Vegetative Structure of Seed Plants: The Leaf

A leaf is an above-ground plant organ specialized for photosynthesis. For this purpose, a leaf is typically flat (laminar) and thin. There is continued debate about whether the flatness of leaves evolved to expose the chloroplasts to more light or to increase the absorption of carbon dioxide. In either case, the adaption was made at the expense of water loss.

In the Devonian period, when carbon dioxide concentration was at several times its present value, plants did not have leaves or flat stems. The leaves of gymnosperms, and angiosperms are variously referred to as macrophyll, megaphylls, or euphylls.

Leaves are also the sites in most plants where transpiration and guttation take place. Leaves can store food and water, and are modified in some plants for other purposes. The comparable structures of ferns are correctly referred to as fronds.

Furthermore, leaves are prominent in the human diet as leaf vegetables.

Anatomy of Leaf

A structurally complete leaf of an angiosperm consists of a petiole (leaf stem), a lamina(leaf blade), and stipules (small processes located to either side of the base of the petiole). The petiole attaches to the stem at a point called the “leaf axil.” Not every species produces leaves with all of the aforementioned structural components.

In some 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 (Fig 3.7). After a period of time (i.e.seasonally, during the autumn), deciduous trees shed their leaves. These leaves then decompose into the soil.

A leaf is considered a plant organ and typically consists of the following tissues as shown in fig. 6.7:

An epidermis that covers the upper and lower surfaces.

An interior chlorenchymacalled the mesophyll.

An arrangement of veins (the vascular tissue).

Epidermis

The epidermis is the outer multi-layered group 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, 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 thicker on leaves from dry climates as compared with those from wet climates.

The epidermis tissue includes several differentiated cell types: epidermal cells, guard cells, subsidiary cells, and epidermal hairs (trichomes). The epidermal cells are the most numerous, largest, and least specialized. 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.

The stoma complex regulates the exchange of gases and water vapor between the outside air and the interior of the leaf. Typically, the stomata are more numerous over the abaxial (lower) epidermis than the adaxial (upper) epidermis.

Mesophyll

Most of the interior of the leaf between the upper and lower layers of epidermis is a parenchyma(ground tissue) or chlorenchymatissue 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 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 chloroplastsclose 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 environments (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 stomataof the epidermis open into substomatal chambers, connecting to 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.

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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.

Veins

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 brings 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 over the phloem. Both are embedded in a dense parenchyma tissue, called “pith”, with usually some structural collenchyma tissue present.

Leaf morphology

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.

Basic leaf types

Fig 6.9 Leaves of the White Spruce (Piceaglauca) are needle-shaped and their arrangement is spiral.

Conifer leaves are typically needle-, awl-, or scale-shaped

Angiosperm (flowering plant) leaves: the standard form includes stipules, a petiole, and a lamina

Arrangement on the stem

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

Fig 6.10a 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.

Fig 6.10b alternate leaf arrangement: angiosperm leaf morphology.

Alternate — leaf attachments are singular at nodes, and leaves alternate direction, to a greater or lesser degree, along the stem.

Opposite — 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 centred around the stem, either clockwise or counterclockwise, with (depending upon the species) the same angle of divergence.

Divisions of the lamina (blade)

Two basic forms of leaves can be described considering the way the blade 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.

Fig 6.11: A leaf with laminar structure and pinnate venation .

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).

Pinnatelycompoundleaves 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: a pinnate leaf with just three leaflets, e.g. Trifolium (clover), Laburnum (Laburnum).

Pinnatifid: pinnately dissected to the midrib, but with the leaflets not entirely separate, e.g. Polypodium, some Sorbus(whitebeams).

Venation (arrangement of the veins)

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 — 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.

Pinnate-netted, penniribbed, penninerved, penniveined; the leaf has usually one main vein (called the mid-vein), with veinlets, smaller veins branching off laterally, usually somewhat parallel to each other; eg Malus(apples).

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 Ginkgoand some pteridophytes.

Fig 6.12a The lower epidermis of Tilia x europea. Fig 6.12b Palmate-veined leaf .

Leaf morphology changes within a single plant

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.

In conclusion, leaves are the photosynthetic organ of a plant. They vary in shape and sizes depending on the species and are also arranged differently in the plant. Some leaves are modified. Some bear thorns or hairlike appendages.

A leaf is an above-ground plant organ specialized for photosynthesis. A structurally complete leaf of an angiosperm consists of a petiole (leaf stem), a lamina(leaf blade), and stipules. A leaf is considered a plant organ and typically consists of the epidermis, mesophyll and vascular tissues.

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