In the Beech, the leaf is plicate, or plaited on the veins. In the Elm, Magnolia, and Tulip-tree, it is conduplicate, that is, folded on the midrib with the inner face within. In the Tulip-tree, it is also inflexed, the blade bent forwards on the petiole. In the Balm of Gilead, the leaf is involute, rolled towards the midrib on the upper face.
Other kinds of vernation are revolute, the opposite of involute, where the leaf is rolled backwards towards the midrib; circinate, rolled from the apex downwards, as we see in ferns; and corrugate, when the leaf is crumpled in the bud.
[Illustration: FIG. 20.—Branch of Norway Spruce.]
In all the trees we have studied, the leaves simply succeed each other, each leaf, or pair of leaves, overlapping the next in order. The names of the overlapping of the leaves among themselves, imbricated, convolute, etc., will not be treated here, as they are not needed. They will come under æstivation, the term used to describe the overlapping of the modified leaves, which make up the flower.[1]
[Footnote 1: Reader in Botany. VIII. Young and Old Leaves.]
3. Phyllotaxy. The subject of leaf-arrangement is an extremely difficult one, and it is best, even with the older pupils, to touch it lightly. The point to be especially brought out is the disposition of the leaves so that each can get the benefit of the light. This can be seen in any plant and there are many ways in which the desired result is brought about. The chief way is the distribution of the leaves about the stem, and this is well studied from the leaf-scars.
The scholars should keep the branches they have studied. It is well to have them marked with the respective names, that the teacher may examine and return them without fear of mistakes.
In the various branches that the pupils have studied, they have seen that the arrangement of the leaves differs greatly. The arrangement of leaves is usually classed under three modes: the alternate, the opposite, and the whorled; but the opposite is the simplest form of the whorled arrangement, the leaves being in circles of two. In this arrangement, the leaves of each whorl stand over the spaces of the whorl just below. The pupils have observed and noted this in Horsechestnut and Lilac. In these there are four vertical rows or ranks of leaves. In whorls of three leaves there would be six ranks, in whorls of four, eight, and so on.
When the leaves are alternate, or single at each node of the stem, they are arranged in many different ways. Ask the pupils to look at all the branches with alternate leaves that they have studied, and determine in each case what leaves stand directly over each other. That is, beginning with any leaf, count the number of leaves passed on the stem, till one is reached that stands directly over the first.[1] In the Beech and the Elm the leaves are on opposite sides of the stem, so that the third stands directly over the first. This makes two vertical ranks, or rows, of leaves, dividing the circle into halves. It is, therefore, called the 1/2 arrangement. Another way of expressing it is to say that the angular divergence between the leaves is 180°, or one-half the circumference.