FLORA OF SCOTTISH LAKES 
167 
descriptions. Besides the above-mentioned, another reason for 
variation is presented in the case of aquatic plants that have within 
comparatively recent times undergone the transformation from 
terrestrial to aquatic habit. We may well suppose the character of 
such to be very variable and unstable — to be, in fact, veritable puzzles 
to the botanical collector. When we find in a plant such instability 
of character for no apparent reason, we may a posterioi^i assume the 
probability that a somewhat recent evolution has taken place from 
terrestrial progenitors. 
In many districts mountain lochs may be distinguished by the 
presence of certain plants, as, for example, Isoetes lacustris. Lobelia 
Dortmanna, Juncus fluitans, Callitriche hamulata, Sparganium 
minimum, etc., and by the absence of reeds at the margin. But in 
the liOch Ness Area such is not always the case, the presence or 
absence of any such plants being no criterion of the elevation of a 
loch. All the plants enumerated are to be found at so low a level as 
Loch Ness (52 feet above sea) ; and a reedy margin sometimes occurs 
in quite highland situations, whilst it is almost absent in such low- 
lying lochs as Oich and Ness. The reason for the presence or absence 
of certain plants is not altogether one of elevation, but is rather due 
to the supply of food-salts, and the amount of exposure of the water 
to winds, coupled with the nature of the shore. The mountain lochs 
usually drain a very small area, poor in food-salts and rich in acid 
humus ; consequently, only those plants are found in them that can 
obtain their requirements from an apparently scanty food-supply, 
combined with the presence of humic acids. Such plants are those 
that have been associated with mountain lochs. Lowland lakes 
usually drain a wider area, and soils poor in peat and rich in food- 
salts, which, although indispensable to most plants, are poison to 
others. In the area of Lochs Ness and Oich there is but a small 
amount of soil rich in food-salts available for drainage, compared 
with the soil poor in food-salts and rich in acid humus. Consequently, 
the effect of drainage from a small, rich food-area is almost ex- 
tinguished by the humic acids, and in such lowland lochs the 
vegetation is identical in species with that of the highest mountain 
lochs. Again, in Lochs Oich and Ness (and, of course, others) there is 
practically no reedy margin, neither does such a formation occur in 
many mountain lochs. The reason for this is the nature of the shore, 
combined with the erosive power of the waves, leaving food-supply 
altogether out of the question. On the other hand, in mountain 
lochs with a sheltered peaty or muddy shore, as in lowland lakes of 
like nature, there is a reedy or sedgy margin. Highland lochs are 
usually in situations fully exposed to the fierce winds, and their shores 
are rocky or stony ; consequently, they have few plants about their 
