Periodicity of Freshwater Algce in Nature 213 
is indicated by the fact that upon its removal by the sudden decrease 
of Zygnema (usually in early June) the Edogonium- curve again 
ascends to a higher level. In this connection the behaviour of the 
two curves in early 1921 is interesting. Owing to the very mild 
January of 1921 both Algae succeeded in becoming “common” by 
the end of that month, but mutual competition checked further 
increase until the end of March (shown by the flattening of both 
curves during January—March), when the (Edogonium assumed the 
upper hand and became “very common” during April, and, appar¬ 
ently as a consequence, the Zygnema suddenly decreased in amount. 
Little relation is shown between the concentration of the water 
and the growth of this wide species of Edogonium, although active 
development generally took place with moderately strong concen¬ 
trations between 16 and 20 degrees (1918, 1920); but in 1919 the 
Alga was “very common” in August when the concentration was as 
high as 27 degrees (cf. Fig. 3). 
Of the narrower species of Edogonium (of Group 2) probably the 
most abundant was 0 . eryptoporum, the frequency-curve of which 
is also given in Fig. 8. The curves for the other species of this group, 
enumerated at the beginning of this section, are practically identical 
in form with that of 0 . eryptoporum, although the height of the 
May—June maximum varies with the different species (the result, 
doubtless, of mutual competition), the least abundant species being 
0 . Borisianum, 0 . Braunii, and 0 . rugulosum. The spring-maximum 
(end of May and first half of June) was observed in each of the four 
years, and occurred with the average temperature of the air rising 
between io° and 13*5° C., while the decline towards the end of June, 
after fruiting, coincided with the setting in of summer conditions. 
These species of the genus thus prefer a distinctly lower temperature 
than the wide species already considered. 
The relation to the sunshine-data (Fig. 2) is very marked, the 
maximum always falling after a spell of very sunny weather. This 
agrees with the observations of Fritsch and Rich(i8) who found that 
the maximum development of the species of Edogonium in Barton’s 
pond was “determined by the first month with plenty of bright 
sunshine.” The second conclusion of these authors regarding these 
species, namely that “dilute water is more favourable to their 
development than concentrated water” ( l.c . p. 78), is hardly con¬ 
firmed by the present observations. The maximum of the species of 
Edogonium which have fruited in Hawkesley Hall pond always 
occurred with medium concentrations between 16 and 20 degrees 
(cf. Figs. 3 and 8); and it may be noted that a sudden increase in 
