34 
STUDIES OF BRITISH FRESHWATER ALGiE. 
that water conta.iihng carbonate of lime, which is known to be un- 
favourable for an abundant development of Desmids, drains into the 
pond. Finally it may be noticed that both as regards number of 
individuals and number of species the flora was richest in 1905 ; since 
that time there has been a steady decrease in both respects. It is not 
at all unlikely that the richness of the flora in 1905 is due to the 
uniform temperature of the water (cf. p. 31) and to the remarkably 
low rainfall of that year (cf. p. 31). 
A few words may be added on the animal life found in the pond. 
Fishes would seem to be not uncommon, rudd and carp in particular 
being mentioned by those who collected the samples. We have also 
noticed Dytiscus, Notonecta, worms, Rotifers, Vaginicola^ Melicerta 
ringens^ and Vorticella. 
Z)._GENERAL consideration of the LIFE-CYCLE 
IN THE POND. 
It has already been pointed out in the previous section that species 
of Cladophora and Spivogyra^ and various Diatoms are the dominant 
constituents of the algal flora of the pond under consideration. We 
can roughly distinguish the following four phases in the annual 
cycle ( cf. also the two charts and plate) : — 
(i.) — Winter-phase (from the middle of December to about the 
end of February) : — At this time we have Cladophora fracta, Melosira 
varians., Fragilaria virescens, Synedra radianti^ and numerous other 
free Diatoms, the latter often being by far the most abundant forms. 
Moiigeotia is frequently also quite common during the winter-phase. 
Its most striking characteristic is the enormous prevalence of filamentous 
and other free-living Diatoms, so that we might term this the Diatom- 
phase. 
(ii.) — Vernal-phase (from about the beginning of March to the end 
of May or middle of June) : — This abounds in species of Spirogyra, 
Cladophora fracta being a subsidiary form in the earlier part of the 
phase, but becoming more important in the second half. This might 
well be styled the Spirogyra-p\\-a.s>Q. 
(iii.) — Summer-phase (from the first weeks in June to the middle of 
September) : — This is stamped by the dominance of Cladophora with 
increasingly abundant epiphytes (especially Cocconeis, also Epithemia 
and Synedra radians, numerous others in smaller numbers). The 
aspect of the summer-phase alters slightly as the weeks pass on. At 
first we often still have Spirogyra competing with Cladophora for 
dominance ; then we have Cladophora quite dominant, with many 
healthy young branches and a relative scarcity of epiphytes, but 
already in July the latter begin to spread to a marked extent, and 
towards the end of the summer-phase a large proportion of the surface 
of the Cladophora is covered with epiphytic growth. The summer- 
phase is really a Cladophora-phn.^Q with epiphytic Diatoms playing an 
