FLORA OF SCOTTISH LAKES 
219 
yellow, and the zone to which its abundance is restricted presents a 
remarkable appearance. This zone is in reality at the ancient water- 
level of the loch previous to a reduction of its level by about 7 feet 
some 150 years ago. This lowering of the level was brought about 
by the construction of two tunnels for the effluent, instead of the 
natural outflow, for the double purpose of reclaiming land at the 
margin and admitting salmon to the loch. Why the Lecidea should 
be so abundant at the old water-level I am unable to explain. 
I dredged this loch in a great many places from end to end, but 
beyond an average depth of about 7 feet I could obtain no evidence of 
the existence of any living plants at the bottom. Yet in suitable 
places the bottom from 2 to 7 feet deep often bears an abundant 
vegetation, which occasionally may be continued into shallower 
water ; instance Littorella lacustris in a few little sheltered sandy 
creeks. The extinction of the submersed bottom flora at so shallow a 
depth as 7 feet is distinctly remarkable, because it is not brought 
about, as in some cases (p. 208), by the discoloration of the water. 
Here the bottom can be seen at a depth of 7 feet, when looking 
over the side of a boat, without the use of any apparatus beyond 
shading the eyes with one's hat in order to shut out the light reflected 
from the surface of the water. Reasoning, therefore, from similar cases 
of translucency, some vegetation should extend to a depth of 28 feet 
or more (p. 163). It has already been indicated (p. 217) that the grass- 
like associations of plants which cover the moors and mountains have 
an influence upon the flora of the lochs in this Area. At the first 
consideration one would imagine that the influence exercised upon 
the bottom flora of a loch by the substitution of grass-like plants 
over the moors for associations of Ericaceiis would be that of less 
peat extract getting into the water. Such, however, is scarcely the 
case, because the moors have an abundance of ancient peat below the 
grass, formed there previous to the development of the sheep-rearing 
industry. The influence is caused in a way one would little expect. 
In winter the dead leaves of the grass-like plants covering the moors, 
chiefly Molinia casrulea, which grows very luxuriantly here, but also 
Nardus stricta, Scirpus c^espitosus, etc., are blown or washed into the 
burns and drains, and are thence carried into the lochs. There, 
owing to the antiseptic action of the peaty water, these remains do 
not readily decay, but accumulate from year to year, and become 
spread out over the loch bottom in enormous quantity, and, of 
course, this stratum of dead grass prevents the growth of a bottom 
flora wherever it lies. The depth to which its influence extends 
varies somewhat in different lochs, and even in any one particular loch. 
In Loch Doon, at the south end, where the loch receives its principal 
supply of this grass from the rivers Gala Lane and Carrick Lane^ 
