Bay leaf
Laurus nobilis, known as bay leaf, from William Woodville, Medical Botany, 1793.
Scientific classification
Kingdom:	Plantae
(unranked):	Magnoliophyta
(unranked):	Magnoliopsida
Order:	Laurales
Family:	Lauraceae
Genus:	Laurus
Species:	L. nobilis
Binomial name
Laurus nobilis

Bay leaf (plural bay leaves) refers to the aromatic leaf of the bay laurel (Laurus nobilis, Lauraceae). Fresh or dried bay leaves are used in cooking for their distinctive flavor and fragrance. The leaves are often used to flavor soups, stews, braises and pâtés in Mediterranean cuisine. The fresh leaves are very mild and do not develop their full flavor until several weeks after picking and drying.^[1]

Taxonomy[link]

The term "bay leaf" is used to refer to several other plants besides the leaves of L. nobilis. These include:

• California bay leaf – the leaf of the California bay tree (Umbellularia californica), also known as California laurel, Oregon myrtle, and pepperwood, is similar to the Mediterranean bay, but has a stronger flavor.
• Indian bay leaf or Malabathrum (Cinnamomum tejpata; also tej pat, tejpat, tejpata or palav aaku in Telugu or punnai ilai in Tamil or daalchini in Kannada), in appearance to bay leaves, but is culinarily quite different, having a fragrance and taste similar to cinnamon (cassia) bark, but milder. In culinary terms, it is misleading to call it bay leaf because it is of a genus other than that of the bay laurel tree, it does not taste the same as the bay laurel leaf, and cannot be used in cooking as a substitute for the bay laurel leaf.
• Indonesian bay leaf or Indonesian laurel (salam leaf) – the leaf of Syzygium polyanthum is not commonly found outside of Indonesia; this herb is applied to meat and, less often vegetables. Like Indian bay leaf, it is also inaccurately named because the plant is actually a member of the Myrtaceae family.^[2]

History[link]

The bay laurel tree has been cultivated since the beginning of recorded history;^[3] it originated in Asia Minor, and spread to the Mediterranean and other countries with suitable climates. Bay leaf is not grown in northern regions, as the plants do not thrive in cold climates. Turkey is one of the main exporters of bay leaves, although they are also grown in areas of Albania, France, Belgium, Italy, Russia, Central America, North America, and India.^[1] The laurel tree was very important both symbolically and literally in both Greece and Rome. The laurel can be found as a central component in many ancient mythologies that glorify the tree as a symbol of honor.^[4] Bay leaves are one of the most widely used culinary herbs in Europe and North America. In the Elizabethan era, some people believed pinning bay leaves to one's pillow on the eve of Saint Valentine's Day would permit one to see one's future spouse in a dream.

Taste and aroma[link]

If eaten whole, bay leaves are pungent and have a sharp, bitter taste. As with many spices and flavorings, the fragrance of the bay leaf is more noticeable than its taste. When dried, the fragrance is herbal, slightly floral, and somewhat similar to oregano and thyme. Myrcene, which is a component of many essential oils used in perfumery, can be extracted from the bay leaf. They also contain the essential oil eugenol.^[4]

Uses[link]

Bay leaves are a fixture in the cooking of many European cuisines (particularly those of the Mediterranean), as well as in North America. They are used in soups, stews, meat, seafood and vegetable dishes. The leaves also flavor many classic French dishes. The leaves are most often used whole (sometimes in a bouquet garni) and removed before serving (they can be abrasive in the digestive tract). In Indian (Sanskrit name tamaalpatra, Hindi tejpatta) and Pakistani cuisine, bay leaves are often used in biryani and other rich, spicy dishes - although not as an everyday ingredient in home cuisine - and as an ingredient in garam masala.

Bay leaves can also be crushed or ground before cooking. Crushed bay leaves impart more of their desired fragrance than whole leaves, but are more difficult to remove, and thus they are often used in a muslin bag or tea infuser. Ground bay laurel may be substituted for whole leaves, and does not need to be removed, but it is much stronger due to the increased surface area and in some dishes the texture may not be desirable.

Bay leaves can also be scattered in a pantry to repel meal moths,^[5] flies, roaches, and silverfish.

Bay leaves have been used in entomology as the active ingredient in killing jars. The crushed, fresh, young leaves are put into the jar under a layer of paper. The vapours they release kill insects slowly but effectively, and keep the specimens relaxed and easy to mount. The leaves discourage the growth of moulds. They are not effective for killing large beetles and similar specimens, but insects that have been killed in a cyanide killing jar can be transferred to a laurel jar to await mounting.^[6] It is not clear to what extent the effect is due to cyanide released by the crushed leaves, and to what extent other volatile products are responsible.

Medicinal value[link]

This unreferenced section requires citations to ensure verifiability.

