more killing out the overshadowed undershrubs.
One must remember that wherever nature has free play, instead of being
controlled by the hand of man, dense forest covers every acre of ground
where the soil is deep enough; gorse, whins, and heather, or their
equivalents grow wherever the forest fails; and herbs can only hold
their own in the rare intervals where these domineering lords of the
vegetable creation can find no foothold. Meadows or prairies occur
nowhere in nature, except in places where the liability to destructive
fires over wide areas together crushes out forest trees, or else where
goats, bison, deer, and other large herbivores browse them ceaselessly
down in the stage of seedlings. Competition for sunlight is thus even
keener perhaps than competition for foodstuffs. Alike on trees, shrubs,
and herbs, accordingly the arrangement of the leaves is always exactly
calculated so as to allow the largest possible horizontal surface, and
the greatest exposure of the blade to the open sunshine. In trees this
arrangement can often be very well observed, all the leaves being
placed at the extremities of the branches, and forming a great
dome-shaped or umbrella-shaped mass, every part of which stands an even
chance of catching its fair share of carbonic acid and solar energy.

The shapes of the leaves themselves are also largely due to the same
cause, every leaf being so designed in form and outline as to interfere
as little as possible with the other leaves on the same stem, as
regards supply both of light and of carbonaceous foodstuffs. It is only
in rare cases, like that of the water-lily, that perfectly round leaves
occur, because the conditions are seldom equal all round, and the
incidence of light and the supply of carbon are seldom unlimited. But
wherever leaves rise free and solitary into the air, without mutual
interference, they are always circular, as may be well seen in the
common nasturtium and the English pennywort. On the other hand, among
dense hedgerows and thickets, where the silent, invisible struggle for
life is fierce indeed, and where sunlight and carbonic acid are
intercepted by a thousand competing mouths and arms, the prevailing
types of leaf are extremely cut up and minutely subdivided into small
lace-like fragments. The plant in such cases can't afford material to
fill up the interstices between the veins and ribs which determine its
underlying architectural structure. Often indeed species w