e, 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 _aestivation_, 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