Periodicity of Freshwater Algce in Nature 163 
these being (1) the central region of the pond, corresponding roughly 
with the central area of Potamogeton natans (see Section II), and 
(2) the peripheral region of shallow water. The second region is more 
especially developed on the east side of the pond (cf. Fig. 1) where 
the bottom slopes very gradually, producing a rather wide marshy 
zone of shallow water, which at times is particularly favourable for 
the development of many algal species. 
The Algae of these two provinces are for the most part remarkably 
distinct throughout the greater part of the year, although of course 
there is no marked line of separation between the two regions, and 
many species are found in both, although the dominant Algae in the 
two provinces are generally different. It is not considered that the 
algal vegetation of all small ponds exhibits this distinction; it only 
happens to obtain in the present case. 
Two communities may thus be distinguished in the algal vege¬ 
tation, namely a Central Community inhabiting the deeper water, 
and a Shallow-water Community 1 near the sides of the pond. In 
the Central Province of the deeper water four phases may be 
distinguished in the annual cycle, these phases corresponding roughly 
with the four seasons, although the spring- and summer-phases are 
the most marked. These four phases are: 
(a) a Spring-phase (February to May) with Zygnema the dominant 
species; 
(b) a Summer-phase (June to August) with CEdogonium dominant, 
and Anabc&na oscillarioides common (but Zygnema rare); 
(c) an Autumn-phase (September to November) marked by a 
decided decrease in CEdogonium and a slight increase in Zygnema ; and 
(d) a Winter-phase (December to January), transitional in 
character, with filamentous Algae generally scarce, but Zygnema 
usually commoner than CEdogonium. 
The Zygnema (see Section VIII) usually increases rapidly during 
February, and by May has attained a great maximum, masses of 
the Alga occupying nearly all the free water-surface between the 
floating leaves of Potamogeton natans. During the spring, however, 
a wide species of CEdogonium (see Section X) gradually increases in 
amount and by June replaces the Zygnema , which rapidly declines 
during this month. From June to August CEdogonium is at a 
maximum, and is often even more abundant than was the Zygnema 
in spring; while Anabeena is usually common amongst the CEdogonium 
1 At certain times of the year several more or less distinct communities or 
facies may be distinguished in this shallow-water region. 
f 
