Page:Popular Science Monthly Volume 27.djvu/367

Rh somewhat to overlap; but it must be remembered that in temperate regions the sun is never vertical. Moreover, while alternate leaves are more convenient in such an arrangement as that of the beech, where there would be no room for a second leaf, it is more suitable in such cases as the sycamores and maples that the leaves should be opposite, because, if, other things remaining the same, the leaves of the sycamore were alternate, the sixth leaf would require an inconvenient length of petiole.

Perhaps it will be said that the plane-tree, which has leaves so like a maple that one species of the latter genus is named after it (Acer platanoides), has, nevertheless, alternate leaves. In reality, however, I think this rather supports my argument, because the leaves of the plane, instead of being at right angles to the stem, lie more nearly parallel with it. Moreover, as any one can see, the leaves are not arranged so successfully with reference to exposure as those of the species we have hitherto been considering, perhaps because, living as it does in more southern localities, the economy of sunshine is less important than in more northern regions.

The shoot of the horse-chestnut is even stouter than that of the sycamore, and has a diameter below the sixth leaf of no less than three tenths of an inch. With this increase of strength is, I think, connected the greater size of the leaves, which attain to as much as eighteen inches in diameter, and this greater size, again, has perhaps led to the dissection of the leaves into five or seven distinct segments, each of which has a form somewhat peculiar in itself, but which fits in admirably with the other leaflets. However this may be, we have in the horse-chestnut, as in the sycamores and maples, a beautiful dome of leaves, each standing free from the rest, and expanding to the fresh air and sunlight a surface of foliage in proportion to the stout, bold stem on which they are borne.

Now, if we place the leaves of one tree on the branches of another, we shall at once see how unsuitable they would be. I do not speak of putting a small leaf such as that of a beech on a large-leaved tree such as the horse-chestnut; but if we place, for instance, beech on lime, or vice versa, the contrast is sufficiently striking.

The lime-leaves would overlap one another, while, on the other hand, the beech-leaves would leave considerable interspaces. Or let us in the same way transpose those of the Spanish chestnut (Castanea) and those of Acer platanoides, a species of maple. I have taken specimens in which the six terminal leaves of a shoot of the two species occupy approximately the same area. Figs. 4 and 11 show the leaves in their natural position, those of Castanea lying along the stalk, while those of Acer are ranged round it. In both cases it will be seen that there is practically no overlapping, and very little waste of space. In Castanea the stalks are just long enough to give a certain play to the leaves. In Acer they are much longer, bringing the leaves