36 
STUDIES OF BRITISH FRESHWATER ALGiE. 
commenced much earlier than usual ; the conditions which probably 
led to this abnormality are considered in detail below [see p. 38). 
Again, in the autumn-phase of 1904 S'pirogyra played an important 
part, with the result that Cladophora was not as prominent as usual, 
wiiile in the autumn of 1905 (Edogonium attained an exceptional 
development. 
Of the subordinate constituents of the algal vegetation (Edogonium 
and Mougeotia are the most important. The former, which is 
represented by a number of species, is practically always present, though 
only rarely attaining any considerable development. Mougeotia is 
also generally to be found in small quantity, and tends to be rather 
commoner during the winter than the summer months (exception : 
June, 1907) ; in some years (especially 1906) it even becomes quite a 
characteristic form during December — February. The genus Zygnema 
is only a very occasional form. A species of Microspora is generally 
to be found during the greater part of the year (commonest during 
spring and early summer), but like the species of Edogonium it rarely 
attains any pronounced maximum. The same may be said of the 
blue-green forms and of Coleochmte scutata (the latter mostly rare). 
Both Vaucheria and Stigeodonium were very irregular in their 
occurrence, which in the former at least may possibly be due to its 
frequently having been overlooked in collecting. 
The above remarks will have shown that there is a very definite 
periodicity in Abbot’s Pool with a good deal of minor variation, while 
occasionally exceptional conditions lead to very marked abnormalities. 
In order to study the nature of the latter more completely, and to 
obtain some insight into the causes determining the normal periodicity, 
we propose now to consider the occurrence of a number of the more 
important forms separately. 
(i.) — Spirogyra. 
In the paper on the occurrence and reproduction of Spirogyra in 
nature, published by us two years ago,i a considerable number of the 
data brought forward were derived from the Fish Pond, Abbot’s 
Leigh ; since that time, however, further valuable points have been 
determined, which cast light on a number of the problems raised in 
that paper. As stated above, species of Spirogyra are, as a rule, quite 
the dominant feature of the algal vegetation from March — May. This 
is, however, not only true of Abbot’s Pool, but of all other pieces of 
water of a similar kind containing Spirogyra that we have examined. 
In addition to this vernal phase many species of the genus also exhibit 
a (far less pronounced) autumnal phase. The latter is best shown by 
S. affinis, Petit, S. jugalis, Kiitz. and S. rivularis, Rabh., whilst 
S. varians^ Kiitz. may be mentioned as an example of a species that 
appears to be quite confined to the vernal phase. 
^ Fritscli and Rich, loc. cit. 
