Periodicity of Freshwater Algce in Nature 219 
spora and Tribonema in this pond is somewhat analogous to that 
of Bulbochcete and (Edogonium (see Section X). 
Certain filaments of M.floccosa always passed into the akinete- 
condition towards the close of the vegetative period (usually during 
March), persisting in this way throughout the summer months. 
M. stagnorum. generally occurred mixed with M. floccosa t and like 
the latter is a cold-water species, although it is clear that it was 
affected very adversely by competition with M. floccosa and also 
Tribonema. It exhibited a slight maximum in October 1918 (mean 
temperature 9 0 C.), but the abundance of Tribonema (and M. 
floccosa) caused a quick decline in early November. A further 
maximum occurred in November—December 1919, the development 
of the species coinciding with that of M . floccosa. Both the latter 
species and M. stagnorum show no relation to the concentration of 
the water. 
Here it may be noted that it seems a general rule that the 
occurrence of any Alga which is struggling to develop under con¬ 
ditions in which it is adversely influenced by competition with other 
species (of animals or plants) will tend to show little or no relation 
to such factors as concentration of the water and bright sunshine 
(although it may still exhibit some relation towards temperature), 
since it is competition which is the particular factor which sets a 
limit to the growth of the species, and restrains its development even 
though all other factors may be favourable for its increased growth. 
In small bodies of water, such as Hawkesley Hall pond, and with 
numerous species present, it is very probable that more or less 
competition is going on at all times of the year. 
XIII. VAUCHERIA 
This genus was represented in the pond by two species, V. aversa 
and V. sessilis, although the latter was present in quantity only 
upon one occasion. The occurrence of V. aversa is given in the 
following table: 
Table 1 showing the Occurrence of Vaucheria aversa in Hawkesley 
Hall Pond during 1918-1920. 
Jan. 
Feb. 
Mar. 
Apr. 
May 
June 
Aug. 
-P 
a, 
<D 
in 
-p 
0 
O 
> 
0 
£ 
Dec. 
1918 
r 
rr 
rr 
r 
c! 
1919 
rc! 
rr! 
rr! 
vr! 
vr 
— 
— 
— 
— 
vr 
vr 
rr 
1920 
rc! 
rc! 
r! 
vr 
vr 
- s 
1 For symbols see footnote on p. 210. An exclamation mark denotes oospore- 
production. 