In the Middle Ages, bay leaves were believed to induce abortions and to have many magical qualities. They were once used to keep moths away, owing to the leaf's lauric acid content that gives it insecticidal properties. They have many properties that make them useful for treating high blood sugar, migraine headaches, bacterial and fungal infections, and gastric ulcers. Bay leaves and berries have been used for their astringent, carminative, diaphoretic, digestive, diuretic, emetic and stomachic properties. Bay oil, or oil of bays (oleum lauri) is used in liniments for bruises and sprains. Bay leaf has been used as an herbal remedy for headaches. It contains compounds, called parthenolides, which have proven useful in the treatment of migraines. Bay leaf has also been shown to help the body process insulin more efficiently, which leads to lower blood sugar levels, and to reduce the effects of stomach ulcers. Bay leaf contains eugenol, which has anti-inflammatory and antioxidant properties. Bay leaf is also an antifungal and antibacterial, and has also been used to treat rheumatism, amenorrhea, and colic.

Safety[link]

Some members of the laurel family, as well as the unrelated, but visually similar mountain laurel and cherry laurel, have leaves that are poisonous to humans and livestock. While these plants are not sold anywhere for culinary use, their visual similarity to bay leaves has led to the oft-repeated belief bay leaves should be removed from food after cooking because they are poisonous. This is not true - bay leaves may be eaten without toxic effect. However, they remain very stiff even after thorough cooking, and if swallowed whole or in large pieces, they may pose a risk of scratching the digestive tract or even causing choking. Thus, most recipes that use bay leaves will recommend their removal after the cooking process has finished.^[7]

Cultivation[link]

Gardeners in frost-free or light frost areas will find that bay laurel seedlings planted in the ground easily grow into large trees, 38 feet (12 m) and taller; but when kept pruned, it can thrive as a small bush. Bay laurel can also be grown in containers, the size of which limits the ultimate size of the trees. New plants are often started via layering, or from cuttings, since growing from seed can be difficult.

Bay trees are difficult to start from seed, due in part to the seed's low germination rate, and long germination period. Fresh seeds with the pericarp removed typically have a 40% germination rate, while dried seeds and/or seeds with an intact pericarp have yet lower germination rates. In addition, the seed germination period can be 50 days or more, which increases the risk of the seeds rotting before they germinate. Treating the seeds with gibberellic acid can be useful in increasing seed germination, as is careful monitoring of moisture levels in the rooting medium.^[8]

Gallery[link]

• 

  Bay leaf

• 

  Leaves and flower buds

• BayLeaves.jpg

  Bay leaves

• 

  Laurus nobilis bush

• 

  A closeup of several Laurus nobilis leaves

• 

  Dried bay leaves in a bowl

References[link]

Wikimedia Commons has media related to: Laurus nobilis

The bay at San Sebastián, Spain

The Bay of Bengal in South Asia

The bay of Baracoa, Cuba

The bay of İzmir, in Turkey

A bay is a large body of water connected to an ocean or sea formed by an inlet of land due to the surrounding land blocking some waves and often reducing winds.^[1] Bays also exist as an inlet in a lake or pond. A large bay may be called a gulf, a sea, a sound, or a bight. A cove is a circular or oval coastal inlet with a narrow entrance; some coves may be referred to as bays.

Bays were significant in the history of human settlement because they can provide a safe place for fishing. Later they were important in the development of sea trade as the safe anchorage they provide encouraged their selection as ports.^[2] Any bay may contain fish and other sea creatures or be adjacent to other bays. For example, James Bay is adjacent to Hudson Bay. Large bays, such as the Bay of Bengal and the Hudson Bay, have varied marine geology.

Formation[link]

There are various ways that bays can be created. The largest bays have developed as a result of continental drift.^[2] As the super-continent Gondwana broke up along curved and indented fault lines, the continents moved apart and the world's largest bays formed. These include the Gulf of Guinea, Persian Gulf, Gulf of Alaska, Gulf of Mexico and the Bay of Bengal which is the largest bay in the world.^[2]

Another way bays are formed is via glacial and river erosion.^[2] If formed by glaciers a bay is known as a fjord. Rias are created by rivers and are characterised by more gradual slopes. Most gulf and bays are formed by the folding of the earth's crust as well as coastal erosion due to waves and currents.^[2] Currents can make waves more constant, while soft rocks will allow erosion to have a stronger effect. Any hard rock is eroded less quickly, leaving headlands. The Gulf of California is an example of a bay created by the geological process of folding.

