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. In the Black Poplar the arrangement of the leaves is again quite different. The leaf stalk is flattened, so that the leaves hang Fig. 16. - Acer platanoides. 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 arrangement. Common cases are,,,, 13. and In the first the leaves are generally broad, in the arrangement they are elliptic, in the and more complicated arrangements nearly linear. The Willows afford a very interesting series. Salix herbacea has the arrangement and rounded leaves, Salix caprea elliptic leaves and 3, Salix pentandra lancetshaped leaves and 3, and S. incana linear leaves and a 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 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. |