164 THE FRESH- WATER LOCHS .OF SCOTLAND 
tribution of submerged plants over the bottom of a lake is the amount 
of suitable light available. It is, therefore, usual to distinguish three 
different zones according to the intensity of the received light. These 
zones vary in magnitude in different localities, in accordance with the 
amount of suspended matter in the water, together with the declivity 
of the sides of the lake-basin. The following are recognisable : — 
(1) A photic zone, throughout which there exists sufficient light 
for the proper development of the higher flora. This zone may extend 
to a depth of 40 feet as at Lismore, to only 12 feet as at Loch Kemp, 
or in many lochs even much less. Deeper than this zone there is — 
(2) A dysphotic zone, in which a few of the higher plants, stragglers 
from the photic zone, may struggle to exist. This zone is normally 
occupied by members of the lower cryptogamic flora that are able to 
thrive with a minimum of light. Deeper still there is — 
(3) An aphotic zone, in which no light-demanding organism can 
exist. 
The last glacial epoch, after destroying the vegetation of Scot- 
land, immediately began the formation of more numerous lake-basins for 
the reception of a greater aquatic flora after its disappearance. Not 
only this, but other results of glaciation are found actually dominat- 
ing the vegetation in certain of the lochs at the present moment. 
At liOchs Oich and Lochy, for example, the sides of the adjacent 
mountains are coated with glacial drift-gravel. This gravel is brought 
into the lochs by the numerous burns in great abundance, and 
deposited upon the shores. Under the erosive power of the waves, 
the constant movement of this gravel upon the littoral entirely prevents 
the growth of aquatic phanerogams over a considerable area of the 
margin of the lochs. Again, in many places a steep escarpment, due 
to glacial action, enters a lake immediately, so that water too deep 
for phanerogams occurs without any shore whatever ; instance Loch 
Ness opposite Invermoriston, where a depth of 652 feet may be sounded 
at 120 yards from the margin. Here we see the indirect effect of a 
past epoch upon the flora of existing lakes, the lakes themselves being 
the direct result of that period. 
Climate also affects the local distribution of the plants in each loch 
more or less. The prevailing and frequently strong winds are 
westerly ; consequently there is upon the eastern shore of a lake a very 
considerable and oft-recurring wave-action. Acting upon a rocky or 
stony shore, this erosive power entirely prevents the growth of the 
higher plants in the shallow water where its influence is felt. Unless 
sheltered by adjacent hills, all the lakes will therefore be almost or 
quite devoid of vegetation on their eastern shores, whilst the western 
shores, and bays sheltered from the prevailing wind, may have an 
abundant vegetation. The Algae of the seashore may be cited as an 