See also[link]

• Bay platform
• Great capes
• Headlands and bays

References[link]

v t e Coastal geography Landforms Anchialine pool Avulsion Archipelago Atoll Ayre Barrier bar Barrier island Bay Baymouth bar Bight Brackish marsh Cape Channel Cliff Cliffed coast Cliff-top dune Coast Coastal plain Coastal waterfall Continental margin Continental shelf Coral reef Cove Dune Estuary Firth Fjard Fjord Flat coast Freshwater marsh Fundus Gat Geo Graded shoreline Gulf Headland Inlet Intertidal wetland Island Islet Isthmus Lagoon Marine terrace Mouth bar Raised shorelines Machair Mega delta Mudflat Natural arch Peninsula Reef Regressive delta Ria River delta Rocky shore Salt marsh Shore Sound Stack Steep coast Strait Strand plain Submarine canyon Tied island Tidal island Tidal marsh Tide pool Tombolo More... Beaches Beach cusps Beach evolution Beach ridge Beachrock Modern recession of beaches Pocket beach Raised beach Storm beach Shingle beach Processes Blowhole Coastal erosion Concordant coastline Current Cuspate foreland Discordant coastline Emergent coastline Feeder bluff Fetch Headlands and bays Large scale coastal behaviour Longshore drift Marine regression Marine transgression Rip current Sea cave Sea foam Shoal Spit Submergent coastline Surf break Surf zone Surge channel Swash Volcanic arc Wave-cut platform Wave shoaling Wind wave Wrack zone Management Coastal management Accretion Integrated coastal zone management Submersion Related Bulkhead line Coastline length Intertidal zone Littoral zone Particle size boulder shingle pebble sand silt clay Physical oceanography Region of freshwater influence Scree More...

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The leaves of a Beech tree.

3D rendering of a µCT scan of a leaf piece, resolution circa 40 µm/voxel.

Lichen Dolichousnea longissima. The photosynthetic appendages on the strands are leaf-like in appearance and function but ontogenically are not leaves.

A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants.^[1][2]

Typically a leaf is a thin, flattened organ borne above ground and specialized for photosynthesis, but many types of leaves are adapted in ways almost unrecognisable in those terms: some are not flat (for example many succulent leaves and conifers), some are not above ground (such as bulb scales), and some are without major photosynthetic function (consider for example cataphylls, spines, and cotyledons).

Conversely, many structures of non-vascular plants, or even of some lichens, which are not plants at all (in the sense of being members of the kingdom Plantae), do look and function much like leaves. Furthermore, several structures found in vascular plants look like leaves but are not actually leaves; they differ from leaves in their structures and origins. Examples include phyllodes, cladodes, and phylloclades.^[2]

General nature of leaves[link]

Typically leaves are flat and thin, thereby maximising the surface area directly exposed to light and promoting photosynthetic function. Externally they commonly are arranged on the plant in such ways as to expose their surfaces to light as efficiently as possible without shading each other, but there are many exceptions and complications; for instance plants adapted to windy conditions may have pendent leaves, such as in many willows and Eucalyptus.

Likewise, the internal organisation of most kinds of leaves has evolved to maximise exposure of the photosynthetic organelles, the chloroplasts, to light and to increase the absorption of carbon dioxide. Most leaves have stomata, which open or narrow to regulate the exchange of carbon dioxide, oxygen, and water vapour with the atmosphere.

In contrast however, some leaf forms are adapted to modulate the amount of light they absorb to avoid or mitigate excessive heat, ultraviolet damage, or desiccation, or to sacrifice light-absorption efficiency in favour of protection from herbivorous enemies. Among these forms the leaves of many xerophytes are conspicuous. For such plants their major constraint is not light flux or intensity, but heat, cold, drought, wind, herbivory, and various other hazards.^[3] Typical examples among such strategies are so-called window plants such as Fenestraria species, some Haworthia species such as Haworthia tesselata and Haworthia truncata^[4] and Bulbine mesembryanthemoides.^[5]

The shape and structure of leaves vary considerably from species to species of plant, depending largely on their adaptation to climate and available light, but also to other factors such as grazing animals, available nutrients, and ecological competition from other plants. Considerable changes in leaf type occur within species too, for example as a plant matures; as a case in point Eucalyptus species commonly have isobilateral, pendent leaves when mature and dominating their neighbours; however, such trees tend to have erect or horizontal dorsiventral leaves as seedlings, when their growth is limited by the available light.^[6] Other factors include the need to balance water loss at high temperature and low humidity against the need to absorb atmospheric carbon dioxide. In most plants leaves also are the primary organs responsible for transpiration and guttation.

Leaves can also store food and water, and are modified accordingly to meet these functions, for example in the leaves of succulent plants and in bulb scales. The concentration of photosynthetic structures in leaves requires that they are richer in protein, minerals, and sugars, than say, woody stem tissues. Accordingly leaves are prominent in the diet of many animals. This is true for humans, for whom leaf vegetables commonly are food staples.

A leaf shed in autumn.

Correspondingly, leaves represent heavy investment on the part of the plants bearing them, and their retention or disposition are the subject of elaborate strategies for dealing with pest pressures, seasonal conditions, and protective measures such as the growth of thorns and the production of phytoliths, lignins, tannins and poisons.

