Periodicity of Freshwater Algce in Nature 21 1 
August 1920 had only a moderate amount of bright sunshine it 
seems that this species rather tends to follow Staurastmm brevispinum 
and 5 . Dickiei in not requiring abundant sunlight for conjugation, 
while differing in this respect (as in the matter of temperature) from 
Cl. acerosum. The same applies to Cl. rostratum, which was occasion¬ 
ally seen in the pond, although always very rare, the zygospores of 
which were observed in April 1919, a month which had only a 
moderate amount of bright sunshine. The Desmids of the pond which 
have conjugated thus fall into two classes, (1) those which require 
abundant bright sunshine before conjugation can take place (Clos¬ 
terium acerosum , Cosmarium Botrytis), and (2) those which apparently 
are able to conjugate given only a moderate amount of sunshine 
(Staurastmm brevispinum , 5 . Dickiei , Closterium lineatum, Cl. ros¬ 
tratum) . The evidence in the case of the two species of Closterium 
in the second class, however, is not quite conclusive, since these 
Desmids were never present in any quantity. 
X. THE (EDOGONIACE/E 
Species of CEdogonium are always present in Hawkesley Hall 
pond, and as regards their periodicity can be sharply separated into 
two groups. 
Group i contains probably only one species of CEdogonium , a 
wide species (width veg. fil. 30-35/x), which since it has never re¬ 
produced sexually during the period of observation cannot be named, 
but which nevertheless is the most important of all the species of 
the genus observed in the pond, and is the form referred to simply 
as CEdogonium in the account (in Section VI) of the annual cycle 
observed in the central region of the pond. As already mentioned 
this species attains a huge maximum in the central deeper water of 
the pond generally during June—August, large masses of its filaments 
during these months occupying sometimes all the available free 
water-surface between the floating leaves of Potamogeton natans (cf. 
Section II). 
Group 2 includes the rest of the species of CEdogonium occurring 
in the pond, and which even when taken all together were never as 
abundant as the species of the first group, and were never more than 
moderately wide (up to 20 /x wide). Their maximum growth was 
always attained during May or early June, a rapid decline taking 
place in the latter month after oospore-production. This group 
includes 0 . cryptoporum, 0 . crisp urn, 0 . Borisianum , 0 . Braunii, 
0 . echinospermum and 0 . rugulosum. Several species of this group 
rather tended to show a preference for the shallower water at the 
5—i4 
