of a far more 
extreme nature than those pertaining to the other biological groups 
referred to above, consisting as they do in a total change from a 
gaseous to a liquid environment; and from this point of view, even 
if it should be found that an aquatic flora is to some extent 
dependent on the nature of the surrounding terrestrial vegetation, 
it will be better, in my opinion, to look upon the former as a distinct 
formation. 
Different pieces of water differ among themselves very 
materially in the nature of the submerged flora, so that we must 
distinguish a number of aquatic formations. Until a large number 
of data have been collected by periodical and occasional collection 
it is impossible precisely to define these formations. But a number 
of useful points may be indicated. 
In the first place we can roughly classify waters into those 
containing Cladophoraceae and those in which members of this 
order are wanting. This is probably a fundamental distinction and 
is particularly useful, because where Cladophoraceae occur, they 
are generally to be met with in some quantity all the year round, 
so that this point is very readily determined. The presence or 
absence of these Algae is probably intimately related to the aeration 
of the water, Cladophoraceae only occurring in waters, which are 
not absolutely stagnant. This is a point which is very evident in 
the Tropics, where, owing to the higher temperature, there is a 
proportionally smaller amount of oxygen dissolved in the water, so 
that Cladophoraceae (except Pithophora ‘) are strikingly rare in these 
parts of the world and generally occur only in well aerated water 
(c.g., wells, rivers, etc.) There is some evidence that certain species 
of Cladophoraceae are able to exist in almost stagnant tropical 
waters, but the floristic records are too imperfect and unsatisfactory 
to admit of certainty on this point. The presence or absence of 
Cladophoraceae. involves other differences, since they constitute one 
1 Pithophora is much better able to meet the exigencies of tropical 
conditions owing to its thin walls, relatively narrow filaments 
and perhaps also its akinetes. This subject will be dealt with 
fully elsewhere. 