Deciduous plants in frigid or cold temperate regions typically shed their leaves in autumn, whereas in areas with a severe dry season, some plants may shed their leaves until the dry season ends. In either case the shed leaves may be expected to contribute their retained nutrients to the soil where they fall.

In contrast, many other non-seasonal plants, such as palms and conifers, retain their leaves for long periods; Welwitschia retains its two main leaves throughout a lifetime that may exceed a thousand years.

Not all plants have true leaves. Bryophytes (e.g., mosses and liverworts) are non-vascular plants, and, although they produce flattened, leaf-like structures that are rich in chlorophyll, these organs differ morphologically from the leaves of vascular plants; For one thing, they lack vascular tissue. Vascularised leaves first evolved following the Devonian period, when carbon dioxide concentration in the atmosphere dropped significantly. This occurred independently in two separate lineages of vascular plants: the microphylls of lycophytes and the euphylls ("true leaves") of ferns, gymnosperms, and angiosperms. Euphylls are also referred to as macrophylls or megaphylls ("large leaves").

Large-scale features (leaf morphology)[link]

A structurally complete leaf of an angiosperm consists of a petiole (leaf stalk), a lamina (leaf blade), and stipules (small structures located to either side of the base of the petiole). Not every species produces leaves with all of these structural components. In certain species, paired stipules are not obvious or are absent altogether. A petiole may be absent, or the blade may not be laminar (flattened). The tremendous variety shown in leaf structure (anatomy) from species to species is presented in detail below under morphology.

The petiole mechanically links the leaf to the plant and provides the route for transfer of water and sugars to and from the leaf. The lamina is typically the location of the majority of photosynthesis.

Anatomy[link]

This section requires expansion.

Medium scale features[link]

Leaves are normally extensively vascularised and are typically covered by a dense network of xylem, which supply water for photosynthesis, and phloem, which remove the sugars produced by photosynthesis. Many leaves are covered in trichomes (small hairs) which have a diverse range of structures and functions.

Small-scale features[link]

A leaf is a plant organ and is made up of a collection of tissues in a regular organisation. The major tissue systems present are:

1. The epidermis that covers the upper and lower surfaces
2. The mesophyll inside the leaf that is rich in chloroplasts (also called chlorenchyma)
3. The arrangement of veins (the vascular tissue)

These three tissue systems typically form a regular organisation at the cellular scale.

Major leaf tissues[link]

Epidermis[link]

SEM image of Nicotiana alata leaf's epidermis, showing trichomes (hair-like appendages) and stomata (eye-shaped slits, visible at full resolution).

The epidermis is the outer layer of cells covering the leaf. It forms the boundary separating the plant's inner cells from the external world. The epidermis serves several functions: protection against water loss by way of transpiration, regulation of gas exchange, secretion of metabolic compounds, and (in some species) absorption of water. Most leaves show dorsoventral anatomy: The upper (adaxial) and lower (abaxial) surfaces have somewhat different construction and may serve different functions.

The epidermis is usually transparent (epidermal cells lack chloroplasts) and coated on the outer side with a waxy cuticle that prevents water loss. The cuticle is in some cases thinner on the lower epidermis than on the upper epidermis, and is generally thicker on leaves from dry climates as compared with those from wet climates.

The epidermis tissue includes several differentiated cell types: epidermal cells, epidermal hair cells (trichomes) cells in the stomate complex; guard cells and subsidiary cells. The epidermal cells are the most numerous, largest, and least specialized and form the majority of the epidermis. These are typically more elongated in the leaves of monocots than in those of dicots.

The epidermis is covered with pores called stomata, part of a stoma complex consisting of a pore surrounded on each side by chloroplast-containing guard cells, and two to four subsidiary cells that lack chloroplasts. Opening and closing of the stoma complex regulates the exchange of gases and water vapor between the outside air and the interior of the leaf and plays an important role in allowing photosynthesis without letting the leaf dry out. In a typical leaf, the stomata are more numerous over the abaxial (lower) epidermis than the adaxial (upper) epidermis and more numerous in plants from cooler climates.

Mesophyll[link]

Most of the interior of the leaf between the upper and lower layers of epidermis is a parenchyma (ground tissue) or chlorenchyma tissue called the mesophyll (Greek for "middle leaf"). This assimilation tissue is the primary location of photosynthesis in the plant. The products of photosynthesis are called "assimilates".

