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 complicated 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.
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.