Jan. 1911.] BOTANICAL SOCIETY OF EDINBURGH ret 
with a certain amount of sand and silt. The stones are 
of all sizes, very large ones, and indeed rocks, being quite 
common. They are arranged most unevenly, though the 
general surface is flat. Such a soil is deficient in available 
nutritive salts, and in itself provides merely desert con- 
ditions for plant-life, no matter how frequent the downpour. 
With the drying of the river-bed, and especially of those 
parts liable to frequent submersion, the silt, which is com- 
posed of very fine particles, is moved by quite a gentle 
breeze, while, during the frequent gales, it, together with 
the finer sand, is deposited in the lee of rocks and caught 
by the vegetation. The silt supply being constantly re- 
newed by floods, the amount of light soil which is conveyed 
by the wind to the old flood-plain is very considerable and 
has led to a thin coating of light silky loam which, though 
far from being a fertile soil, is quite sufficient to support, 
with the addition of the self-supplied humus, a closed 
formation of those plants which are provided with certain 
“ adaptations.” 
The stony soil is porous to the extreme; the shallow 
silty soil, too, dries rapidly, and its water-content is usually 
small. But the frequent rain renders the surface wetter 
than might be expected, and the humus, which finally 
becomes an important constituent, is able to absorb and 
hold abundant water. Beneath the surface, just under 
the top layer of stones, the soil is generally more or less 
inoist, the stones hindering evaporation. The larger stones 
and rocks also play an important part in checking evapora- 
tion, both from the shelter from wind and the shade from 
sun which they afford. These aids towards conserving the 
rain-water are of special importance, since, as will be 
shown, it possibly forms the chief water-supply of the 
plants. 
There is obviously no lack of underground water, the 
water-table, except in winter, lying very near the surface, 
the distance, however, depending on the height of the 
older flood-plain and the distance from the sides of the 
valley. This abundance of water arises from the fact that 
the streams of the river, numerous and rapid though they 
are, represent only a small portion of the actual river- 
water, so porous are their beds; in fact, the greater part 
TRANS. BOT. SOC. EDIN. VOL. XXIV. . 
