210 
William J. Hodgetts 
example of a high concentration causing a depression in the fre¬ 
quency-curve is apparent in July and August 1919; but the fall in 
the concentration during September 1919 caused the frequency-curve 
to rise again, although the further increased concentration (and also 
the falling temperature) during October soon caused the Alga to 
decline once more. 
The following table gives the frequencies of the three remaining 
Desmids which were of importance in the pond. 
Table 1 showing the Occurrence of Three of the Desmids present in 
Hawkesley Hall Pond during 1918-1921. 
d 
oj 
1—> 
22 
<D 
Mar. 
Apr. 
a 
2 
June 
July 
Aug. 
Sept. 
Oct. 
Nov. 
Dec. 
1 
r 1918 
vr 
vr 
r 
r 
r 
rr 
rc 
rc 
rc 
r 
rr 
rc 
Pleurotcenium J 
1 1919 
rr 
rr 
rr 
rr 
rr 
rr 
r 
vr 
vr 
vr 
vr 
vr 
Ehrenbergii 1 
l 1920 
vr 
r 
r 
r 
r 
rr 
rr 
rr 
rc 
rr 
rr 
r 
1 
[ 1921 
r 
r 
r 
r 
r 
rr 
1 
r 1918 
vr 
r 
rr 
rr 
rr 
rr 
rr 
rc 
rc 
rr 
rr 
rr 
Closterium j 
1 1919 
rr 
rr 
rr 
r 
rr 
rr 
r 
r 
vr 
vr 
vr 
vr 
monilifemm 1 
1 1920 
vr 
vr 
vr 
vr 
r 
rr 
r 
rr 
rr 
r 
r 
r 
1 
11921 
r 
vr 
r 
r 
r 
r 
j 
r 1918 
vr 
vr 
vr 
vr 
vr 
r 
rr 
vr 
vr 
vr 
vr 
r 
Closterium | 
' 1919 
vr 
vr 
vr 
r 
r 
rr 
vr 
vr 
vr 
r 
vr 
vr 
lineatum j 
1 1920 
vr 
vr 
vr 
vr 
vr 
r 
r 
r 
rr! 
r 
vr 
vr 
1 
1 1921 
vr 
vr 
vr 
vr 
vr 
vr 
All three it will be noted tend to have their maximum develop¬ 
ment during the summer or early autumn. Temperature seems to be 
the main determining factor, although sunshine may, in a lesser 
degree, be important. In general concentrations of the water 
between 14 and 20 degrees are most favourable; and, as with most 
of the other Desmids, the high concentrations attained in the second 
half of 1919 proved harmful. The curious secondary maximum of 
Pleurotcenium in December 1918 was probably due to the fact that* 
this month was abnormally mild (see Fig. 2). Both species of 
Closterium given in the table prefer distinctly higher temperatures 
than Cl. acerosum, the occurrence of which has already been discussed, 
although Cl. monilifemm (like Cl. acerosum) persisted as “rather 
rare” throughout the winter of 1918-1919. 
The curious double zygospores of Cl. lineatum were observed in 
the pond (but were scarce) in early September 1920, and since 
1 The meaning of the symbols in this and in subsequent tables, and in the 
figures, is as follows: vr = very rare, r = rare, rr = rather rare, rc = rather 
common, c = common, vc = very common, - = absent; + (in the figures) and 
! (in the tables) indicate sexual reproduction. 
