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Leaves
The leaf is the third basic organ of plants (the others are roots and stems). The leaf is usually defined by its position-that is, it originates as a small projection at the apex or tip of the stem; it is attached to the stem; and in temperate plants there nearly always is a vegetative bud near where it is attached to the stem.
The basic role of the leaf is photosynthesis. Photosynthesis is the process in which energy from sunlight is combined with carbon dioxide and water to produce sugar and oxygen. The structure of the leaf is tailored to this process. Leaves are generally flat to provide as much surface area as possible for the absorption of sunlight. Leaves also have small openings on their lower surface, called stomata, which allow the carbon dioxide in and the oxygen out. What is more, the leaf is supplied with a system of veins that link up with the veins in the roots and stems. These veins supply the water for photosynthesis and other nutrients needed by the photosynthesizing cells, and they carry the sugars formed via photosynthesis to other parts of the plant, where they feed the living cells of the roots and stems or are stored for future use.
Parts of the Leaf [Illustration]
The leaf consists of two main parts: the lamina and the petiole.
- Lamina: The flat portion of a leaf, also called the leaf blade.
- Petiole: The stalk to which the lamina is attached. There are
several types of petiole:
- Phyllode: A broadened, leaf-like petiole. In this case, the leaf has lost its lamina during evolution and the petiole has taken its place in photosynthesis. Some acacias have phyllodes.
- Pulvinus: A swollen area at the base of a petiole. The pulvinus often acts to lower the leaf when it is wilted. It is also the part of the sensitive plant (Mimosa pudica) that makes the leaves close up when touched.
- Sheath: A broadened petiole base that surrounds the stem. Most grasses have leaf sheaths.
Other leaf parts include:
- Stipule: A leaf-like growth at the base of the petiole. The presence of stipules is characteristic of a number of species. Often, they protect the buds and developing leaves and then fall off. Some can be large and conspicuous. They are typically paired in dicots, when they are present, and absent in monocots.
- Buds: Small shoots with scale-like leaves which, when given the proper conditions, burst out in growth. Although not technically leaves, buds are nearly always found associated with the base of a leaf petiole.
Leaf Division [Illustration]
Leaves can consist of a single blade or can be divided into two or more blades. The way these divisions are arranged is characteristic of individual species.
- Simple leaf: A leaf with only one lamina.
- Compound leaf: A leaf with two or more lamina (called
leaflets). It is often difficult to distinguish a leaflet from a simple
leaf; they look for all intents and purposes the same. It is important to keep
in mind that a leaf is defined by its development and position, not just by its
appearance. In general, compound leaves can be recognized by the absence of a
bud at the base of the leaflet. There are several different types of compound
leaves, including:
- Palmate: Leaves with the leaflets attached to a common point like the fingers of a human hand.
- Pinnate/bipinnate/tripinnate: Leaves with the leaflets aligned along a central stalk (called a rachis) like the tines of a comb. Pinnately compound leaves may have only a single rachis or the rachis may branch once, in which case it is called bipinnate; twice, in which case it is called tripinnate; or three times, in which case it is called quadripinnate. These leaves can be further classified into paripinnate, in which all the leaflets are more or less paired, and there is no single terminal leaflet, and imparipinnate, in which there is a single terminal leaflet.
- Ternate/biternate: Leaves with three leaflets and in which the terminal leaf is not stalked. In other words, this is a palmately compound leaf with three leaflets. It follows that biternate leaves are doubly ternate, with the ternate divisions again ternately divided.
- Trifoliate: Leaves with three leaflets and in which the terminal leaflet is stalked. In other words, this is a pinnately compound leaf with three leaflets.
Evergreen vs. Deciduous
One way that plants have become adapted to their climates is by either retaining their leaves year-round or shedding them as a difficult season approaches. In cold-winter temperate areas, for example, many plants drop their leaves in autumn because the loss of energy that went into creating the leaves and the loss of energy that might be manufactured by photosynthesis in winter is outweighed by even greater energy that would be needed to support the leaves during the cold months and the potential threat of leaf death due to freezing. In plants that retain most of their leaves year-round, even through difficult seasons, the leaves have acquired certain characteristics that enable them to survive the extremes.
Evergreen plants retain their leaves for more than one year. This does not mean that the leaves are retained forever but rather that the plant is always green. Individual leaves of most evergreen plants are shed after a few years. Deciduous plants deliberately drop their leaves at the onset of a difficult season-winter in temperate and boreal areas and the dry season in desert areas of the world.
