40 
STUDIES OF BRITISH FRESHWATER ALG^. 
absent in autumn of 1906.^ S. rivularis on the other hand was found 
in the autumn of all three years. 
There is undoubtedly also in many years a decrease in the quantity * 
of Sjnrogyy'a in midwinter, but if there is a proper vernal phase in the 
following year some species of the genus are always to be found 
throughout the winter-months. In this respect Abbot’s Pool differs 
from certain other ponds we have examined in which there is often an 
almost complete disappearance of Spirogyra during midwinter between 
the autumnal and the ensuing vernal phases. 
In our earlier paper on Spirogyra it was suggested that the realisa- 
tion of a certain group of conditions was necessary for the development 
of the autumnal phase, and that one of these was dilution of the water 
back to its ordinary degree of concentration. ^ In support of this the 
late and scanty appearance of Spirogyras in the autumn of 1906 was 
brought forward, and this was correlated with the occurrence of a very 
dry and hot summer, with rain only setting in at a late date. Although 
this particular feature was not as noticeable in Abbot’s Pool as in 
certain other ponds, it was sufficiently pronounced to lend further 
support to the view that the amount of rainfall has an important, 
though probably indirect, bearing on the occurrence of Spirogyra in 
nature (cf. p. 38). 
(ii.) — Cladophova. 
As above indicated Cladophora fracta is to be found in the pond 
under consideration all the year round, and this would appear to apply 
with equal truth to any other pond containing species of this genus. 
It decreases in amount very considerably at certain times of the year, 
noticeably during the winter and early spring, while its period of 
maximum abundance falls into the summer and autumn. During the 
winter, and to some extent also during the spring-months, the filaments 
of the Cladophora are in what may be called the winter-condition ; 
they are not very prominently branched, and the cells have thick 
stratified walls, dark green contents loaded with starch, and often show 
a somewhat irregular inflated shape. In the latter part of the spring, 
however, numerous young green branches having a healthy, normal 
aspect are put out;^ these branch themselves, and thus we find in the 
summer months a richly branched growth of healthy bright green 
Cladophora (the summer-condition). This prominent development of 
Cladophora during the warmer months of the year is, in our opinion, 
due to the fact that the water of the Fish Pond does not become very 
strongly heated in the summer ; for data are rapidly accumulating to 
1 This may have something to do with the hob weather prevalent during 
August and September of 1906. The weather for the previous fortnight in the 
two cases is described by the collector as “ fine on the whole, very warm,” and 
“ very fine and hot, with very little rain.” 
■2 Fritsch & Rich, p. 427. 
•^cf. Comere, loc. cit., p. 399. 
