Morphology

A structurally complete leaf of an angiosperm consists of a petiole (leaf stalk), a lamina (leaf blade), stipules (small structures located to either side of the base of the petiole) and a sheath. Not every species produces leaves with all of these structural components. The proximal stalk or petiole is called a stipe in ferns. The lamina is the expanded, flat component of the leaf which contains the chloroplasts. The sheath is a structure, typically at the base that fully or partially clasps the stem above the node, where the latter is attached. Leaf sheathes typically occur in grasses and Apiaceae (umbellifers). Between the sheath and the lamina, there may be a pseudopetiole, a petiole like structure. Pseudopetioles occur in some monocotyledons including bananas, palms and bamboos. Stipules may be conspicuous (e.g. beans and roses), soon falling or otherwise not obvious as in Moraceae or absent altogether as in the Magnoliaceae. A petiole may be absent (apetiolate), 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. The upper (adaxial) angle between a leaf and a stem is known as the axil of the leaf. It is often the location of a bud. Structures located there are called "axillary".

External leaf characteristics, such as shape, margin, hairs, the petiole, and the presence of stipules and glands, are frequently important for identifying plants to family, genus or species levels, and botanists have developed a rich terminology for describing leaf characteristics. Leaves almost always have determinate growth. They grow to a specific pattern and shape and then stop. Other plant parts like stems or roots have non-determinate growth, and will usually continue to grow as long as they have the resources to do so.

The type of leaf is usually characteristic of a species (monomorphic), although some species produce more than one type of leaf (dimorphic or polymorphic). The longest leaves are those of the Raffia palm, R. regalis which may be up to 25 m (82 ft) long and 3 m (9.8 ft) wide. The terminology associated with the description of leaf morphology is presented, in illustrated form, at Wikibooks.

Where leaves are basal, and lie on the ground, they are referred to as prostrate.

Basic leaf typesedit

Perennial plants whose leaves are shed annually are said to have deciduous leaves, while leaves that remain through winter are evergreens. Leaves attached to stems by stalks (known as petioles) are called petiolate, and if attached directly to the stem with no petiole they are called sessile.

• Ferns have fronds.
• Conifer leaves are typically needle- or awl-shaped or scale-like, they are usually evergreen, but can sometimes be deciduous. Usually, they have a single vein.
• Flowering plant (Angiosperm) leaves: the standard form includes stipules, a petiole, and a lamina.
• Lycophytes have microphylls.
• Sheath leaves are the type found in most grasses and many other monocots.
• Other specialized leaves include those of Nepenthes, a pitcher plant.

Dicot leaves have blades with pinnate vegetation (where major veins diverge from one large mid-vein and have smaller connecting networks between them). Less commonly, dicot leaf blades may have palmate venation (several large veins diverging from petiole to leaf edges). Finally, some exhibit parallel venation.

Monocot leaves in temperate climates usually have narrow blades, and usually parallel venation converging at leaf tips or edges. Some also have pinnate venation.

Arrangement on the stemedit

Different terms are usually used to describe the arrangement of leaves on the stem (phyllotaxis):

Alternate

One leaf, branch, or flower part attaches at each point or node on the stem, and leaves alternate direction, to a greater or lesser degree, along the stem.

Basal

Arising from the base of the stem.

Cauline

Arising from the aerial stem.

Opposite

Two leaves, branches, or flower parts attach at each point or node on the stem. Leaf attachments are paired at each node and decussate if, as typical, each successive pair is rotated 90° progressing along the stem.

Whorled, or verticillate

Three or more leaves, branches, or flower parts 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. Pseudoverticillate describes an arrangement only appearing whorled, but not actually so.

Rosulate

Leaves form a rosette.

Rows

The term, distichous, literally means two rows. Leaves in this arrangement may be alternate or opposite in their attachment. The term, 2-ranked, is equivalent. The terms, tristichous and tetrastichous, are sometimes encountered. For example, the "leaves" (actually microphylls) of most species of Selaginella are tetrastichous, but not decussate.

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 the golden angle, which is approximately 360° × 34/89 ≈ 137.52° ≈ 137° 30′. In the series, the numerator indicates the number of complete turns or "gyres" until a leaf arrives at the initial position and 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 bladeedit

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 may be dissected to form 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 being separated along a main or secondary vein. The leaflets may have petiolules and stipels, the equivalents of the petioles and stipules of leaves. 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 of the palm of a hand; for example, Cannabis (hemp) and Aesculus (buckeyes).

Pinnately compound

Leaves have the leaflets arranged along the main or mid-vein.

Odd pinnate

With a terminal leaflet; for example, Fraxinus (ash).

Even pinnate

Lacking a terminal leaflet; for example, Swietenia (mahogany). A specific type of even pinnate is bipinnate, where leaves only consist of two leaflets; for example, Hymenaea.

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 group of pinnules on each secondary vein forms a pinna; for example, Albizia (silk tree).

