[Illustration: FIG. 10.--Five-partite leaf of Aconite.]
[Illustration: FIG. 11.--Pedate leaf of Stinking Hellebore
(_Helleborus foetidus_). The venation is radiating. It is a
palmately-partite leaf, in which the lateral lobes are deeply divided.
When the leaf hangs down it resembles the foot of a bird, and hence
the name.]
When in a radiating leaf there are three primary partitions, and the
two lateral lobes are again cleft, as in hellebore (fig. 11), the leaf
is called _pedate_ or _pedatifid_, from a fancied resemblance to the
claw of a bird. In all the instances already alluded to the leaves
have been considered as flat expansions, in which the ribs or veins
spread out on the same plane with the stalk. In some cases, however,
the veins spread at right angles to the stalk, forming a _peltate_
leaf as in Indian cress (fig. 12).
The form of the leaf shows a very great variety ranging from the
narrow _linear_ form with parallel sides, as in grasses or the
needle-like leaves of pines and firs to more or less rounded or
_orbicular_--descriptions of these will be found in works on
descriptive botany--a few examples are illustrated here (figs. 7, 13,
14, 15). The apex also varies considerably, being rounded, or
_obtuse_, sharp or _acute_ (fig. 7), notched (fig. 15), &c. Similarly
the shape of the base may vary, when rounded lobes are formed, as in
dog-violet, the leaf is cordate or heart-shaped; or kidney-shaped or
_reniform_ (fig. 16), when the apex is rounded as in ground ivy. When
the lobes are prolonged downwards and are acute, the leaf is
_sagittate_ (fig. 17); when they proceed at right angles, as in _Rumex
Acetosella_, the leaf is _hastate_ or halbert-shaped. When a simple
leaf is divided at the base into two leaf-like appendages, it is
called _auriculate_. When the development of parenchyma is such that
it more than fills up the spaces between the veins, the margins become
_wavy_, _crisp_ or _undulated_, as in _Rumex crispus_ and _Rheum
undulatum_. By cultivation the cellular tissue is often much
increased, giving rise to the _curled_ leaves of greens, savoys,
cresses, lettuce, &c.
[Illustration: FIG. 12.--Peltate leaves of Indian Cress (_Tropaeolum
majus_).]
[Illustration: FIG. 13.--Lanceolate leaf of a species of Senna.]
Compound leaves.
Compound leaves are those in which the divisions extend to the midrib
or petiole, and the separated portions become each articulated with
it, and receive the name of _leaflets_. The midrib, or petiole, has
thus the appearance of a branch with separate leaves attached to it,
but it is considered properly as one leaf, because in its earliest
state it arises from the axis as a single piece, and its subsequent
divisions in the form of leaflets are all in one plane. The leaflets
are either sessile (fig. 18) or have stalks, called _petiolules_ (fig.
19). Compound leaves are pinnate (fig. 19) or palmate (fig. 18)
according to the arrangement of leaflets. When a pinnate leaf ends in
a pair of pinnae it is _equally_ or _abruptly pinnate_ (paripinnate);
when there is a single terminal leaflet (fig. 19), the leaf is
_unequally pinnate_ (imparipinnate); when the leaflets or pinnae are
placed alternately on either side of the midrib, and not directly
opposite to each other, the leaf is _alternately pinnate_; and when
the pinnae are of different sizes, the leaf is _interruptedly
pinnate_. When the division is carried into the second degree, and the
pinnae of a compound leaf are themselves pinnately compound, a
bipinnate leaf is formed.
[Illustration:
FIG. 14.--Oblong leaf of a species of Senna.
FIG. 15.--Emarginate leaf of a species of Senna. The leaf in its
contour is somewhat obovate, or inversely egg-shaped, and its base
is oblique.
FIG. 16.--Reniform leaf of _Nepeta Glechoma_, margin crenate.