Leaf Texture
The texture of the leaf is often an reflection of the duration of the leaf. Evergreen leaves are usually coriaceous, while deciduous leaves are usually membranaceous or chartaceous.
- Membranaceous: Thin and flexible.
- Chartaceous: Thin and papery.
- Coriaceous: Thick and leathery.
Leaf Shape [Illustration]
When describing leaf shapes, botanists use a variety of terms often taken from Latin. These terms can be used for describing any flat shape on a plant (not only the leaf, but also the stipule, sepal, petal, and so on). These terms take into account the length-to-width ratio, the curving of the margin or edge, and the position of the widest point. For additional precision, words like "narrowly," "widely," and "depressed" are often used to qualify the terms. The following are some of the more common leaf shapes:
- Deltoid (deltate): About the same length as width. Shaped like a triangle in which all sides are the same length. Cottonwood (Populus grandidentata) has deltoid leaves.
- Elliptic: One and a half to two times longer than wide. Shaped like a narrow oval. The margins are symmetrically curved, and the leaf is widest at the middle. Chestnuts (Castanea species) have elliptic leaves.
- Lanceolate: Three to six times longer than wide. Shaped like the tip of a lance. The margins are symmetrically curved, and the leaf is widest toward the base. Willows (Salix species) have lanceolate leaves.
- Linear: More than twelve times longer than wide. Long and narrow with more or less parallel margins. Most grasses have linear leaves.
- Oblong: One and a half to two times longer than wide. Similar to elliptic but the margins are parallel, not curved. Willow oak (Quercus phellos) has oblong leaves.
- Ovate: One and a half to two times longer than wide. Shaped like an egg. The margins are symmetrically curved, and the leaf is widest below the middle. Beaked hazelnut (Corylus cornuta) has ovate leaves.
Leaf Base [Illustration]
The base of the leaf blade can have a wide variety of shapes. The following are a few common leaf base shapes:
- Acute: Tapering to a sharp point with convex sides.
- Auriculate: A base with rounded projections that have a concave inner margin.
- Cordate: Heart-shaped; the base has a notch similar to that at the top of a heart.
- Cuneate: Tapering to a sharp point. Similar to acute but with straight, not convex, sides.
- Hastate: A base with outwardly pointed lobes.
- Oblique: A base that is not symmetrical on both sides.
- Obtuse: Tapering to a blunt point.
- Peltate: Borne on a stalk attached to the lower surface rather than to the base or margin of the leaf. Nasturtiums (Tropaeolum majus) have peltate leaves.
- Perfoliate: A leaf in which the bases of two opposite leaves are fused around the stem, so that the stem appears to pass through the leaf.
- Sagittate: With a downwardly pointed lobe on each side of the base.
- Truncate: A base that is squared off at the ends, as if cut off.
Leaf Margin [Illustration]
The margin or edge of a leaf can have teeth, hairs, or other type of indentation. Some of the more common margin types are:
- Ciliate: With hairs along the margin.
- Crenate: With low, rounded teeth that have no point.
- Dentate: With teeth along the margin that are pointed outward from the margin. Similar to serrate, but in serrate the teeth are pointed at least slightly upward towards the tip.
- Double serrate: With serrate teeth along the margins of other, larger serrate teeth.
- Entire: Without any indentations or teeth.
- Serrate: With teeth along the margin that are pointed upwards towards the tip of the leaf.
- Undulate: With a wavy margin.
Leaf Lobe [Illustration]
Leaves can have shallow indentations or deeper indentations, forming lobes. It can be difficult to distinguish between deeply incised teeth and shallow lobes. Some of the more common types of lobing are:
- Cleft: The margin is cut in towards the midvein.
- Incised: The leaf is cut sharply and deeply, into usually regularly shaped lobes.
- Lacerate: Irregularly lobed with deep incisions that look as if they were torn.
- Laciniate: Slashed; similar to incised, but the cuts are sharply angled.
- Palmate: Palm-shaped; with several lobes from a single point.
- Pectinate: Comb-shaped; the same as pinnatifid but with more numerous, straighter, and narrower lobes.
- Pedate: Similar to palmate, but the lateral lobes are further dissected.
- Pinnatifid: Pinnately lobed half the distance or more to the midrib, but not all the way to the midrib.
Leaf Apex [Illustration]
The apex or tip of the leaf, like the base, comes in a wide variety of shapes that are characteristic of different species. Some of the more common leaf apex shapes are:
- Acuminate: A sharp apex with less than a 90 degree angle and concave sides.