Trifoliate (or trifoliolate)

A pinnate leaf with just three leaflets; for example, Trifolium (clover), Laburnum (laburnum), and some species of Toxicodendron (for instance, poison ivy).

Pinnatifid

Pinnately dissected to the central vein, but with the leaflets not entirely separate; for example, Polypodium, some Sorbus (whitebeams). In pinnately veined leaves the central vein in known as the midrib.

Characteristics of the petioleedit

Petiolated leaves have a petiole (leaf stalk), and are said to be petiolate.

Sessile (epetiolate) leaves have no petiole and the blade attaches directly to the stem. Subpetiolate leaves are nearly petiolate or have an extremely short petiole and may appear to be sessile.

In clasping or decurrent leaves, the blade partially surrounds the stem.

When the leaf base completely surrounds the stem, the leaves are said to be perfoliate, such as in Eupatorium perfoliatum.

In peltate leaves, the petiole attaches to the blade inside 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, lateral

As in Hibiscus.

Adnate

Fused to the petiole base, as in Rosa.

Ochreate

Provided with ochrea, or sheath-formed stipules, as in Polygonaceae; e.g., rhubarb.

Encircling the petiole base

Interpetiolar

Between the petioles of two opposite leaves, as in Rubiaceae.

Intrapetiolar

Between the petiole and the subtending stem, as in Malpighiaceae.

Veinsedit

Veins (sometimes referred to as nerves) constitute one of the more visible leaf traits or characteristics. The veins in a leaf represent the vascular structure of the organ, extending into the leaf via the petiole and providing transportation of water and nutrients between leaf and stem, and play a crucial role in the maintenance of leaf water status and photosynthetic capacity.They also play a role in the mechanical support of the leaf. Within the lamina of the leaf, while some vascular plants possess only a single vein, in most this vasculature generally divides (ramifies) according to a variety of patterns (venation) and form cylindrical bundles, usually lying in the median plane of the mesophyll, between the two layers of epidermis. This pattern is often specific to taxa, and of which angiosperms possess two main types, parallel and reticulate (net like). In general, parallel venation is typical of monocots, while reticulate is more typical of eudicots and magnoliids ("dicots"), though there are many exceptions.

The vein or veins entering the leaf from the petiole are called primary or first-order veins. The veins branching from these are secondary or second-order veins. These primary and secondary veins are considered major veins or lower order veins, though some authors include third order. Each subsequent branching is sequentially numbered, and these are the higher order veins, each branching being associated with a narrower vein diameter. In parallel veined leaves, the primary veins run parallel and equidistant to each other for most of the length of the leaf and then converge or fuse (anastomose) towards the apex. Usually, many smaller minor veins interconnect these primary veins, but may terminate with very fine vein endings in the mesophyll. Minor veins are more typical of angiosperms, which may have as many as four higher orders. In contrast, leaves with reticulate venation there is a single (sometimes more) primary vein in the centre of the leaf, referred to as the midrib or costa and is continuous with the vasculature of the petiole more proximally. The midrib then branches to a number of smaller secondary veins, also known as second order veins, that extend toward the leaf margins. These often terminate in a hydathode, a secretory organ, at the margin. In turn, smaller veins branch from the secondary veins, known as tertiary or third order (or higher order) veins, forming a dense reticulate pattern. The areas or islands of mesophyll lying between the higher order veins, are called areoles. Some of the smallest veins (veinlets) may have their endings in the areoles, a process known as areolation. These minor veins act as the sites of exchange between the mesophyll and the plant's vascular system. Thus, minor veins collect the products of photosynthesis (photosynthate) from the cells where it takes place, while major veins are responsible for its transport outside of the leaf. At the same time water is being transported in the opposite direction.

The number of vein endings is very variable, as is whether second order veins end at the margin, or link back to other veins. There are many elaborate variations on the patterns that the leaf veins form, and these have functional implications. Of these, angiosperms have the greatest diversity. Within these the major veins function as the support and distribution network for leaves and are correlated with leaf shape. For instance, the parallel venation found in most monocots correlates with their elongated leaf shape and wide leaf base, while reticulate venation is seen in simple entire leaves, while digitate leaves typically have venation in which three or more primary veins diverge radially from a single point.

In evolutionary terms, early emerging taxa tend to have dichotomous branching with reticulate systems emerging later. Veins appeared in the Permian period (299–252 mya), prior to the appearance of angiosperms in the Triassic (252–201 mya), during which vein hierarchy appeared enabling higher function, larger leaf size and adaption to a wider variety of climatic conditions. 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). In leaves with reticulate venation, veins form a scaffolding matrix imparting mechanical rigidity to leaves.

Morphology changes within a single plantedit

Homoblasty

Characteristic in which a plant has small changes in leaf size, shape, and growth habit between juvenile and adult stages, in contrast to;

Heteroblasty

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