Leaf

I

Introduction

Leaf, principal food-making organ of a plant, a lateral outgrowth of the stem and branches. Leaves that have not undergone modification for specialized purposes normally consist of two main parts: a stalk, called a petiole, and an expanded, flattened portion, called a blade. The green colour of the blades of most foliage leaves is caused by chlorophyll, which is used by the plant to manufacture carbohydrate sugars from water and carbon dioxide (see photosynthesis). Not all leaves are green; many have additional pigments that produce colours other than green despite the presence of chlorophyll, and some may lack chlorophyll in all or part of the blade. Most of the autumn coloration in leaves occurs when, as a result of the decomposition of chlorophyll, additional pigments are revealed. The internal structure of leaves, like that of roots and stems, is a modification of a basic pattern common to most vascular plant parts. The surface of leaves are covered in tiny openings, known as stomata. They allow the exchange of gases, such as oxygen and carbon dioxide, and water vapour, which are necessary for transpiration.

II

Forms of Leaves

Many of the characteristics used to identify plants are external structural features of leaves. The two basic types of leaves are (1) the simple leaf, which has a single, undivided blade, such as an oak leaf, and (2) the compound leaf, which has a blade composed of several leaflets, such as a clover leaf. Compound leaves that bear leaflets in two opposite rows arising from a central midrib, such as leaves of ash, are pinnately compound; those that have leaflets arising from a single point and radiating fanwise, such as horse chestnut, are palmately compound.

The arrangement of veins within a single leaf or leaflet of a dicotyledonous plant (see Dicots) follows a similar pattern. A pinnately veined leaf, such as elm, has a relatively thick main vein running from the base to the tip, with smaller veins radiating from the main vein and branching into veinlets. A palmately veined leaf, such as maple, has several almost equally thick veins radiating from the leaf base, branching into veinlets. A peltate leaf, such as nasturtium, which has the petiole attached near the centre of the lower surface of a round or shield-shaped blade, has several main veins that radiate from the centre to the edges, giving rise to smaller branched veinlets. Leaves of most monocotyledonous plants have almost equal parallel veins, which arise at the base of the leaf and end at or near the tip. Leaves of most ferns and of a few higher plants, such as ginkgo, have dichotomous veins: several small veins, about equal in thickness, arise from one or several points at the base of the leaf or leaflet and fork repeatedly in twos to the edge of the blade.

The manner in which the leaf is attached to the stem varies widely. In leaves of most dicotyledonous plants the base of the petiole is the point of attachment, but in leaves of most monocotyledonous plants the petiole is lacking, and the base of the leaf is a broad, flat sheath wrapped round the stem. Leaves having petioles may produce a leaf-like or scale-like structure, called a stipule, at the point of attachment to the stem, as in roses or sweet peas (see Laurel).

The arrangement of leaves, known as phyllotaxy, varies widely among plants. Basically, two main types exist: alternate, in which the leaves are arranged round a stem in a spiral, and opposite, in which pairs of leaves are joined to the stem at the same level. If successive pairs of leaves are arranged at right angles, as in the mint, the leaves are said to be decussate. If more than two leaves are attached to the stem at the same level, the arrangement is called verticillate, or whorled.

III

Leaf Adaptations

The shapes and structure of leaves are adapted to the conditions under which the plant lives. Typical leaves of temperate regions having moderate humidity are quite different from those of humid, tropical regions or cold, dry regions. Most leaves have flat blades that expose as much surface as possible to sunlight, but conifers, adapted to cold, windy regions, have needle-like leaves that offer a minimum surface to the drying, winter winds. The leaves have one or two veins deeply imbedded in the middle and a layer of strong supporting tissue just beneath the thick and heavily cutinized outer layer. In plants of arid regions, such as aloe, the leaves are often much more spongy and can retain large amounts of water. They are often referred to as succulents. Leaves of many tropical forest plants are adapted to encourage excess moisture to run off at the tips.

Many plant organs that bear little resemblance to foliage leaves originate, embryologically, as leaves. The two large cotyledons, or seed leaves, that constitute the bulk of a bean seed, for example, function as food-supplying leaves for the young bean seedling. Scales that cover young developing buds are leaves modified for the function of protection. Tendrils of many climbing plants are really leaves with undeveloped blades. Some spines, such as those of cactus, are also modified leaves. Petal-like bracts, such as the showy, so-called petals of dogwood and poinsettia, which surround the base of a flower or an inflorescence, are leaves that may carry on photosynthesis. Even sepals, petals, stamens, and carpels are regarded as leaves modified for purposes of reproduction.