- Acute: Similar to acuminate but with straight to convex sides.
- Apiculate: Similar to mucronate in that it ends in a sharp point but the point includes tissue from the midvein and the lamina.
- Aristate: Ending in a hard,bristle-shaped tip.
- Caudate: Very acuminate, with a long and thin tip.
- Cuspidate: A form of acuminate in which the tip tapers gradually to a point.
- Emarginate: Broadly notched at the apex. Similar to retuse, which is only slightly notched.
- Mucronate: A sharp point (mucro) formed by a continuation of the midvein.
- Obcordate: Heart-shaped; notched at the tip like the top of a heart.
- Obtuse: A blunt apex with more than a 90 degree angle.
- Retuse: Slightly notched at the apex.
Leaf Folding
Because the leaves are often fully formed but not expanded, they need to be packed in the bud. The exact form these contorted leaves take is useful in the identification of many species. All leaves show folding during some part of their development but most eventually become flat or only slightly curled. The folding of mature leaves is called ptyxis. Two common types of ptyxis are:
- Circinate: The apex of the leaf is curled towards the base. Many ferns exhibit circinate vernation in their "fiddleheads."
- Revolute: The margins of the leaf are curled under. An example is labrador tea (Rhododendron groenlandicum).
Leaf Modifications
Leaves seem to be the most malleable plant organ. They have been modified to function in a wide variety of ways. Some of the most common or interesting are:
- Bract and bracteole: Bracts are leaves associated with flowers. The colorful parts of a shrimp plant (Justicia brandegeana) are bracts. Bracteoles are small bracts found on the pedicel (the stalk supporting the flower). Bracteoles are usually paired in dicots and borne singly in monocots.
- Prophyll: The leaves found at the first node of a shoot.
- Tendril: See stems.
- Spine: See stems.
- Insect traps: Leaves modified to trap insects and often absorb nutrients (particularly nitrogen) from the decaying insect carcasses. Plants from very unrelated parts of the plant kingdom have become insectivorous, and have done so by evolving "traps" such as sticky or epiascidiate leaves.
- Sticky leaves: This type of trap, exemplified by the sundews (Drosera species), catches insects in goo either on the leaf surface or on hairs.
- Epiascidiate leaves (container leaves): In this type of trap, exemplified by the pitcher plants (Sarracenia species), the leaves form containers in which the insects are caught. Other types of epiascidiate leaves are the bladders of the bladderworts (Utricularia species), which suck the unsuspecting insect into the trap, and the Venus's flytrap (Dionaea muscipula), with leaves that snap shut when an insect than lands on them.
- Emergence, prickle: A sharp, spiny structure formed on the surface of a leaf (or stem). It is similar to a spine, which is a modified stem, leaf, or root.
- Food bodies: Many plants produce food for visiting insects that benefit the plants in some way. This food is found in food bodies, which have a wide variety of names. The most famous are the Beltian bodies found on ant-plants (Acacia species), which attract ants that bite any intruder.
- Bulb: An underground, swollen bud. Food is stored in the leaves of the bud. A bulb resembles a corm, but in a corm the food is stored in the stem.
- Succulent: Noticeably fleshy; used to store water. This term can also be used for roots and stems.
- Indeterminate growth: Most leaves are determinate -- that is, once they reach a certain size, they stop growing in length. The leaves of a few plants can continue to grow indefinitely. For example, many grass leaves grow from the base and can continue to grow after the tops of the leaves are cut, making it possible to grow and mow lawns.
Leaf Arrangement [Illustration]
Leaves can be arranged on a stem in a variety of ways. Some of the more common arrangements are:
- Alternate: One leaf per node. Oaks have alternate leaves.
- Basal: All the leaves coming from the base. Tulips have basal leaves.
- Equitant: Overlapping one another into two vertical rows or ranks, forming a fan, as the leaves of irises.
- Fascicled: Clustered, as in most species of pine.
- Imbricate: Overlapping one another. The leaves of Norfolk Island pine (Araucaria heterophylla) are imbricate.
- Opposite: Borne across from one another at the same node; paired. Maples have opposite leaves.
- Rosulate: Arranged in a basal rosette, with little or no stem. Sedums have rosulate leaves.
- Whorled: With three or more leaves at the same node, arranged in whorls. Joe-pye weed (Eupatorium maculatum) has whorled leaves.