The main function of leaves is to capture sunlight and use that light to create food through a process called photosynthesis. Once the photosynthates are produced, they are transported from the leaves to other parts of the plant via the phloem in the vascular bundle.
The leaf blade makes up most of a leaf and is used to capture and collect light energy. For some plants, the leaf blade connects to and is held away from the stem by the leaf’s petiole which also allows the leaf to rotate throughout the day to maximize the leaf blade's surface area exposed to the sun. Water, nutrients and other compounds needed for photosynthesis can travel from the stem into the leaf via xylem in the petiole’s vascular bundle, and food produced can flow out of the leaf to other parts of the plant via phloem in the petiole’s vascular bundle. There are, however, some plants called whose leaves called sessile or stalkless leaves do not have petioles - their leaves connect directly to the stem with no petiole.
As previously mentioned, the outside of the leaf blade on its top and bottom sides are covered by a thin layer of protective cells called the epidermis. This layer not only protects the tissue inside the leaf, but also regulates water loss. The epidermis might also be covered with a cuticle layer made of cutin, a waxy substance prevents water loss and can give leaves a shiny appearance.
The layer that exists between the lower and upper epidermis is called the mesophyll. The mesophyll contains chloroplasts, which are plastids or organelles that conduct photosynthesis. Chloroplasts contain the photosynthetic pigment chlorophyll, which allow them to capture energy from sunlight.
Plants that need only a moderate amount of water are called mesophytes. In these plants there is upper and lower mesophyll layer. The upper layer’s cells are densely packed palisade parenchyma cells, while the lower layer’s cells are loosely packed spongy parenchyma cells with lots of air spaces between them. These air spaces allow carbon dioxide, water, and oxygen to move freely with the lower layer.
There is also a huge network of vascular bundles running throughout the mesophyll, which connect to the vascular system of the stem. As with stems, the vascular bundles in leaves contain both xylem for transporting water to the leaves and phloem for transporting the photosynthates from the leaves to other parts of the plant.
Leaves have microscopic pores typically on the underside of the leaf called stomata (the singular form is stoma) that allow gases to enter and exit the leaf. In some plants like water lilies, the stomata can appear primarily on top of the leaf since it would otherwise be underwater. Different species have varying numbers and sizes of stomata. The stomata are typically open during the day when photosynthesis is occurring, but they can close during the day if more water is being lost by the leaves through transpiration than is being absorbed by the roots. The stomata close at night, because photosynthesis cannot occur without sunlight. The opening and closing of the stomata are controlled by a pair of specialized guard cells next to each stoma.
Light energy is captured by the chloroplasts in leaves. Carbon dioxide is collected by the leaves through their stomata. The light energy, carbon dioxide and water are used by the chloroplasts to produce chemical energy or food like carbohydrates through the process of photosynthesis. The food is then transferred from the leaves to the rest of the plant via the phloem in the vascular network that connects all stems, leaves, and roots. The photosynthetic reaction produces oxygen as a byproduct. The oxygen and water vapor escape from the leaves via the stomata.
Types of Leaves
A simple leaf has a single continuous leaf blade that is never divided into multiple leaflets. A compound leaf has multiple leaf blades called pinnae or leaflets, connected to the stem with a single petiole.
A compound leaf is said to be pinnately compound if the leaflets are arranged along a extension of the petiole, or rachis. There are three types of pinnately compound arrangements:
- Even-pinnate leaflet arrangement (also called paripinnate): Leaflets sprout in pairs opposite one another along the rachis without a single terminal leaflet at the end of the midrib (i.e., it has an even number of leaflets).
- Odd-pinnate leaflet arrangement (also called imparipinnate): Leaflets sprout in pairs opposite one another along the rachis with a single terminal leaflet (i.e., it has an odd number of leaflets).
- Alternate-pinnate leaflet arrangement: Leaflets sprout alternately on opposite sides of the rachis, typically with a single terminal leaflet.
Some pinnately compound leaves branch again to become bipinnate or double pinnate. This occurs in the honey locust, the mimosa, and the Kentucky Coffeetree.
A leaf is said to be palmately compound if the leaflets all attach to the petiole at a common center point at the distal end of the petiole. They are shaped like a palm frond or hand.
The shape of a leaf or leaflet is another characteristic used to identify and classify plants. While there are many overall leaf shapes, some of the more common ones are listed below.
Linear leaves are very narrow, and many times longer than they are wide. The sides of the leaf are almost parallel and come together at the tip. Linear leaves are shaped like a long sword blade. Most grasses have linear leaf blades.
Lanceolate leaves are similar to linear leaves but are wider in the center and taper towards both the apex and base. Daffodils have lanceolate leaves.
Ovate leaves are shaped like eggs, wider near the base and narrower near the apex. Magnolia trees tend to have ovate leaves.
Cordate leaves are heart-shaped with the point of the heart at the apex of the leaves. Redbud trees have cordate leaves. Plants with heart-shaped leaves with the point of the heart occurring at the base of the leaf are called obcordate. Yellow wood sorrels have obcordate leaves.
Orbicular leaves are circular, or almost circular, in shape. Some examples of plants with orbicular leaves are the Arrowwood Viburnum, the Cranberry Cotoneaster, and the Quaking Aspen.
