THB NATURAL REGENERATION OF WOODS. 65 
where the surface is thickly strewn with large boulders, or where 
the bare rock projects in large masses, because so long as a close 
wood is preserved the leaves accumulate to form valuable plant- 
food, and, in coniferous woods, moss grows in great luxuriance, 
so that a good seed-bed is provided even on what would otherwise 
be a bare rock-surface, and the young seedlings ultimately push 
their roots into cracks or crevices, or ramify over the rocky 
surface till they get at better soil, and so large well-grown trees 
are not unfrequently to be seen in situations which they could 
never have occupied had they not been self-sown. Let such an 
area be cleared of its trees even for a few years, and the moss and 
humus will, under the combined influence of sun, wind, and 
water, entirely disappear from the rocks and boulders, for shade 
and a humid atmosphere, such as are to be found in a dense 
wood, are absolutely necessary for the formation of forest humus, 
and the production of a rank growth of mosses. The fact must 
not be overlooked that rocks covered by decaying vegetable 
matter or mosses weather much more rapidly than where 
unprovided with a covering, for the carbonic acid gas resulting 
from decomposition of the humus, and the solutions exuded from 
the roots of plants, act far more powerfully than rain or the 
atmosphere in corroding rocks and rendering their ingredients 
soluble, 
When a locality, in the enjoyment of unrestricted sunlight, is 
apt to produce a very rank growth of ground vegetation, such as 
ferns, heather, blackberries, large grasses, etc., it is often 
advantageous to restock by self-sowing before felling all the 
old trees, for in this way the weeds are kept in check till the 
young trees have had time to surmount them, ‘This is often 
the case on moist, rich, loamy soil, especially if calcareous. Here 
such a rank growth of herbaceous vegetation would spring up 
after clear-felling, that small slowing-growing trees would be apt 
to be smothered, or one would be compelled to give them light 
and air by mowing the weeds, unless recourse was had to the 
expensive operation of restocking with extra large trees. 
3. A third case where natural regeneration may be advan- 
tageously resorted to occurs where we have to deal with species of 
trees which are very liable to be injured in youth by too powerful 
atmospheric agencies. This is the case more especially with the 
beech and silver fir. When young both these trees are very 
sensitive to frost, heat, and drying winds, from which they can 
VOL. XIII. PART I. E 
