THE FRESHWATER PLANKTON OF THE SCOTTISH LOCHS. ili 
of the lochs. This Desmid occurred in small subspherical or ellipsoidal colonies 
containing upwards of 500 individuals imbedded in a copious mucus. ‘These colonies 
were freely floating in the water, although the natural habitat of the plant is on wet 
‘rocks. It would be of interest to determine whether this species is or is not a constant 
feature of the plankton of these lochs. It would thus furnish a case of a species 
normally confined to wet rocks having become adapted to a pelagic existence. It 
normally occurs in irregular mucous colonies, and the only modification has been the 
reduction in size of the colonies, and their assumption of a more regular shape. 
The remarkable abundance of Desmids is one of the most salient features of the 
Scottish plankton, and requires further comment. At the same time, it must be 
remembered that the plankton of the lakes in the extreme west of Ireland and North 
Wales, and that of some of the lakes in the English Lake District,* is almost equally rich 
in Desmids; and therefore this feature is not an isolated phenomenon confined to the 
lakes of the Scottish Highlands. We attribute the abundance and variety of these 
Conjugates entirely to the geological character of the districts in which the lakes are 
situated. Most of them are in rocky hollows in the old formations, either Precambrian 
or Older Palzeozoic, and this fact always increases very considerably the richness of the 
Desmid-flora of any area, whether it be the flora of the bogs or of the lakes. In 
conjunction with this it must be remembered that these areas are suitable for the 
existence of peat-bogs and peaty pools, and that the water which drains into the lakes 
is mostly distinctly peaty and slightly acid. This is a condition eminently favourable 
for the multiplication of Desmids ; but although it accounts for the abundance of certain 
of these plants, it does not account for their variety. Many of the Carboniferous areas 
are equally suitable for the formation of peat, and are likewise studded with peat-bogs, 
peaty pools and tarns; but neither the bogs nor the plankton of the tarns and lakes, 
although containing numerous Desmids, contain the great diversity of species met with 
on the older formations. In districts in which the geological formations are of the 
Secondary or Tertiary Age there is a still further reduction in the quality of the 
Desmid-flora. There are slight exceptions to this on the Lower Greensand of Surrey 
and on the Middle Hocene of Hampshire, but as a general rule the Desmid-flora of 
Secondary and Tertiary areas is relatively very poor, except where there is a consider- 
able amount of intrusive igneous material. 
Many of the most handsome of the Scottish plankton-Desmids, such as Staurastrum 
Arctiscon, St. longispinum, St. Ophiura, St. sexangulare, St. aversum, St. Brasiliense, 
Micrasterias furcata and others, are exclusively confined to Older Paleozoic or 
Precambrian areas. As an instance, Micrasterias furcata is confined in the British 
Islands to North Wales, Sutherland, the Outer Hebrides, Donegal, and the extreme 
west of Galway. 
Probably most of the Desmids of the plankton were originally derived from the 
* We hope in a short time to give a full account of the phytoplankton of the loughs of the west and south-west of 
Treland, and also of the lakes of the English Lake District. 
