162 THE FRESH-WATER LOCHS OF "SCOTLAND 
acts inimically towards trees and most lowland grasses. These 
natural conditions have undoubtedly been unwittingly hastened 
during the past two thousand years by the destructive influence of 
man on forest. Then, in this country, forest is antagonistic to the 
grass-land of the lowlands because of climatic influences.^ 
It is the presence of the peat extract in the water that is the 
dominating factor governing the flora in peaty lochs. Its presence 
excludes directly a number of aquatic or semi-aquatic plants that 
might otherwise thrive there. It obliterates any small quantity of 
calcium carbonate that might be present, and thus renders the water 
untenable to calciphilous plants. On the other hand, certain calci- 
fugal plants, having become accustomed to tolerate the presence of 
humic acids, abound. I scarcely know that one should say the latter 
thrive the better through lack of competition with the former, for 
commonly it is not because competition for available space is so great, 
but because the local conditions favour the dominant production of a 
few individual species. 
By reason of the preserving action of humic acids, the organic 
remains in shallow water about the shores of the lakes do not readily 
decay, but undergo a slow process of disintegration, and form a sort 
of liquid peat. Owing to this action, suitably situated shallow places 
about peaty lakes become reclaimed by the growth of land-winning 
plants quicker than is the case in lakes that are free of humic acids. 
A similar preserving action prevents the rapid decay of organic remains 
at the bottom of the lakes, and in cases where the latter are com- 
paratively shallow, and where a large amount of foreign vegetable 
detritus is carried in by streams, etc., this substance accumulates at the 
bottom and prevents the development of plants that would otherwise 
thrive there. This is a common feature in the lochs of Area IV. 
The peat extract darkens the water, and this restricts the depth 
zone to which submersed aquatics will grow, because they are unable 
to carry on photosynthesis beyond a very limited depth, owing to 
want of light. In peaty water, therefore, the photic zone, throughout 
which there exists sufficient light for the proper development of the 
higher plants, does not extend to a greater depth than about 30 feet, 
and is frequently very much less than that. 
The extreme depth to which such plants as Nitella opaca and 
Fontinalis antipyretica will flourish in peaty water may roughly be 
estimated by multiplying by four the greatest depth at which one 
can see the gravel at the bottom, when looking over the shaded side 
of a boat about midday in the summer, when the sun is shining 
brilliantly, the water being perfectly calm, and the boat still. Such a 
1 Tlie involved complications bronglit about by these factors cannot be 
explained here, for want of space. 