Elliptic leaves are shaped like an oval or ellipse (a squashed circle). The Calathea ‘Eclipse’ plant has elliptic leaves.
The outer edge of a leaf blade is referred to as the leaf margin. This is another characteristic used to identify and classify plants. There are four basic types of leaf margins: entire, toothed, lobed, and parted or cleft.
Leaves that have an even and smooth edge with no teeth or notches all the way around the leaf is said to have an entire leaf margin. The leaves of the Southern Magnolia have an entire leaf margin.
Toothed margins have teeth like a saw. These can vary in size, sharpness, and shape. There are three basic types of toothed margins: serrate, dentate, and crenate.
Serrate leaf margins have pointed saw like teeth directed forward towards the apex of the leaf. Most poplar trees, for example, have serrate leaf margins.
Dentate margins have triangular teeth like serrate, only the teeth point outward from the leaf rather than leaning toward the apex. Strawberry leaves have dentate margins.
Crenate leaf margins have rounded teeth. Katsura trees have leaves with crenate margins.
Lobed margins have notches or incisions extending inward less than halfway to the midrib.
Parted or cleft margins have notches or incisions extending inward more than halfway to the midrib.
There are many other types of leaf margins, but these often are variations of the basic types above, like serrulate (with fine serration) and doubly serrate (serrate with sub-teeth).
Veins in leaves are an extension of the vascular system that exists in roots and stems. These veins extend from the stem through the petiole into the leaf. Once in the leaf blade, the veins extend throughout the lamina mesophyll.
The veins in leaves as in stems and roots are vascular bundles composed of xylem and phloem cells embedded typically in parenchyma (sometimes sclerenchyma) and enclosed in bundle sheath cells. The xylem transports water and needed minerals from the stem into the leaf. The phloem transports sugars produced by photosynthesis out of the leaf to the rest of the plant.
The veins in leaves occur in a variety of arrangements. The arrangement of veins in a leaf or leaflet is called the venation pattern, of which there are two primary types: parallel venation and reticulated venation.
In parallel venation, the veins run parallel to each other through the leaf blade. This is characteristic of most monocots. Parallel venation can be further broken down into two types:
- Pinnately parallel venation (or unicostate parallel venation) occurs when many veins originate at a prominent midrib and run parallel to each other from the midrib to the edge of the leaf.
- Palmately parallel venation (or multicostate parallel venation) can occur in two different patterns. When the veins originate at the tip of the petiole and run parallel to each other the length of the leaf blade meeting at the apex of the leaf, it is called divergent parallel venation. When the veins originate from the midrib and run parallel to each other through the leaf blade meeting at the apex of the leaf, it is called convergent parallel venation.
In reticulated venation or net venation, the primary veins extend out from one or more main ribs, and then branch multiple times into finer veinlets. The veinlets interconnect to form a complex network. Reticulated venation is a common characteristic of dicots, and can be further broken down into two types:
- Pinnate reticulate venation occurs when a leaf has a single main rib or midrib and the remaining veins branch or reticulate to form a network.
- Palmate reticulate venation occurs when a leaf has multiple main ribs from which the remaining veins branch or reticulate to form a network.
There are many textures of leaves that occur in nature. These textures are useful in identifying and classifying plants, but it is also important in garden design. Botanists have developed many terms for describing the various textures of leaf surfaces. Below are a sample of such terms divided in two groups: those without hairs and those with hairs.
- Leaf textures without hairs:
- Glabrous: Having a smooth, hairless surface
- Coriaceous: Having a tough, leather like surface
- Glutinous: Having a sticky surface
- Scabrose: Having a surface that is rough like sandpaper
- Farinose: Having a mealy surface with a covering of waxy, white powder
- Glaucous: Having a whitish or blueish waxy covering
- Leaf textures with hairs:
- Pubescent: Having a hairy surface
- Arachnoid: Having fine, entangled cobweb like hairs
- Downy: Having a surface covered with short, soft hairs
- Tomentose: Having a surface covered with matted, wooly hairs
- Hirsute: Having course, stiff hairs
- Hispid: Having a surface that is rough with bristles, stiff hairs, or minute prickles
While some difference in hairs can be observed with the naked eye, others require a hand lens, microscope, or even electron microscope to discern.
The arrangement of leaves on a stem is known as phyllotaxy. Leaves can be arranged along a plant’s stem in a variety of ways. How the leaves are arranged on the stem is another useful characteristic in the identification and classification of plants. Leaves are commonly arranged in one of four patterns: alternate, spiral, opposite, whirled.
Alternate arrangement occurs where there is only one leaf per node on the stem, and the leaves at the nodes directly above and below occur on the opposite side of the stem in a flat plane.
Spiral arrangement also occurs when there is one leaf per node - however, the leaf at each node is attached at one of several places around the stem (not just opposite) not in a flat plane creating a spiral pattern from the base or axil of the stem to the tip.
Opposite arrangement occurs when there are two leaves per node on the stem, and the pair of leaves are on opposite sides of the stem.
Whorled arrangement occurs when three or more leaves occur per node on the stem.
Some other less common leaf arrangements include basal, rosette, and distichous.