STUDIES OF BRITISH FRESHWATER ALG^. 
53 
A second group of forms are the species of Spirogyra^ whose annual 
cycle also seems little affected by that of the other Algae in the pond ; 
and the same may be said of the third group, constituted by the free- 
living Diatoms. These three groups of forms, which apparently have 
an independent development, may perhaps be looked upon as suh- 
associations ; these form an integral part of the whole algal association 
(formation ?) in the pond, although not much connected with one 
another owing to the fact that they succeed one another in time. 
For this reason the whole algal flora of the pond is above described 
as a successive association (formation %). Our study of the other algal 
constituents has not been based on sufficient data to enable us to 
say whether they work in with one or other of the three sub- 
associations distinguished above, or themselves constitute further 
sub-associations. We should restrict the latter term to a group of 
forms which in their relative development influence one another ( e.g. 
Cladophora and its epiphytes) or flourish simultaneously apparently 
in response to the same group of influencing factors (e.g. the free- 
living Diatoms), but except in the three cases above noted such 
relations have been very difficult to determine. Cladophora and its 
epiphytes may be described as a competitive sub-associatio7i^ while 
the other two cases are examples of non-competitive sub- associations. 
The factors operating in the pond are of three kinds — seasonal, 
irregular, and correlated,^ and in the preceding pages we hav^e dis- 
cussed many of them. The seasonal factors are mainly constituted 
by changes in the concentration of the water, in the temperature of 
the water, in the quantity of dissolved gases and the amount of 
organic substance in the water, and in the light-intensity. The 
irregular factors comprise abnormal periods of low and high tempera- 
ture, periods of drought or heavy rainfall, and abnormal spells of 
dull weather. The correlated factors are no doubt numerous, but 
we have only been able to clearly recognise the competition between 
Cladophora and its epiphytes. All the numerous changes in the 
vegetation of the pond must be due to the simultaneous operation of 
a number of these factors, and in many cases we have at least been 
able to suggest the factors that may be responsible. 
Frequently, no doubt, the condition of all but one factor is suit- 
able, and the given change has to await the realisation of the necessary 
condition of this limiting factor before the change takes place. As 
already pointed out in our earlier paper,^ all our observations tend 
to indicate that the doctrine of limiting factors^ will probably be 
found to underlie the whole scheme of intricate changes that are so 
striking a feature of freshwater algal vegetation. 
^cf. Fritsch, in New PhytoL, vol. v., 1906, p. 161. 
■•^Loc. cit., p. 432. 
^cf. F. F. Blackman, Optima and Limiting Factors, Ann. of Bot., 
vol. xix., 1905, pp. 281—295. 
