ated arrangements nearly linear. The Willows afford a
very interesting series. Salix herbacea has the 1/3 arrangement and
rounded leaves, Salix caprea elliptic leaves and 2/5, Salix pentandra
lancet-shaped leaves and 3/8, and S. incana linear leaves and a 5/13
arrangement. The result is that whether the series consists of 2, 3, 5,
8, or 13 leaves, in every case, if we look perpendicularly at a twig the
leaves occupy the whole circle.

In herbaceous plants upright leaves as a rule are narrow, which is
obviously an advantage, while prostrate ones are broad.

[Illustration: AQUATIC VEGETATION, BRAZIL. _To face page 145._]

AQUATIC PLANTS

Many aquatic plants have two kinds of leaves; some more or less rounded,
which float on the surface; and others cut up into narrow segments,
which remain below. The latter thus present a greater extent of surface.
In air such leaves would be unable even to support their own weight,
much less to resist the force of the wind. In still air, however, for
the same reason, finely-divided leaves may be an advantage, while in
exposed positions compact and entire leaves are more suitable. Hence
herbaceous plants tend to have divided, bushes and trees entire, leaves.
There are many cases when even in the same family low and herb-like
species have finely-cut leaves, while in shrubby or ligneous ones they
more or less resemble those of the Laurel or Beech.

These considerations affect trees more than herbs, because trees stand
more alone, while herbaceous plants are more affected by surrounding
plants. Upright leaves tend to be narrow, as in the case of grasses;
horizontal leaves, on the contrary, wider. Large leaves are more or less
broken up into leaflets, as in the Ash, Mountain-Ash, Horse-Chestnut,
etc.

The forms of leaves depend also much on the manner in which they are
packed into the buds.

The leaves of our English trees, as I have already said, are so arranged
as to secure the maximum of light; in very hot countries the reverse is
the case. Hence, in Australia, for instance, the leaves are arranged not
horizontally, but vertically, so as to present, not their surfaces, but
their edges, to the sun. One English plant, a species of lettuce, has
the same habit. This consideration has led also to other changes. In
many species the leaves are arranged directly under, so as to shelter,
one another. The Australian species of Acacia have lost their true
leaves, and the parts which in them