70 
E. Marion Delf. 
month of March, and disappearing gradually in May and June, 1912, 
and more abruptly in May, 1913. These months had a nearly equal 
rainfall in the two years, thus accounting, perhaps, for the longer 
vegetative period in the former year. The summer temperatures 
are certainly unfavourable to most of the Protococcales. Very 
similar results are recorded by Fritsch and Rich for the Protococ¬ 
cales of Barton’s Pond, but there the maximum falls later in the 
spring (April to June). In the Hampstead ponds there is also a 
secondary maximum in the early winter (October to December): 
although this is never very striking. Another point of interest is 
the spasmodic increase of certain genera for unknown reasons; this 
has been recorded for Tetraspora, Volvox, Chlamydomonas and 
Carteria. 
The Ulotrichales. 
The commonest and the most abundant members of this group 
found in the ponds are undoubtedly Ulotlirix subtilis, Microspora 
floccosa and Draparnnldia , but Chcztophora, Coleochcete and others 
are also present at times. On the whole, there is a well-marked 
diurnal periodicity, the principal phase occurring from February to 
the end of April, and the secondary phase occurring from the end 
of October to December. 
The genus Ulotlirix has been found in greatest abundance dur¬ 
ing the months of February and March, 1912. It appears to increase 
in amount either gradually (V, February, 1912) or suddenly (VI, 
December 2nd, 1913), but in either case is apt to disappear very 
rapidly (III, V, VII, 1912). The rapid increase is readily intelligible 
from the ease with which whole filaments break up into zoospores 
and the abrupt disappearance may perhaps be accounted for by the 
attacks of water insects, or animals, which are themselves apt to 
increase rapidly at this time of year. 1 
The genus Microspora is more gradual both in becoming 
dominant and in disappearing. It is, on the whole, a later spring 
form than Ulotlirix, becoming dominant in March and April, 1912, 
in most of the ponds and in two cases even persisting into June and 
July. In a recent paper, the periodicity of another species of 
Microspora is fully discussed by Fritsch, 2 and he concludes that the 
principal factors which condition its dominance in Barton’s Pond, 
1 Tadpoles were numerous in ponds III and VII in May, 1913. In their 
early stages these are voracious vegetable feeders, and this may account for 
the abrupt disappearance of both the Protococcales and the Ulothricales in that 
year. 
* Fritsch, F. E., and Rich, F. Loc. cit., pp. 30-35. 
