[ 15 J 
A STUDY OF SOME OF THE FACTORS 
CONTROLLING THE PERIODICITY OF 
FRESHWATER ALG JE IN NATURE 
By WILLIAM J. HODGETTS, M.Sc. 
(With 2 figures in the text) 
XVII. THE VOLVOCALES 
T he periodicities of four members of the Volvocales have been 
worked out, namely Chlamydomonas Reinhardi, Pandorina 
Morum, Eudorina elegans, and Volvox aureus. The first and last have 
their periodicities given in the table below, while Pandorina and 
Eudorina have their frequency-curves represented in Fig. io. All 
these species, when present, generally had a scattered distribution 
in the pond, not being confined to any particular region. 
Table showing the Occurrence of Chlamydomonas Reinhardi and 
Volvox aureus in Hawkesley Hall Pond during 1918-1921. 
d 
1—) 
rO 
<D 
<3 
% 
Sh 
ft 
< 
May 
June 
j>» 
3 
t-n 
bo 
3 
< 
Sept. 
Oct. 
> 
0 
Jz. 
d 
CD 
Q 
j' 
1918 
— 
— 
— 
— 
vr 
r 
r 
rr 
c 
vr 
— 
— 
Chlamydomonas } 
Reinhardi | 
1919 
1920 
— 
vr 
vr 
- - 
vr 
vr 
vr 
vr 
r 
r 
rc 
rr 
vr 
vr 
r 
vr 
r 
\ 
.1921 
r 
vr 
vr 
vr 
rr 
r 
t 
1918 
— 
— 
—. 
rr 
rr! 
— 
—| 
— 
r 
vr 
vr 
rr 
Volvox | 
1919 
vr 
r 
vr 
vr 
rr! 
— 
— r 
aureus | 
1920 
.1921 
vr 
vr 
rc 
c! 
vr! 
Of the three species of Chlamydomonas ( C . Reinhardi , C. De- 
baryana, C. monadina) which were identified with certainty in the 
pond only one (C. Reinhardi) has been at all abundant, the other 
two being observed only occasionally, and were never present in any 
quantity. The above table thus practically gives the frequency of the 
genus as a whole in the pond. Three out of the four maxima shown 
by C. Reinhardi (cf. table above) fall rather curiously in the month 
of September, and in each of the three complete years development 
was practically ponfined to late summer and early autumn, while in 
both 1918 and 1919, the maximum occurred immediately after the 
