Algal Vegetation of Ponds on Hampstead Heath. 79 
also the aeration and dilution effected by a good rainfall, seem 
to favour the development of these forms. Navicula persists into 
the summer months although possibly disappearing in July and 
August. 
There is little comparative evidence as to the periodicity of 
diatoms, but the above remarks shew that there is a general 
resemblance between the diatomic flora of the Hampstead ponds 
and of Barton’s Pond. 1 On the other hand, in the lakes of the 
north-west, 2 West and West found that the Bacillariae were princi¬ 
pally autumn forms, sometimes shewing summer maxima. 
The Cyanophyce/E. 
This group is chiefly represented by the genus Oscillatoria, 
but Nostoc, Lyngbya and Arthrospira have also been seen. The 
occurrences are somewhat irregular, but all fall within the colder 
months of the year. Nostoc has been recorded for October, 1912, 
and for February and March in 1912 and 1913. The remaining 
genera have only been found in the months of February and March 
and are never very constant constituents of the samples obtained. 
It is probable that they are inhabitants of the deeper water, possibly 
on the mud floor, and so frequently escape detection. 
Summary and General Conclusions. 
There is a well-marked periodicity in the occurrence of the 
majority of the algae in the ponds on Hampstead Heath. The 
season of greatest diversity and abundance was from February 
to April or May in the years recorded. This corresponds to a 
period of variable rainfall, gradually ascending temperatures, 
increasing light intensity, and of comparatively slight development 
of animal life. The rainfall is probably less directly influential in 
these ponds than in some, for there is usually a good supply of 
water from underground sources. As the temperature rises from 
May to July the algae become greatly diminished, and many forms 
altogether disappear until late in the following October, or early 
November. They reappear first in those ponds which are slightly 
warmer and bear more phanerogamic vegetation and about two or 
three weeks later in the remaining ponds (I, II, VI and VIII). 
In 1912 there was a secondary maximum in October and 
November, falling off again in December and January, but in 1913 
1 Fritsch, F. E., and Rich, F. Loc. cit. 
3 West, W., and West, G. S. Loc. cit. 
