Page:Popular Science Monthly Volume 27.djvu/363

Rh situated some distance apart, and though, as shown in the table, they are small in comparison to the diameter of the stem, still they attain a size of fifteen square inches, or even more. Now, if they were of the same form as the ordinary pear-leaf, they would be about seven inches long by two to three in breadth. The mountain-ash, as we know, lives in mountainous and exposed localities, and such a leaf would be unsuitable to withstand the force of the wind in such situations. From this point of view, the division into leaflets seems a manifest advantage.

Perhaps it will be said that in some trees the leaves are much more uniform in size than in others. This is true. The sycamore, for instance, varies greatly; in the specimen tabulated, the stem was ·13 in diameter, and the area of the six upper leaves was sixty square inches. In another, the six upper leaves had an area of rather over one hundred inches, but in this case the diameter of the stem was ·18.

Another point is the length of the internode. In such trees as the beech, elm, hornbeam, etc., the distance from bud to bud varies comparatively little, and bears a tolerably close relation to the size of the leaf. In the sycamore, maple, etc., on the contrary, the length varies greatly.

Now, if, instead of looking merely at a single leaf, we consider the whole bough of any tree, we shall, I think, see the reason of their differences of form.

Let us begin, for instance, with the common lime (Fig. 1). The leaf-stalks are arranged at an angle of about 40° with the branch, and the upper surfaces of the leaves are in the same plane with it. The



result is, that they are admirably adapted to secure the maximum of light and air. Let us take, for instance, the second or third leaf in Fig. 1. They are four and a half inches long and very nearly as broad. The distance between the two leaves on each side is also just four and a half inches, so that they exactly fill up the interval. In Tilia parvifolia the arrangement is similar, but leaves and internodes are both less, the leaves, say, one and a half inch, and the internodes ·6.