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.
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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 Compound leaves. 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.
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| Fig. 14. | Fig. 15. | Fig. 16. | Fig. 17. |
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.
Fig. 17.—Sagittate leaf of Convolvulus.
The petiole or leaf-stalk is the part which unites the limb or blade of the leaf to the stem. It is absent in sessile leaves, and this is also frequently the case when a sheath is present, as in grasses (fig. 5). It consists of the fibro-vascular bundles with a Petiole. varying amount of cellular tissue. When the vascular bundles reach the base of the lamina they separate and spread out in various ways, as already described under venation. The lower part of the petiole is often swollen (fig. 20, p), forming the pulvinus, formed of cellular tissue, the cells of which exhibit the phenomenon of irritability. In Mimosa pudica (fig. 20) a sensitiveness is located in the pulvinus which upon irritation induces a depression of the whole bipinnate leaf, a similar property exists in the pulvini at the base of the leaflets which fold upwards. The petiole varies in length, being usually shorter than the lamina, but sometimes much longer. In some palms it is 15 or 20 ft. long, and is so firm as to be used for poles or walking-sticks. In general, the petiole is more or less rounded in its form, the upper surface being flattened or grooved. Sometimes it is compressed laterally, as in the aspen, and to this peculiarity the trembling of the leaves of this tree is due. In aquatic plants the leaf-stalk is sometimes distended with air, as in Pontederia and Trapa, so as to float the leaf. At other times it is winged, and is either leafy, as in the orange (fig. 21, p), lemon and Dionaea, or pitcher-like, as in Sarracenia (fig. 22). In some Australian acacias, and in some species of Oxalis and Bupleurum, the petiole is flattened in a vertical direction, the vascular bundles separating immediately after quitting the stem and running nearly parallel from base to apex. This kind of petiole (fig. 23, p) has been called a phyllode. In these plants the laminae or blades of the leaves are pinnate or bipinnate, and are produced at the extremities of the phyllodes in a horizontal direction; but in many instances they are not developed, and the phyllode serves the purpose of a leaf. These phyllodes, by their vertical position and their peculiar form, give a remarkable aspect to vegetation. On the same acacia there occur leaves with the petiole and lamina perfect; others having the petiole slightly expanded or winged, and the lamina imperfectly developed; and others in which there is no lamina, and the petiole becomes large and broad. Some petioles are long, slender and sensitive to contact, and function as tendrils by means of which the plant climbs; as in the nasturtiums (Tropaeolum), clematis and others; and in compound leaves the midrib and some of the leaflets may similarly be transformed into tendrils, as in the pea and vetch.
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The leaf base is often developed as a sheath (vagina), which embraces the whole or part of the circumference of the stem (fig. 5). This sheath is comparatively rare in dicotyledons, but is seen in umbelliferous plants. It is much more common amongst monocotyledons. In sedges the Leaf base. sheath forms a complete investment of the stem, whilst in grasses it is split on one side. In the latter plants there is also a membranous outgrowth, the ligule, at right angles to the median plane of the leaf from the point where the sheath passes into the lamina, there being no petiole (fig. 5, l).
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Fig. 20.—Branch and leaves of the Sensitive plant (Mimosa pudica), showing the petiole in its erect state, a, and in its depressed state, b; also the leaflets closed, c, and the leaflets expanded, d. Irritability resides in the pulvinus, p. |
In leaves in which no sheath is produced we not infrequently find small foliar organs, stipules, at the base of the petiole (fig. 24, s). The stipules are generally two in number, and they are important as supplying characters in certain natural orders. Thus they occur
