Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/119

Rh LEAVES.] BOTANY 09 elongated in a direction parallel to the surface of the leaf, so as to be fibre-like. These constitute a hypodermal layer, beneath which the chlorophyll cells of the parenchyma are densely packed together, and are elongated in a direction vertical to the surface of the leaf ; this has been termed palisade tissue. The form and arrangement of the cells, however, depend much on the nature of the plant, and its exposure to light and air. Sometimes the arrangement of the cells on both sides of the leaf is similar, as occurs in leaves which have their edges presented to the sky. In very succulent plants the cells form a compact mass, and those in the centre are often colourless. In some cases the cellular tissue is deficient at certain points, giving rise to distinct holes in the leaf, as in Monstera Adansonii ; such a leaf has been called perfuse. In Victoria regia perfora tions in the leaf seem to be subservient to the purposes of nutrition, by permitting the gases collected beneath the large expanded leaf to escape, and thus allowing its under surface to be brought into immediate contact with the water. The fibre-vascular system in the leaf constitutes the venation. The fibre-vascular bundles from the stem bend out into the leaf, and are there arranged in a definite manner. They usually form two layers, which may be separated by maceration. In skeleton leaves, or leaves in which the parenchyma is removed, these layers are well seen. In some leaves, as in the Barberry, the vessels forming the veins are hardened, producing spines without any parenchyma. The hardening of the extre mities of the vascular tissue is the cause of the spiny margin of many leaves, such as the Holly, of the sharp- pointed leaves of Madder, and of mucronate leaves, or those having a blunt end with a hard projection in the centre. Submerged leaves, or leaves which are developed under water, differ in structure from aerial leaves. They have usually no nbro-vascular system, but consist of a congeries of cells, which sometimes become elongated and compressed so as to resemble veins. They have a layer of compact cells on their surface, but no true epidermis, and no stomata. Their internal structure consists of cells, disposed irregularly, and sometimes leaving spaces which are filled with air for the purpose of floating the leaf. When exposed to the air these leaves easily part with their moisture, and become shrivelled and dry. In some instances there is oidy a network of filamentous-like cells formed, the spaces between which are not filled with parenchyma, giving a peculiar skeleton appearance to the leaf, as in Ouvirandra fenestralis (Lattice plant). Such a leaf has been called fcnestrate. A leaf, whether aerial. or submerged, generally consists of a flat expanded portion (fig. 99, I), called the blade, limb, or lamina, of a narrower portion called the petiole or stfilk (fig. 99, p), and sometimes of a portion at the base of the petiole, which forms a sheath or vagina (fig. 99, &amp;lt;-/), or is developed in the form of leaflet?, called stipules (fig. 123, *). The sheathing portion is some times incorporated with the stem, and is then called t if/diary. These portions are not always present. The sheathing or stipulary por tion is frequently wanting, and occasionally only one of the other two is developed. When a leaf has a distinct stalk it is called petiolatc ; Fig. 99. when it has none, it is Leaf of rolygomim, with part of stem. sessile, and if in this case it embraces the stem it is said to be amplexicaul. The part of the leaf next the petiole or the axis is the base, while the opposite extremity is the apex. The surfaces of the leaf are called the paying, and its edges or margins form the circumscription of the leaf. The leaf is usually flattened and expanded horizon tally, i.e., at right angles to the longitudinal axis of the shoot, so that the upper pagina is directed towards the heavens, and the lower pagina towards the earth. In some cases leaves, as in Iris, or leaf-like petioles, as in Australian Acacias and Eucalypti, have their plane of expansion parallel to the axis of the shoot ; or the leaf may have a cylin drical or polyhedral form, as in Mesembryanthemum. In other instances, as in Alstromeria, the leaf becomes twisted in its course, so that what is superior at one part becomes inferior at another. The upper angle formed between the leaf and the stem is called its axil, and everything arising at that point is called axillary. It is there that leaf-buds are usually developed. The leaf is sometimes articulated with the stem, and, when it falls off, a scar or cicatricula remains ; at other times it is continuous with it, and then decays, while still attached to the axis. In their early state all leaves arc continuous with the stem, and it is only in their after growth that articulations are formed. When leaves fall off annually they are called deciduous ; when they remain for two or more years they are persistent, and the plant is called evergreen. The laminar portion of a leaf is occasionally articulated with the petiole, as in the Orange, and a joint at times exists between the vaginal or stipulary portion and the petiole. The arrangement of the fibro-vascular system in the Venation lamina constitutes the venation or nervation. In an ordinary leaf, as that of the Elm, there is observed a large central vein running from the base to the apex of the leaf, this is the midrib (fig. 100) ; it gives off veins laterally (pri- Fig. 101. FIG. 100. Loaf of Ulmus effusa. Reticulated venaf ion ; primary veins going to the margin, which is serrated. Leaf unequal at the base. FIG. 101. Multicostate divergent leaf of Castor-oil plant (Hicinus communis). It is palmately-clcfr, and exhibits seven lobes at the margin. The petiole is in serted a little above the base, and hence the leaf is called peltate or shield-like. mary veins), which either end in a curvature within the margin (curve-veined), as in Lilac and Belladonna, or go directly to the edge of the leaf (feather-veined], as in Elm (fig. 100) and Chestnut. If they are curved, then external veins and marginal veinlets are interspersed through the parenchyma external to the curvature. There are also other veins of less extent (costal veins) given off by the midrib, and these give origin to small vtinlets. A leaf with only a single midrib is said to be unicostate (fig. 100). In some cases, as Sycamore and Cinnamon, in place of there being only a single midrib there are several large veins (ribs) of nearly equal size, which diverge from the point where the blade joins the petiole or stem, giving oiF lateral veins. The leaf in this case is multicostate (fig. 101). When there are three prominent ribs, as in Cinnamon and Cassia, the leaf is tricostate ; when five, quinquecostate. When the midrib gives off two ribs a little above the base, the leaf becomes triplicostatf : when it gives off five, quintiqnicostate.