Leaf

From Wikipedia, the free encyclopedia
Jump to: navigation, search
The leaves of a Beech tree
SEM image of Nicotiana alata leaf's epidermis, showing trichomes (hair-like appendages) and stomata (eye-shaped slits, visible at full resolution).
Medium scale diagram of leaf internal anatomy
Fine scale diagram of leaf structure
Leaves can have different shapes. The part of biology that studies the shapes of things is called Morphology

A leaf is an above-ground plant organ. Its main functions are photosynthesis and gas exchange. A leaf is often flat, so it absorbs the most light, and thin, so that the sunlight can get to the chloroplasts in the cells. Most leaves have stomata, which open and close. They regulate carbon dioxide, oxygen, and water vapour exchange with the atmosphere.

Plants with leaves all year round are evergreens, and those that shed their leaves are deciduous. Deciduous trees and shrubs generally lose their leaves in autumn. Before this happens, the leaves change colour. The leaves will grow back in spring.

Leaves come in many shapes and sizes. The biggest undivided leaf is that of a giant edible aroid. This lives in marshy parts of the tropical rain forest of Borneo. One of its leaves can be ten feet across and have a surface area of over 30 square feet (~2.8 sq. metres).[1]

Leaf anatomy[change | change source]

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)

Epidermis[change | change source]

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 is covered with pores called stomata. They are part of a complex with 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.

Mesophyll[change | change source]

Most of the interior of the leaf between the upper and lower layers of epidermis is a tissue called the mesophyll (Greek for "middle leaf"). This assimilation tissue is the main place photosynthesis takes place in the plant. The products of photosynthesis are sugars, which can be turned into other products in plant cells.

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 good advantage of light. The slight separation of the cells provides maximum absorption of carbon dioxide. 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 chambers, which are connected to the air spaces between the spongy layer cells.

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

Veins[change | change source]

The 'veins' are 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 wide range of structures and functions.

Morphology[change | change source]

What leaves look like on the plant varies greatly. Closely related plants have the same kind of leaves because they have all descended from a common ancestor. The terms for describing leaf shape and pattern is shown, in illustrated form, at Wikibooks.

Basic types[change | change source]

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

Arrangement on the stem[change | change source]

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 — succeessive leaves in alternate direction along the stem.
  • Opposite — Two structures, one on each opposite side of the stem, typically leaves, branches, or flower parts.
  • Whorled — three or more leaves attach at each point or node on the stem.

Leaves form a helix pattern centered around the stem, 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. This tends to give the best chance for the leaves to catch light.

Divisions of the blade[change | change source]

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.

Petioles[change | change source]

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

Some leaves have a petiole (leaf stem). Sessile leaves do not: the blade attaches directly to the stem. Sometimes the leaf blad surrounds the stem, giving the impression that the shoot grows through the leaf.

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.

Stipules[change | change source]

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 shed or not shed.

Venation[change | change source]

Branching veins on underside of taro leaf
The lower epidermis of Tilia × europaea

There are two subtypes of venation:

  1. the major veins stretch up to the margin of the leaf, and
  2. when major veins extend close to the margin, but bend before they intersect with the margin.

Leaf adaptations[change | change source]

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, many species have leaves which are adapted to other functions.

  • Thorns help protect the plant from being eaten.
  • Vines help the plant to attach to surfaces, and to climb trees.
  • Some leaves are used to store energy in bulbs. An example is the onion.
  • Many succulents store water in some of their leaves.
  • Some plants (called epiphytes) grow on other plants. They do not have roots in the ground. Their capture rainwater.
  • Carnivorous plants use adapted leaves to capture their prey.
  • Sliced leaves reduce wind resistance.
  • Hairs on the leaf surface trap humidity in dry climates.
  • Waxy leaf surfaces reduce water loss.
  • Large surface area provides 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 let the light in.
  • Succulent leaves store water and organic acids.
  • Aromatic oils, poisons or pheromones produced by leaf borne glands deter herbivores (e.g. eucalyptus).
  • Crystalline minerals may herbivores (e.g. silica phytoliths in grasses, raphides in Araceae).
  • Petals attracts pollinators.
  • Tendrils allow the plant to climb (e.g. peas).
  • Bracts and 'false flowers' replace normal flower structures when the true flowers are greatly reduced (e.g. Spurges).

References[change | change source]

  1. Attenborough, David 1995. The private lives of plants: a natural history of plant behaviour. BBC Books, London. p47
  2. They are leaves which are supplied by a single unbranched vein.