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 corzlate or heart-shaped; or kidney-shaped or renzforrn (fig. 16), when the apex is rounded as in ground ivy. When the lobes are prolonged downwards and are acute, the leaf is sagtttate (fig. I7); when they proceed at right angles, as in Rurnex Acetosella, the leaf is hastate or halbertshaped. 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 'wat/y, crisp or undulated, as in Rumex rrispus and Rheum undulatum. By cultivation the cellular tissue is often much increased, giving rise to the curled leaves of greens, savoys, cresses, lettuce, &c. Compound leaves are those in which the divisions extend to the Com Gund midrib or petiole, and the sepahav; rated portions become each articulated with it, and receive the name of leaflets. The midrib, or petiole, has thus the appearance of a branch with 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 leafstalk is sometimes distended with air, as in 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 Dtouuea, or pitcher like, as in Sarracenta (fig. 22). In some I Australian acacias, and in some species of Oxalis and Bupleururn, the petiole is flattened f 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 pin f nate or bi pinnate, and are produced at the Porttederza and T rapa,

FIG. 13.-Lanceolate

FIG.12.*PClt&t€lC&V€SOftlI1(li3, l'1CI'€SS leaf of a species of (Tropaeolum majus). Senna. V 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. IQ). 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 pirmate (paripinnate); when there is a single terminal leaflet (fig. 19), the leaf is unequally pirmate (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 pirmate; and when the pinnae are of different sizes, the leaf is tnterruptedty ptnrtate. When the division W J FIG. 14. FIG. 15. FIG. 16. FIG. 17. FIG. 14.-Oblong leaf of a species of Senna. FIG. I 5.-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. .-Reniform leaf of Nepeta Gtechoma, margin crenate. FIG. 17.-Sagittate leaf of Convolvulus. is carried into the second degree, and the pinnae of a compound leaf are themselves innately compound, a bi pinnate leaf is formed. 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 Pe” I frequently the case when a sheath is present, as in grasses oe' (fig. 5). It consists of the fibro-vascular bundles with a tissue. When the vascular bundles reach separate and spread out in various ways, venation. The lower part of the petiole forming the pulvinus, formed of cellular varying amount of cellular the base of the lamina they as already described under is often swollen (fig. 20, p), tissue, the cells of which exhibit the phenomenon of irritability. a sensitiveness is located in the pulvinus In .Mimosa pudica (fig. 20) which upon irritation induces a depression of the whole bi pinnate 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 "fn' " Pt?

i/H?

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/Al Q % /3 ”@ 1 % ” / ll. @ fit;, Q 1 ' ' '” V

 // / /

<f;l¢f 1 I if K FIG. 1 9.- Imparipinnate (unequally pinnate) leaf of Robinia. There are nine pairs of shortly-stalked leaflets (foliola, pinnae), and an odd one at the extremity. At the base of the leaf the spiny stipules are seen. 4 2 FIG. 18.-Palmately compound leaf of the Horse-chestnut (Aesculus Htppocastanurn). 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 211asturt;um)s » ropaeo um, 5~°° /ya. 4-. Clematis and r /' /“ ' . others; and in / compound leaves / .b the midrib and / 1' ' g some of the leaf / 4 7lets may similarly be transformed '§ into tendrils, as in ' Q the pea and vetch. The leaf base is often de- ~, fsveloped as a . k sheath (vagina), t which embraces the whole or part of the circumference of the stem

§§ £lth5l}S Irr1tab1l1ty resides in the pulvinus, p.,

FIG.20.-B1'8.I1Cl1 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 tl. paratively rare in dicotyledons, but is seen in umbelliferous plants. It is much more common amongst monocotyledons. In sedges the sheath forms a complete investment of the stem, whilst in L fb grasses it is split on one side. In the latter plants there is ee 'He also a membranous outgrowth, the ltgule, 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). 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