6 TRAP-FLORA OF RENFREWSHIRE. 
Next the rock is a thin layer of dead peat covered by white or 
grey lichen; a moss tuft comes next, and, through the mosses, 
stems of grass or blaeberry appear. 
4. Herbaceous stage.—At the end of this third stage (in the case 
of these rocks on the lower ground) we find the fourth stage 
beginning. 
In situations such as are these rock slopes on the low grounds, 
sphagnum does not set in. The growth of the herbaceous plants 
is aided by the disintegration of the rock, and also by rock-falls, 
and by rapid drainage. The net result is the mixture of the 
moss humus with sandy, stony, and other particles, and the 
formation of a very steep slope. The vegetation covering it is 
very thick, and a close not an open one. A whole series of 
plants can develop under these conditions. The most important 
are—Scabiosa, Galium verum, thyme, bluebell, Hypericum pul- 
chrum, Teucrium, Meum, violas, etc. All the plants of the 
Vaccinium group maintain their ground except Cad/una which 
generally vanishes. Some of the above list, notably JZewm, have 
special preferences. eum prefers the edge of a buried escarp- 
ment, where its long root stock can pierce deeply into the rock. 
The crown is protected against cold, heat, and drought by the 
extraordinary bushy rosette of dead leaves. 
5. Ulex stage-——The fifth stage (leguminous and bracken) 
consists of whin and broom. The plants, when young, are often 
crevice plants, and quite inconspicuous, but, as they grow older, 
they branch out so as to overshadow and kill out most of the 
grasses. Practically any one of the fourth group may occur 
amongst them ; many others are also found, e.g., Rumex acetosa, 
yarrow, o/cus, and other grasses, and also invading plants of 
the ordinary meadowland around them, such as ragweed, clovers, 
thistles and buttercups. 
Bracken is very characteristic of this stage, and also of the 
following. Its distribution is very peculiar. Generally it is at 
from four to five hundred feet altitude, and, I think, its presence 
is governed mainly by the exposure. Wind, I fancy, prevents 
bracken developing unless it is protected by other plants. 
I have this year seen the destruction of a very old whin-thicket, 
consisting of plants apparently five feet high, with woody stems 
often three inches in diameter or more. 
