1885.] FORMS OF LEAVES. 333 



inches. The di.stance between the buds is about 1^ inch, and the 

 leaves lie in the general plane of the braucli, which bends slightly 

 at each internode. The basal half of the leaf fits the swell of the 

 twig, while the upper half follows the edge of the leaf above ; and 

 the form of the inner edge being thus determined, decides that of 

 the outer one also. In the lime, the internodes are longer and tlie 

 leaf consequently broader. In the Spanish chestnut, the stem is 

 nearly three times as stout as that of the beech, and consequently 

 can carry a larger leaf surface. But the distances between the 

 buds are often little greater than those in the beech. This deter- 

 mines, then, the width, and by compelling the leaf to lengthen itself, 

 leads to the peculiar form which it assumes. Moreover, not only 

 do the leaves on a single twig admirably fit one another, but they 

 are also adapted to tlie ramification of the twigs themselves, and 

 thus avail themselves of the light and air, as we can see by the 

 shade they cast without large interspaces or much overlai)ping. In 

 the sycamores, maples, and horse chestnuts, the arrangement is 

 altogether different. The shoots are stiff and upright, with leaves 

 placed at right angles to the plane of the branch, instead of being 

 parallel to it. 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 adniii'ably suited ; but in a crown of leaves, such as those of 

 the sycamore, space would be thereby wasted, and it is better that 

 they should expand at once, as soon as their stalks have carried 

 them free from the upper and inner leaves. Hence we see how 

 beautifully the whole form of these leaves is adapted to the mode 

 of growth of, and arrangement of the buds in, the plants them- 

 selves. 



In the black poplar, the arrangement of the leaves is again 

 quite different. Tlie leaf stalk is flattened, so that the leaves hang 

 vertically. In connection with tliis, 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 yellow composite not unlike a small sunflower, is another 

 plant with upriglit leaves, which, growing in the wide, open prairies, 

 tend to point nortli and south, thus exposing both surfaces equally 

 to the light and heat. It was shown by diagrams that this posi- 

 tion also affected the internal structure of the leaf. 



