HELEOPLANKTON OF THREE BERKSHIRE POOLS. 7 
marked differences in the contents of their planktons. It will be seen that 
the adjacent Bulmershe Pools have only four species in common, whereas 
Bulmershe North and Whiteknights have five species in common and 
Bulmershe South and Whiteknights only one. Thirty-eight out of the forty 
- species found in Bulmershe South are confined to that pool, five out of 
thirteen to Bulmershe North, and fourteen out of nineteen to Whiteknights. 
The causes of the distribution are doubtless complex in any case, but the 
complexity will be even greater in a small pool than in a large lake. 
Assuming that the main factor is the nature of the substances dissolved in 
the water, it follows that the variations in the composition and concentration 
of the solution will depend mainly on the volume of water in the pool or 
lake. For in the first place, any given depth of rainfall will dilute the bulk 
of a large lake much less than the same depth of rainfall would that of a 
small pool. The drainage also, bringing in dissolved substances, will have 
a much smaller effect on the composition of the large volume of water in a 
lake than on the lesser volume in a pool. Furthermore, the fringe of aquatic 
vegetation will materially alter composition of the water by the withdrawal 
of substances necessary for growth and by the addition of the products of 
decay ; and as the weed-fringe of a small pool is relatively greater in pro- 
portion to the bulk of water than in the case of a lake, the smaller body of 
water will undergo greater variations in the nature of the substances in it. 
In a small peol, therefore, slight differences in the volume of water, or in 
the size and nature of the drainage of the area, or in the amount of kind of 
aquatic vegetation, will exercise a relatively great effect on the composition 
of the water solution. It is therefore not surprising that the planktons of 
small pools should differ from one another to the extent they do. 
Whatever the chemical effects of the aquatic vegetation may be, what one 
might call the ecological effects are all important in the case of the smaller 
bodies of water. The alg:e collected by the plankton net are in the majority 
of cases not permanent denizens of the surface water, but are derived from 
the benthos. Their true habitat is among the leaves of the macrophytie aquatic 
vegetation or in the mud of the shallower parts, and they are carried out by 
the agitation of the water by the wind. They mingle with the true plank- 
tonic algæ for a time, but as they have no devices with which to counteract 
the pull of gravity, they soon sink below the region of minimum photic 
action and perish. 
Of the algæ shown in the list, only the Peridinieæ (except Glenodinium 
uliginosum), the Volvocaceæ, and perhaps the two desmids Closterium aciculare 
and Xanthidium antilopeum may claim to be planktonic. The rest are 
from the benthos, even though they may be in large numbers. It is clear, 
therefore, that the plankton of a small pool is to a large extent dependent on 
the presence of aquatic vegetation ; and as it is possible that different species 