In ferns and most flowering plants, the mesophyll is divided into two layers:

• An upper palisade layer of tightly packed, vertically elongated cells, one to two cells thick, directly beneath the adaxial epidermis. Its cells contain many more chloroplasts than the spongy layer. These long cylindrical cells are regularly arranged in one to five rows. Cylindrical cells, with the chloroplasts close to the walls of the cell, can take optimal advantage of light. The slight separation of the cells provides maximum absorption of carbon dioxide. This separation must be minimal to afford capillary action for water distribution. In order to adapt to their different environment (such as sun or shade), plants had to adapt this structure to obtain optimal result. Sun leaves have a multi-layered palisade layer, while shade leaves or older leaves closer to the soil are single-layered.
• Beneath the palisade layer is the spongy layer. The cells of the spongy layer are more rounded and not so tightly packed. There are large intercellular air spaces. These cells contain fewer chloroplasts than those of the palisade layer. The pores or stomata of the epidermis open into substomatal chambers, which are connected to the air spaces between the spongy layer cells.

These two different layers of the mesophyll are absent in many aquatic and marsh plants. Even an epidermis and a mesophyll may be lacking. Instead for their gaseous exchanges they use a homogeneous aerenchyma (thin-walled cells separated by large gas-filled spaces). Their stomata are situated at the upper surface.

Leaves are normally green in color, which comes from chlorophyll found in plastids in the chlorenchyma cells. Plants that lack chlorophyll cannot photosynthesize.

Veins[link]

The veins of a bramble leaf.

The veins are the vascular tissue of the leaf and are located in the spongy layer of the mesophyll. They are typical examples of pattern formation through ramification. The pattern of the veins is called venation.

The veins are made up of:

• Xylem: tubes that bring water and minerals from the roots into the leaf.
• Phloem: tubes that usually move sap, with dissolved sucrose, produced by photosynthesis in the leaf, out of the leaf.

The xylem typically lies on the adaxial side of the vascular bundle and the phloem typically lies on the abaxial side. Both are embedded in a dense parenchyma tissue, called the pith or sheath, which usually includes some structural collenchyma tissue.

Seasonal leaf loss[link]

Leaves shifting color in fall

Leaves in temperate, boreal, and seasonally dry zones may be seasonally deciduous (falling off or dying for the inclement season). This mechanism to shed leaves is called abscission. After the leaf is shed, a leaf scar develops on the twig. In cold autumns, they sometimes change color, and turn yellow, bright-orange, or red, as various accessory pigments (carotenoids and xanthophylls) are revealed when the tree responds to cold and reduced sunlight by curtailing chlorophyll production. Red anthocyanin pigments are now thought to be produced in the leaf as it dies, possibly to mask the yellow hue left when the chlorophyll is lost - yellow leaves appear to attract herbivores such as aphids.^[7]

Morphology[link]

The Citrus leaf is identified by the pores and pigments, as well as the margins.

External leaf characteristics (such as shape, margin, hairs, etc.) are important for identifying plant species, and botanists have developed a rich terminology for describing leaf characteristics. These structures are a part of what makes leaves determinant; they grow and achieve a specific pattern and shape, then stop. Other plant parts like stems or roots are non-determinant, and will usually continue to grow as long as they have the resources to do so.

Classification of leaves can occur through many different designative schema, and the type of leaf is usually characteristic of a species, although some species produce more than one type of leaf. The longest type of leaf is a leaf from palm trees, measuring at nine feet long. The terminology associated with the description of leaf morphology is presented, in illustrated form, at Wikibooks.

Basic types[link]

Leaves of the White Spruce (Picea glauca) are needle-shaped and their arrangement is spiral

• Ferns have fronds
• Conifer leaves are typically needle-, awl-, or scale-shaped
• Angiosperm (flowering plant) leaves: the standard form includes stipules, a petiole, and a lamina
• Lycophytes have microphyll leaves.
• Sheath leaves (type found in most grasses)
• Other specialized leaves (such as those of Nepenthes)

Arrangement on the stem[link]

Different terms are usually used to describe leaf placement (phyllotaxis):

The leaves on this plant are arranged in pairs opposite one another, with successive pairs at right angles to each other ("decussate") along the red stem. Note the developing buds in the axils of these leaves.

• Alternate – leaf attachments are singular at nodes, and leaves alternate direction, to a greater or lesser degree, along the stem.
• Opposite – Two structures, one on each opposite side of the stem, typically leaves, branches, or flower parts. Leaf attachments are paired at each node; decussate if, as typical, each successive pair is rotated 90° progressing along the stem; or distichous if not rotated, but two-ranked (in the same geometric flat-plane).
• Whorled – three or more leaves attach at each point or node on the stem. As with opposite leaves, successive whorls may or may not be decussate, rotated by half the angle between the leaves in the whorl (i.e., successive whorls of three rotated 60°, whorls of four rotated 45°, etc.). Opposite leaves may appear whorled near the tip of the stem.
• Rosulate – leaves form a rosette

