Further Observations on Heath Association, i 25 
On the bare areas caused by landslides (3, p. 161) we have 
extreme exposure as a permanent condition, the average daily 
evaporation at the ground-level being 2-12 cc. and at the 3 ft. level 
3 cc. In comparing these figures with those from the burnt stretches 
it must be borne in mind that the areas under consideration are 
situated in the trough formed by valley B. 
Temperature-readings have shown that on sunny days the bare 
surfaces become heated to such an extent that just after midday the 
temperature of the top-crust is, on the average, about 2° C. above 
that of the air, whereas in the closed association it is approximately 
4'5° C. below or, where provided with a covering of moss as much 
as 5*5° C. below. The great heating of the soil on exposed areas 
will naturally lead to much greater loss of water than in the closed 
association, so that even if the soil had the same water-capacity 
and were sheltered, it would necessarily be drier on the exposed 
area. 
(2) Wind-trimming. 
The wind-trimming in the case of Calluna to which we have 
already referred bears testimony to the adverse conditions at some 
height from the ground. The same phenomenon can be observed 
in all the true heath-representatives wherever they are subjected to 
such exposure. 
The most general type of growth-form resulting from exposure 
is that seen on the burnt areas, particularly in the case of Ulex 
minus in which the uprising branches are relatively short and closely 
aggregated together, extension being mainly in the horizontal 
direction. The whole therefore forms a slightly convex cushion or 
hummock. Young plants of most of the shrubby species in the more 
exposed situations almost invariably exhibit a prostrate habit, due 
to the relative unimportance of the main axis (which in the case of 
U. nanus generally dies away) and the development of numerous 
laterals in close contact with the surface. It has been observed that 
seedlings of U. nanus growing in hollows of burnt areas sometimes 
exhibit a vertical development of the main axis until the level of 
the surrounding soil is reached, whereupon growth of the main axis 
is arrested and horizontal laterals are produced. The prostrate type, 
although particularly characteristic of burnt areas, is often retained 
to a more or less marked extent, on the upper slopes, even in the 
closed association. 
