ROBERT SMITH ON PLANT ASSOCIATIONS OF THE TAY BASIN. 209 
The birch is a tree which withstands the cold exceedingly well. 
The average temperature at which its leaves appear is about 9° C. 
(Drude, p. 249), and according to Kihlmann it is not the cold alone 
which limits its distribution, but rather the cold combined with the 
drying wind of winter or of early spring. 
Its general distribution is very wide, including Central and North 
Europe, Russia, and Siberia even to Kamtschatka. Connected with 
this wide distribution we have the fact that its habitat may be very 
varied. In our valley, we see it growing on the sands of Tentsmuir, 
in the moist humus of beech and oak woods, on the slopes of 
Birnam, the rocks of Craigiebarns and other similar places, and on 
the cold, wet Highland moors. 
On enclosed ground, on the moors and hills, the birch springs 
up spontaneously much more freely than any other tree. From 
the amount of remains of birch trunks and branches in the peat¬ 
mosses, this tree must once have covered a very great portion of 
our moorlands. 
The trees afford little shade, and the associated species are largely 
those of the bare heaths; the shelter, however, usually ensures a 
slightly richer growth. 
The Heather, Calluna Erica^ D.C.—This is the most charac¬ 
teristic social species of Scotland. In the Highlands, it covers mile 
after mile with an almost unbroken brown or purple mantle. The 
study of the life-conditions of the heather association is of special 
interest to Scottish botanists, for we are in the region of its maximum 
development. We find the same association in England and Ireland, 
in Scandinavia and Germany, but as we pass eastwards and south¬ 
wards it diminishes in importance (Krause). Outside of this area, 
North-West Europe, the Calluna shrub rarely appears in the long 
unbroken stretches so familiar to us. In Russia, it is a secondary 
element in the pine forests. In the Alps and Carpathians, we find 
it in the woods of coniferous trees. In the Cevennes, Professor 
Flahault reports it as sub-dominant in the Spanish chestnut woods. 
I have found it near its southern limit in the Esterel Mountains in 
the French Riviera. There it was in a forest of Pinus pinaster, Ait., 
and only assumed an important place in the sub-dominant vegetation 
as we ascended the northern slope of the hills. 
Numerous German investigators have studied and attempted to 
explain the great development of Calluna heath in North-West 
Europe. Climate seems partly the cause, for the limits of the plant 
in Russia are marked by a change from the moist conditions obtain¬ 
ing in North-West Europe to those of the continental steppes; and 
the fact that it is mostly found in woods in Russia may be ascribed to 
