STUDIES OF BRITISH FRESHWATER ALG^. 
35 
important part in the latter half. The summer-phase is also noticeable 
in showing an occasional prominent development of some other form ; 
thus, in 1905 species of (Edogonium played quite an important part, 
in 1907 Mougeotia was well represented, while in 1904 and 1906 a 
species of Stigeoclonium was not at all uncommon. 
(iv.) — Autumn-phase (from the middle of September to about the 
middle of December) ; — This phase is not nearly so sharply characterised 
as the other three, and it would be difficult to name it after any 
prevalent algal form. Cladophora is generally the most important 
form, but with it we often have a good deal of Spirogyra, (Edogonium, 
and sometimes a certain amount of Oscillm'ia and Lyngbya. There are 
at first a good many epiphytic forms on the Cladophora, but their 
number steadily decreases as the winter-phase approaches, while free 
Diatoms become more abundant. The autumn-phase is thus largely a 
period of transition between the summer- and winter-phases. 
To put it briefly Cladophora is, as a general rule, the dominant form 
during the summer and autumn months, while Spirogyra is the most 
conspicuous Alga during the spring months, and Diatoms the most 
important forms during the winter. It is not only the change in actual 
composition of the algal vegetation that seems to us to justify the dis- 
tinction of the above four phases, for the transition from one phase to 
another is marked also by changes in the amount of algal growth 
present. There is a very marked increase in passing from the winter- to 
the vernal phase, while at the end of the latter there is again a decrease, 
followed once more by an increase at the commencement of the 
autumn-phase. The decrease in actual amount of algal growth present 
in the summer-phase appears not to be so marked in the Fish Pond, 
Abbot’s Leigh, as in many other pieces of water, and this is probably 
to be attributed to the water never becoming very strongly heated in 
summer (cf. p. 31). 
The very pronounced dominance of Spirogyra during the vernal 
phase appears to be quite independent of the relative abundance of 
the other constituents of the algal vegetation, although it is not 
impossible that the immense preponderance of species of this genus 
during the spring months may tend to crowd out other forms to some 
extent. It is noticeable that the rise of the Spirogyra-GuvvQ (see 
chart I.) generally involves a fall on the part of that of Cladophora, 
and often of that of (Edogonium as well, but there are not sufficient 
data to enable one to come to any definite conclusion on this point. 
The amount of Cladophora on the other hand seems to bear a direct 
relation to the wealth of the epiphytic Diatom-vegetation, and it seems 
that there is a perpetual struggle between the two, which at one time 
of the year leads to the dominance of the one, at another time to the 
success of the other (cf. below, p. 41), 
It is not always however that we get the normal succession of forms 
that has been outlined above. Both in 1904 and 1908 the Spirogyra- 
phase was as good as completely absent during the spring months, with 
the result that the winter-phase was prolonged and the summer-phase 
