only small intercellular spaces, except where stomata happen to
be present (fig. 2, m); they form the palisade tissue. On the other
side of the leaf the cells are irregular, often branched, and are
arranged more or less horizontally (fig. 2, pi), leaving air-spaces
between them, l, which communicate with stomata; on this account
the tissue has received the name of spongy. In leaves having a
very firm texture, as those of Coniferae and Cycadaceae, the cells of
the parenchyma immediately beneath the epidermis are very much
thickened and 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, forming the 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. The fibro-vascular system in the leaf constitutes
the venation. The fibro-vascular bundles from the stem bend out
into the leaf, and are there arranged in a definite manner. In
skeleton leaves, or leaves in which the parenchyma is removed,
this arrangement is well seen. In some leaves, as in the barberry,
the veins are hardened, producing spines without any parenchyma.
The hardening of the extremities of the fibro-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.
The form and arrangement of the parts of a typical foliage leaf are intimately associated with the part played by the leaf in the life of the plant. The flat surface is spread to allow the maximum amount of sunlight to fall upon it, as it is by the absorption of energy from the sun’s rays by means of the chlorophyll contained in the cells of the leaf that the building up of plant food is rendered possible; this process is known as photo-synthesis; the first stage is the combination of carbon dioxide, absorbed from the air taken in through the stomata into the living cells of the leaf, with water which is brought into the leaf by the wood-vessels. The wood-vessels form part of the fibro-vascular bundles or veins of the leaf and are continuous throughout the leaf-stalk and stem with the root by which water is absorbed from the soil. The palisade layers of the mesophyll contain the larger number of chlorophyll grains (or corpuscles) while the absorption of carbon dioxide is carried on chiefly through the lower epidermis which is generally much richer in stomata. The water taken up by the root from the soil contains nitrogenous and mineral salts which combine with the first product of photo-synthesis—a carbohydrate—to form more complicated nitrogen-containing food substances of a proteid nature; these are then distributed by other elements of the vascular bundles (the phloem) through the leaf to the stem and so throughout the plant to wherever growth or development is going on. A large proportion of the water which ascends to the leaf acts merely as a carrier for the other raw food materials and is got rid of from the leaf in the form of water vapour through the stomata—this process is known as transpiration. Hence the extended surface of the leaf exposing a large area to light and air is eminently adapted for the carrying out of the process of photo-synthesis and transpiration. The arrangement of the leaves on the stem and branches (see Phyllotaxy, below) is such as to prevent the upper leaves shading the lower, and the shape of the leaf serves towards the same end—the disposition of leaves on a branch or stem is often seen to form a “mosaic,” each leaf fitting into the space between neighbouring leaves and the branch on which they are borne without overlapping.
Submerged leaves, or leaves which are developed under water, differ in structure from aerial leaves. They have usually no fibro-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 cases there is only a network of filament-like cells, the spaces between which are not filled with parenchyma, giving a skeleton appearance to the leaf, as in Ouvirandra fenestralis (Lattice plant).
A leaf, whether aerial or submerged, generally consists of a flat expanded portion, called the blade, or lamina, of a narrower portion called the petiole or stalk, and sometimes of a portion at the base of the petiole, which forms a sheath or vagina (fig. 5, s), or is developed in the form of outgrowths, called stipules (fig. 24, s). All these portions are not always present. The sheathing or stipulary portion is frequently wanting. When a leaf has a distinct stalk it is petiolate; when it has none, it is 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 leaf is usually flattened and expanded horizontally, i.e. at right angles to the longitudinal axis of the shoot, so that the upper face is directed towards the heavens, and the lower 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, there is then no distinction into an upper and a lower face, but the two sides are developed alike; or the leaf may have a cylindrical or polyhedral form, as in mesembryanthemum. The upper angle formed between the leaf and the stem is called its axil; it is there that leaf-buds are normally developed. The leaf is sometimes articulated with the stem, and when it falls off a scar remains; at other times it is continuous with it, and then decays, while still attached to the axis. In their early state all leaves are 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 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.
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The arrangement of the fibro-vascular system in the 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. 3); it gives off veins laterally (primary veins). A leaf with Venation. only a single midrib is said to be unicostate and the venation is described as pinnate or feather-veined. In some cases, as sycamore or castor oil (fig. 4), 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 off lateral veins. The leaf in this case is multicostate and the venation palmate. The primary veins give off secondary veins, and these in their turn give off tertiary veins, and so on until a complete network of vessels is produced, and those veins usually project on the under surface of the leaf. To a distribution of veins such as this the name of reticulated or netted venation has been applied. In the leaves of some plants there exists a midrib with large veins running nearly parallel to it from the base to the apex of the lamina, as in grasses (fig. 5); or with veins diverging from the base of the lamina in more or less