As a stem grows, leaves tend to appear arranged around the stem in a way that optimizes yield of light. In essence, leaves form a helix pattern centered around the stem, either clockwise or counterclockwise, with (depending upon the species) the same angle of divergence. There is a regularity in these angles and they follow the numbers in a Fibonacci sequence: 1/2, 2/3, 3/5, 5/8, 8/13, 13/21, 21/34, 34/55, 55/89. This series tends to a limit close to 360° x 34/89 = 137.52 or 137° 30', an angle known in mathematics as the golden angle. In the series, the numerator indicates the number of complete turns or "gyres" until a leaf arrives at the initial position. The denominator indicates the number of leaves in the arrangement. This can be demonstrated by the following:

• alternate leaves have an angle of 180° (or 1/2)
• 120° (or 1/3) : three leaves in one circle
• 144° (or 2/5) : five leaves in two gyres
• 135° (or 3/8) : eight leaves in three gyres.

Divisions of the blade[link]

A leaf with laminar structure and pinnate venation

Two basic forms of leaves can be described considering the way the blade (lamina) is divided. A simple leaf has an undivided blade. However, the leaf shape may be formed of lobes, but the gaps between lobes do not reach to the main vein. A compound leaf has a fully subdivided blade, each leaflet of the blade separated along a main or secondary vein. Because each leaflet can appear to be a simple leaf, it is important to recognize where the petiole occurs to identify a compound leaf. Compound leaves are a characteristic of some families of higher plants, such as the Fabaceae. The middle vein of a compound leaf or a frond, when it is present, is called a rachis.

• Palmately compound leaves have the leaflets radiating from the end of the petiole, like fingers off the palm of a hand, e.g. Cannabis (hemp) and Aesculus (buckeyes).
• Pinnately compound leaves have the leaflets arranged along the main or mid-vein.
  • odd pinnate: with a terminal leaflet, e.g. Fraxinus (ash).
  • even pinnate: lacking a terminal leaflet, e.g. Swietenia (mahogany).
• Bipinnately compound leaves are twice divided: the leaflets are arranged along a secondary vein that is one of several branching off the rachis. Each leaflet is called a "pinnule". The pinnules on one secondary vein are called "pinna"; e.g. Albizia (silk tree).
• trifoliate (or trifoliolate): a pinnate leaf with just three leaflets, e.g. Trifolium (clover), Laburnum (laburnum).
• pinnatifid: pinnately dissected to the central vein, but with the leaflets not entirely separate, e.g. Polypodium, some Sorbus (whitebeams). In pinnately veined leaves the central vein in known as the midrib.

Characteristics of the petiole[link]

The overgrown petioles of Rhubarb (Rheum rhabarbarum) are edible.

Petiolated leaves have a petiole (leaf stem). Sessile leaves do not: The blade attaches directly to the stem. In clasping or decurrent leaves, the blade partially or wholly surrounds the stem, often giving the impression that the shoot grows through the leaf. When this is the case, the leaves are called "perfoliate", such as in Claytonia perfoliata. In peltate leaves, the petiole attaches to the blade inside from the blade margin.

In some Acacia species, such as the Koa Tree (Acacia koa), the petioles are expanded or broadened and function like leaf blades; these are called phyllodes. There may or may not be normal pinnate leaves at the tip of the phyllode.

A stipule, present on the leaves of many dicotyledons, is an appendage on each side at the base of the petiole resembling a small leaf. Stipules may be lasting and not be shed (a stipulate leaf, such as in roses and beans), or be shed as the leaf expands, leaving a stipule scar on the twig (an exstipulate leaf).

• The situation, arrangement, and structure of the stipules is called the "stipulation".
  • free
  • adnate : fused to the petiole base
  • ochreate : provided with ochrea, or sheath-formed stipules, e.g. rhubarb,
  • encircling the petiole base
  • interpetiolar : between the petioles of two opposite leaves.
  • intrapetiolar : between the petiole and the subtending stem

Venation[link]

Branching veins on underside of taro leaf

The venation within the bract of a Lime tree.

The lower epidermis of Tilia × europaea

Palmate-veined leaf

There are two subtypes of venation, namely, craspedodromous, where the major veins stretch up to the margin of the leaf, and camptodromous, when major veins extend close to the margin, but bend before they intersect with the margin.

• Feather-veined, reticulate (also called pinnate-netted, penniribbed, penninerved, or penniveined) – the veins arise pinnately from a single mid-vein and subdivide into veinlets. These, in turn, form a complicated network. This type of venation is typical for (but by no means limited to) dicotyledons.
• Three main veins branch at the base of the lamina and run essentially parallel subsequently, as in Ceanothus. A similar pattern (with 3-7 veins) is especially conspicuous in Melastomataceae.
• Palmate-netted, palmate-veined, fan-veined; several main veins diverge from near the leaf base where the petiole attaches, and radiate toward the edge of the leaf, e.g. most Acer (maples).
• Parallel-veined, parallel-ribbed, parallel-nerved, penniparallel – veins run parallel for the length of the leaf, from the base to the apex. Commissural veins (small veins) connect the major parallel veins. Typical for most monocotyledons, such as grasses.
• Dichotomous – There are no dominant bundles, with the veins forking regularly by pairs; found in Ginkgo and some pteridophytes.

