Periodicity of Freshwater Algce in Nature 153 
On the extreme west side of the pond is a patch of marshy ground, 
passing under the fence, and by means of which the pond, during 
times of very high water, is put in temporary communication with 
another smaller pond (not shown in Fig. 1) located on the opposite 
side of the fence. This second pond is very much shaded by trees, 
and has not proved very suitable for a study of algal periodicity; 
in the present paper it is not again referred to. 
The soil of the district overlies Keuper Marl, and round the pond 
is of the nature of a rather marly clay, which is very impervious 
when wet. The general surface of the meadow slopes upwards 
towards the pond, which is situated at the highest point of this 
particular field, but towards the north-west the ground rises gently 
again, and it is mainly water which drains off the land from this 
direction which feeds the pond. Since the latter never completely 
dries up even during the hottest weather—although at such times 
it contracts in volume considerably—it seems likely that there may 
be a bottom-spring which to a certain extent supplies the pond, but 
this point was not definitely settled. When flooded with heavy rain 
the water overflows at the shallow south corner, and drains off down 
the sloping meadow, so that it is impossible for the water-level to 
exceed a certain maximum height, which is determined of course by 
the height of the ground at the southern overflow corner. The pond 
is some distance from a road, and no field-path runs near it, and on 
the whole it is allowed to remain comparatively undisturbed, which 
to some extent may account for the richness of its algal flora. 
During the summer months the pond supports an abundant 
phanerogamic flora, which exhibits a more or less zonal arrangement 
round the pond (Fig. 1). Four well-marked zones can be recognised 
generally throughout the summer, these being, commencing with 
the outermost: 
(1) a zone of Juncus conglomeratus ; 
(2) a zone of Scirpus palustris and Glyceria fluitans (these species 
generally co-dominant); 
(3) a zone of Sparganium ramosum ; and 
(4) a central area of Potamogeton natans. 
The marginal zone of rushes is well-marked and extends all 
round the pond, with the exception of the south overflow corner, 
covering the damp banks just above water-level. J uncus conglo¬ 
meratus being a markedly social species it is generally easily able 
to exclude competitors, but a few marsh-loving plants (e.g. Galium 
palustre) are sometimes present. 
4—9 
