F. E. Fritsch. 
156 
The second of these associations is difficult to separate from 
some of the submerged associations (cf. below). A union of (a) and 
(c) may be suggested and can doubtless be supported, but the 
separation is useful owing to the fact that the free-floating forms 
among the higher plants are more or less confined to the littoral 
region, while the Plankton, as is well known, extends throughout 
the piece of water from shore to shore. Amongst the Plankton I 
only include forms which are never attached at any period of their 
life-history. This is a necessary limitation ; otherwise it would be 
impossible to exclude forms like Spirogyra, Cladophora , etc., which 
I regard as belonging to the submerged associations. It is not 
possible, however (at present at least), to define absolutely the limits 
between the Plankton and the submerged associations in all cases. 
In the case of the marginal (semi-aquatic or marsh) vegetation 
there is much more to be said for looking upon it as a distinct 
formation. The conditions of life are very different from those of 
the true aquatics, since only the lower parts of the plant are sub¬ 
merged, whilst the greater portion is sub-aerial. The presence of 
the marginal zone of vegetation is dependent on different conditions 
to those which influence the submerged or floating forms, and in 
many cases ( e.g ., where the shores are very steep) it may be wanting 
altogether. All these features, I think, make it desirable to separate 
the marginal growth as a distinct formation (reed-marsh formation). 
In many cases it is not easy to delimit it on the land- or on the water¬ 
side, but many of our terrestrial formations have equally indefinite 
boundaries. Every formation is necessarily best developed in the 
middle of the area which it occupies, and near its limits it will 
often grade imperceptibly into another formation. In correspondence 
with the gradually increasing depth of the water the marginal 
growth often shows a well-marked zonal development; the zones 
may be regarded as distinct associations which have already been 
defined by others (Schroter and Kirchner, Magnin) more capable 
of dealing with them. Similar zones can also frequently be distin¬ 
guished in the submerged growth, which succeeds the semi-aquatics 
as the deepening of the water excludes the latter. These associations 
are, however, dominated by and probably only to be characterised 
by Cormophytes. A considerable amount of algal growth occurs 
amongst these latter, but it is very questionable whether there is 
any marked restriction of algal forms to one or other of these 
cormophytic associations and, until adequate evidence in support of 
this is forthcoming, it is better to regard all the littoral algal growth 
as an algal association distinct from the cormophytic ones. 