Note that, although it is the more complex pattern, branching veins appear to be plesiomorphic and in some form were present in ancient seed plants as long as 250 million years ago. A pseudo-reticulate venation that is actually a highly modified penniparallel one is an autapomorphy of some Melanthiaceae, which are monocots, e.g. Paris quadrifolia (True-lover's Knot).

Morphology changes within a single plant[link]

• Homoblasty - Characteristic in which a plant has small changes in leaf size, shape, and growth habit between juvenile and adult stages.
• Heteroblasty - Characteristic in which a plant has marked changes in leaf size, shape, and growth habit between juvenile and adult stages.

Terminology[link]

Chart illustrating some leaf morphology terms

A portion of a coriander leaf

Shape[link]

Edge (margin)[link]

• ciliate: fringed with hairs
• crenate: wavy-toothed; dentate with rounded teeth, such as Fagus (beech)
• crenulate finely or shallowly crenate
• dentate: toothed, such as Castanea (chestnut)
  • coarse-toothed: with large teeth
  • glandular toothed: with teeth that bear glands.
• denticulate: finely toothed
• doubly toothed: each tooth bearing smaller teeth, such as Ulmus (elm)
• entire: even; with a smooth margin; without toothing
• lobate: indented, with the indentations not reaching to the center, such as many Quercus (oaks)
  • palmately lobed: indented with the indentations reaching to the center, such as Humulus (hop).
• serrate: saw-toothed with asymmetrical teeth pointing forward, such as Urtica (nettle)
• serrulate: finely serrate
• sinuate: with deep, wave-like indentations; coarsely crenate, such as many Rumex (docks)
• spiny or pungent: with stiff, sharp points, such as some Ilex (hollies) and Cirsium (thistles).

Tip[link]

Leaves showing various morphologies. Clockwise from upper left: tripartite lobation, elliptic with serrulate margin, peltate with palmate venation, acuminate odd-pinnate (center), pinnatisect, lobed, elliptic with entire margin

• acuminate: long-pointed, prolonged into a narrow, tapering point in a concave manner.
• acute: ending in a sharp, but not prolonged point
• cuspidate: with a sharp, elongated, rigid tip; tipped with a cusp.
• emarginate: indented, with a shallow notch at the tip.
• mucronate: abruptly tipped with a small short point, as a continuation of the midrib; tipped with a mucro.
• mucronulate: mucronate, but with a smaller spine.
• obcordate: inversely heart-shaped, deeply notched at the top.
• obtuse: rounded or blunt
• truncate: ending abruptly with a flat end, that looks cut off.

Base[link]

• acuminate: coming to a sharp, narrow, prolonged point.
• acute: coming to a sharp, but not prolonged point.
• auriculate: ear-shaped.
• cordate: heart-shaped with the notch towards the stalk.
• cuneate: wedge-shaped.
• hastate: shaped like an halberd and with the basal lobes pointing outward.
• oblique: slanting.
• reniform: kidney-shaped but rounder and broader than long.
• rounded: curving shape.
• sagittate: shaped like an arrowhead and with the acute basal lobes pointing downward.
• truncate: ending abruptly with a flat end, that looks cut off.

Surface[link]

• farinose: bearing farina; mealy, covered with a waxy, whitish powder.
• glabrous: smooth, not hairy.
• glaucous: with a whitish bloom; covered with a very fine, bluish-white powder.
• glutinous: sticky, viscid.
• papillate, or papillose: bearing papillae (minute, nipple-shaped protuberances).
• pubescent: covered with erect hairs (especially soft and short ones).
• punctate: marked with dots; dotted with depressions or with translucent glands or colored dots.
• rugose: deeply wrinkled; with veins clearly visible.
• scurfy: covered with tiny, broad scalelike particles.
• tuberculate: covered with tubercles; covered with warty prominences.
• verrucose: warted, with warty outgrowths.
• viscid, or viscous: covered with thick, sticky secretions.

The leaf surface is also host to a large variety of microorganisms; in this context it is referred to as the phyllosphere.

Hairiness[link]

Common Mullein (Verbascum thapsus) leaves are covered in dense, stellate trichomes.

Scanning electron microscope image of trichomes on the lower surface of a Coleus blumei (coleus) leaf.

"Hairs" on plants are properly called trichomes. Leaves can show several degrees of hairiness. The meaning of several of the following terms can overlap.

