iar sword-blade shape.

In the Sycamores, Maples (Fig. 16), and Horse-Chestnuts the arrangement
is altogether different. The shoots are stiff and upright with leaves
placed at right angles to the branches instead of being parallel to
them. The leaves are in pairs and decussate with one another; while the
lower ones have long petioles which bring them almost to the level of
the upper pairs, the whole thus forming a beautiful dome.

For leaves arranged as in the Beech the gentle swell at the base is
admirably suited; but in a crown of leaves such as those of the
Sycamore, space would be wasted, and it is better that they should
expand at once, so soon as their stalks have carried them free from the
upper and inner leaves.

[Illustration: Fig. 16.--Acer platanoides.]

In the Black Poplar the arrangement of the leaves is again quite
different. The leaf stalk is flattened, so that the leaves hang
vertically. In connection with this it will be observed that while in
most leaves the upper and under surfaces are quite unlike, in the Black
Poplar on the contrary they are very similar. The stomata or breathing
holes, moreover, which in the leaves of most trees are confined to the
under surface, are in this species nearly equally numerous on both.

The "Compass" Plant of the American prairies, a plant not unlike a small
sunflower, is another species with upright leaves, which growing in the
wide open prairies tend to point north and south, thus exposing both
surfaces equally to the light and heat. Such a position also affects the
internal structure of the leaf, the two sides becoming similar in
structure, while in other cases the upper and under surfaces are very
different.

In the Yew the leaves are inserted close to one another, and are linear;
while in the Box they are further apart and broader. In other cases the
width of the leaves is determined by what botanists call the
"Phyllotaxy." Some plants have the leaves opposite, each pair being at
right angles with the pairs above and below.

In others they are alternate, and arranged round the stem in a spiral.
In one very common arrangement the sixth leaf stands directly over the
first, the intermediate ones forming a spiral which has passed twice
round the stem. This, therefore, is known as the 2/5 arrangement. Common
cases are 1/2, 1/3, 2/5, 3/8, and 5/13. In the first the leaves are
generally broad, in the 3/8 arrangement they are elliptic, in the 5/13
and more complic