214 William J. Hodgetts 
the concentration, as during May 1920 when the concentration rose 
from 9 to 18 degrees in the course of the month (cf. Fig. 3), did 
not check the development of these species. 
It will be noticed that in regard to the species of the genus which 
were identified there is no evidence that the conditions requisite 
for good vegetative growth differ in any way from those essential 
for oospore-production, although, of course, such a difference may 
possibly exist. Undoubtedly the most curious feature about the 
occurrence of (Edogonium in the pond was the independent way in 
which the wide sterile species continued its great maximum after 
the almost complete disappearance (during June) of the narrower 
species which had fruited. It may be that the wide species, having 
become perennial in the pond, has dispensed with sexual repro¬ 
duction, or only resorts to it very occasionally. 
The genus Bulbochcete is rather scantify represented in the pond, 
but B. minor has been observed during the spring of each of the 
four years, and although never abundant its frequency (Fig. 8) may 
be briefly referred to. Its maximum development fell usually during 
April—May (April—July in 1919), and it is undoubtedly a vernal 
form preferring average temperatures between 8° and 13 0 C., thus 
resembling very closely the species of (Edogonium which fruited in 
the pond. The latter species have doubtless been partly responsible 
for the feeble development of BulbockcBte in the pond, since both 
generally grow together upon the same substratum (submerged stems 
of rushes, etc.) and are thus in competition with each other. In this 
connection it is noteworthy that Bulbochcete (mainly B. minor , but 
B. intermedia was also noted) was rather better developed in April— 
May 1918, as compared with the other years, and the narrower 
species of Edogonium were poorly represented during the spring of 
1918. The dependence of Bulbochcete on abundant bright sunshine 
is very obvious (cf. with sunshine-data in Fig. 2), and this condition 
is very probably essential for the sexual reproduction of this genus. 
XI. TRIBONEMA 
This genus is represented in Hawkesley Hall pond by T. bornby - 
cinum (the type, and also its forma minor) and T. affine. As already 
stated in Section VI Tribonema is most abundant in shallow water 
near the sides of the pond during late autumn and winter, when it is 
often the best-developed filamentous Alga in the pond, although 
during the colder months Spirogyra (see Section VII) may be 
locally common, and Microspora (Section XII) may be more or less 
developed in places. 
