6 MR. B. M. GRIFFITHS ON THE 
Glenodinium uliginosum was found in very small numbers in Bulmershe 
South, but its condition indicated that it was probably a mere casual from 
the small sphagnum bog near the south-west corner of the pool. 
A striking feature of the plankton is the very small number of 
Myxophyceæ present. Only five species were found, and none were in any 
quantity. The Bacillarieæ are also very poorly represented by four species, 
but one of these, Synedra Acus, occurred in quantity in Whiteknights Pool. 
The paucity of these groups may be due to the absence of contamination of 
the water by house-drainage or by drainage from arable land. 
The dominance of Tribonema affine in Bulmershe North Pool is a peculiar 
feature, as its usual habitat is in ditches ete. Species of this genus appear 
to become temporary plankton forms occasionally. Tribonema minus (Wille), 
Hazen, is recorded as suddenly appearing in the plankton of Lake Mendota, 
Wisconsin (Smith, 1920), and 7. bombyeina forma depauperata, Wille, 
occurs in some of the Danish lakes (Wesenburg-Lund, 1908). T. affine ts 
also given as occurring in some of the Scotch lakes (West & West, 1909). 
In the case of the Berkshire pools, the distribution is peculiar, for the 
organism is absent from the adjacent Bulmershe South Pool and present in 
the more distant Whiteknights. ; 
The Desmids are represented by some twenty species—nineteen in 
Bulmershe South, one in Bulmershe North, and none in Whiteknights. 
Most of them were found in very small numbers, and they are most probably 
derived from the small sphagnum bog or from the weeds. In the case of 
Closterium aciculare var. subpronum, however, we probably have a true 
plankton desmid. It formed a considerable part of the plankton of 
Bulmershe South Pool, associated with Nanthidium antilopeum, which, though 
not in great abundance, was in a healthy and dividing condition. Staur- 
astrum teliferum was the sole desmid found in the neighbouring Bulmershe 
North Pool, but only in very small numbers. 
Of the other important plankton organisms, Pandorina morum was con- 
fined to Whiteknights, and Eudorina elegans and Volvoa aureus were 
confined to Bulmershe South. The Flagellate, Dinobryon Sertularia, was 
very plentiful in Bulmershe North, where also Dictyospharium pulchellum 
attained some importance. The latter alga was the sole form common to all 
three pools. 
The above account shows that the planktons even of adjacent pools are 
very dissimilar. The causes of the differences are very obscure, for though 
it has been shown by West and West (1900) that, in general, dominant 
desmid planktons are associated with “ purity " of water, the factors deter- 
mining the distribution of other organisms are practically unknown. In the 
case of the Berkshire pools, their situation on similar subsoils, in closely 
similar environments and in close proximity, does not prevent the most 