• arachnoid, or arachnose: with many fine, entangled hairs giving a cobwebby appearance.
• barbellate: with finely barbed hairs (barbellae).
• bearded: with long, stiff hairs.
• bristly: with stiff hair-like prickles.
• canescent: hoary with dense grayish-white pubescence.
• ciliate: marginally fringed with short hairs (cilia).
• ciliolate: minutely ciliate.
• floccose: with flocks of soft, woolly hairs, which tend to rub off.
• glabrescent: losing hairs with age.
• glabrous: no hairs of any kind present.
• glandular: with a gland at the tip of the hair.
• hirsute: with rather rough or stiff hairs.
• hispid: with rigid, bristly hairs.
• hispidulous: minutely hispid.
• hoary: with a fine, close grayish-white pubescence.
• lanate, or lanose: with woolly hairs.
• pilose: with soft, clearly separated hairs.
• puberulent, or puberulous: with fine, minute hairs.
• pubescent: with soft, short and erect hairs.
• scabrous, or scabrid: rough to the touch.
• sericeous: silky appearance through fine, straight and appressed (lying close and flat) hairs.
• silky: with adpressed, soft and straight pubescence.
• stellate, or stelliform: with star-shaped hairs.
• strigose: with appressed, sharp, straight and stiff hairs.
• tomentose: densely pubescent with matted, soft white woolly hairs.
  • cano-tomentose: between canescent and tomentose.
  • felted-tomentose: woolly and matted with curly hairs.
• tomentulose: minutely or only slightly tomentose.
• villous: with long and soft hairs, usually curved.
• woolly:' with long, soft and tortuous or matted hairs.

Adaptations[link]

The embedded lists in this article may contain items that are not encyclopedic. Please help out by removing such elements and incorporating appropriate items into the main body of the article. (February 2008)

Poinsettia bracts are leaves which have evolved red pigmentation in order to attract insects and birds to the central flowers, an adaptive function normally served by petals (which are themselves leaves highly modified by evolution).

In the course of evolution, leaves have adapted to different environments in the following ways:

• A certain surface structure avoids moistening by rain and contamination (See Lotus effect).
• Sliced leaves reduce wind resistance.
• Hairs on the leaf surface trap humidity in dry climates and create a boundary layer reducing water loss.
• Waxy leaf surfaces reduce water loss.
• Large surface area provides large area for sunlight and shade for plant to minimize heating and reduce water loss.
• In more or less opaque or buried in the soil leaves, translucent windows filter the light before the photosynthesis takes place at the inner leaf surfaces (e.g. Fenestraria).
• Succulent leaves store water and organic acids for use in CAM photosynthesis.
• Aromatic oils, poisons or pheromones produced by leaf borne glands deter herbivores (e.g. eucalypts).
• Inclusions of crystalline minerals deter herbivores (e.g. silica phytoliths in grasses, raphides in Araceae).
• Petals attracts pollinators.
• Spines protect the plants (e.g. cacti).
• Insect traps feed the plants directly (see carnivorous plants).
• Bulbs store food and water (e.g. onions).
• Tendrils allow the plant to climb (e.g. peas).
• Bracts and pseudanthia (false flowers) replace normal flower structures when the true flowers are greatly reduced (e.g. Spurges).

Interactions with other organisms[link]

Some insects mimic leaves (Kallima inachus shown)

A girl playing with leaves

Leaf after being eaten by Caterpillar

Dew on a leaf

Although not as nutritious as other organs such as fruit, leaves provide a food source for many organisms. Animals which eat leaves are known as folivores. The leaf is one of the most vital parts of the plant, and plants have evolved protection against folivores such as tannins, chemicals which hinder the digestion of proteins and have an unpleasant taste.

Some animals have cryptic adaptations to avoid their own predators. For example, some caterpillars will create a small home in the leaf by folding it over themselves, while other herbivores and their prey mimic the appearance of the leaf. Some insects, such as the katydid, take this even further, moving from side to side much like a leaf does in the wind.

Bibliography[link]

• Leaves: The formation, charactistics and uses of hundred of leaves in all parts of the world by Ghillean Tolmie Prance. 324 photographic plates in black and white, and colour by Kjell B Sandved 256 pages^[8]

Footnotes[link]

See also[link]

• Abscission – loss of leaves
• Cladode - a flattened photsynthetic stem, if very leaf-like then it is called a Phylloclade
• Cladophyll – a stem-like leaf part (synonym: phyllode)
• Evolution of leaves
• Guttation – beads of fluid forming at leaf margins
• Leaf area index
• Leaf protein concentrate
• Leaf Sensor – a device that measures the moisture level in plant leaves
• Vernation – sprouting of leaves, also the arrangement of leaves in the bud

External links[link]

• Media related to Leaves at Wikimedia Commons
• The Wiktionary entry for leaf
• VASCULAR PLANT SYSTEMATICS Section B. General Characters and Character States: Position and Arrangement
• Science aid: Leaf Leaf structure and transpiration resource for teens.

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