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William J. Hodgetts 
sides of the pond, often occurring in quantity round the submerged 
parts of rushes, etc., but 0 . cryptoporum and 0 . Braunii were also 
at times rather abundant in deeper water. 
Dealing first with the wide species of the first group, the frequency- 
curve of which is given in Fig. 8, there is obviously a marked relation 
shown towards temperature (cf. Fig. 2), since the various maxima 
always coincide with the higher temperatures (a monthly mean between 
12 0 and 16-5° C.), while the decline after a maximum is simultaneous 
with the lowering of the temperature at the end of summer or in 
early autumn. There was an exceptional smaller secondary maximum 
in the very mild October of 1920; while the maximum was attained 
very early (April) in 1921, probably owing to the warm spring of 
this year. There seems little or no relation towards bright sunshine, 
although growth was most marked in very sunny months in 1918 
and 1920, yet in 1919 pronounced growth took place during March— 
April and again in July, and both these periods were poor in sunshine 
(cf. Fig. 2); and again the marked increase of the species in the 
abnormally mild January of 1921 was due solely to the rather 
favourable temperature, since this month was very dull. 
An important factor affecting the vernal increase of this form is 
competition with Zygnema in the central parts of the pond. It has 
already been stated (Section VI) that the wide species of (Edogonium 
replaces the Zygnema usually during June, so that competition 
between these two genera must be most marked in May and June; 
and it may be that this competition with Zygnema tends to prevent 
the species of (Edogonium now under consideration from attaining its 
maximum early in the year, as do the narrower species of the genus 
in this pond. Although the Zygnema is a vernal type while the wide 
Edogonium is a summer-form, yet during the transition from spring 
to summer there is every reason to believe that there is active 
competition between the two, since both occupy identical positions 
in the pond, and the latter is not large enough for both to attain 
any great maximum at the same time. 
A comparison of the frequency-curves of Zygnema (in Fig. 5) and 
the wide Edogonium (in Fig. 8) rather bears this out. In the first 
place the rise in the Edogonium- curve, just before a maximum, 
generally involves a fall in the Zygnema- curve; and further, in 1918 
and 1919 at the periods of maximum development of the Zygnema 
the upward tendency of the Edogonium- curve is interrupted by a 
horizontal portion—a form of curve which indicates that some 
limiting factor has come into play, and is checking further growth. 
That this particular limiting factor is competition with the Zygnema 
