Periodicity of Freshwater Algce in Nature 221 
Vaucheria sessilis has been found in the pond on three occasions, 
namely October 1919 (rare), September 1920 (very rare), and April— 
May 1921 (rather rare) and was absent, or occasionally perhaps rare 
and vegetative only, at other times. Although the evidence is 
insufficient this species apparently differs in its requirements from 
V. aversa. During its period of greatest development in the pond 
(April—May 1921) the average temperature of the air was between 
7 0 and n° C., the concentration of the water was rising between 
15 and 18 degrees, there was abundant sunshine, and rain was very 
deficient during the preceding three months, so that the aeration of 
the water was probably poor. 
XIV. APHANOCHCETE AND DRAPARNALDIA 
Aphanochcete repens and Draparnaldia glomerata, although not 
very important constituents of the algal flora of the pond may be 
briefly referred to. The occurrence of the former species is given in 
the following table: 
Table showing the Occurrence of Aphanochcete repens in Hawkesley 
Hall Pond during 1918-1921. 
a 
aS 
1 — > 
A 
<D 
fc 
Mar. 
Apr. 
S 
June 
j>> 
13 
' — i 
bio 
3 
< 
Sept. 
-M 
O 
O 
> 
0 
& 
6 
<u 
0 
1918 
vr 
vr 
r 
r 
rr 
r 
vr 
vr 
r 
rr 
rr 
r 
1919 
r 
rr 
rr 
rc 
rr 
rr 
rr 
vr 
vr 
vr 
vr 
vr 
1920 
vr 
vr 
vr 
vr 
rr 
r 
r 
vr 
vr 
rr 
rr 
rr 
1921 
rc 
rc 
rc 
rr 
r 
r 
Aphanochcete 
repens 
was 
observed 
in 
the pond 
as a n 
epiphyte 
upon the wide sterile species of CEdogonium (rarely upon the narrower 
species, and very rarely upon Microspora) which generally formed 
such a conspicuous feature in the central parts of the pond during 
the summer months (cf. Section X and the curve of this species in 
Fig. 8). The occurrence of Aphanochcete it will be noted shows only 
a slight relation to the periodicity of its “host,” since although the 
maximum development of the epiphyte occurred either in May 
(1918, 1920), April (1919), or during January—March (1921), which 
in each case coincided with the vernal increase of the species of 
CEdogonium in question, yet Aphanochcete always decreased in 
amount long before the CEdogonium commenced to decline. In fact 
Aphanochcete is a spring-form (although sometimes showing a certain 
amount of development in the autumn, as in 1918 and 1920), pre¬ 
ferring moderate temperatures (a monthly mean between 4 0 and 
n° C.). The higher temperatures of summer are very unfavourable 
