Leaf

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This article is about leaf, a plant organ. See Leaf (disambiguation) for other meanings.

Fallen leaf of a maple. Note areas where chlorophyll (green) has been destroyed now appear yellow.

In botany, a leaf is a plant organ specialized for photosynthesis. For this purpose, a leaf is typically flat and thin, to expose the chloroplast containing cells (chlorenchyma) to light over a broad area, and to allow light to penetrate fully into the tissues. Leaves are also the sites in most plants where respiration, transpiration, and guttation take place. Leaves can store food and water, and are modified in some plants for other purposes.

Table of contents

1 Leaf structure

1.1 Epidermis
1.2 Mesophyll
1.3 Veins

2 Leaf Types, Arrangements, and Forms

3 Adaptations

Leaf structure

Detailed view of the veins

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). Not all species produce leaves with all of these parts. In some species, stipules are not obvious; or a petiole may be absent. The blade is not always laminar (flattened). External leaf characteristics (shape, margins, hairs, etc.) are important for identifying plant species. The point at which the petiole attaches to the plant stem is called the leaf axil.

A leaf typically consists of the following tissues:

An epidermis that covers the upper and lower surfaces;
An interior chlorenchyma called the mesophyll;
and veins (vascular tissue)

Epidermis

The epidermis is the outer layer of cells covering the leaf blade. The layer is usually transparent (cells lack chloroplasts) and coated on the outer surface with a waxy cuticle that prevents water loss. The cuticle may be 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 is covered with pores called stomata (sing., stoma) that enable oxygen and carbon dioxide to move in and out of the leaf. These pores are more numerous over the lower epidermis than the upper epidermis in most leaves. Water vapor also passes out of the stomata during transpiration. To conserve water, the guard cells at the opening of the stomata may force the opening to close up during the night.

Trichomes or hairs grow out from the epidermis in many species.

Mesophyll

Most of the interior of the leaf between the upper and lower layers of epidermis is a parenchyma or chlorenchyma tissue called the mesophyll. This is the primary photosynthetic tissue of the plant. It is divided into two layers: an upper palisade layer of tightly packed, vertically elongated cells, one to two cells thick. Beneath the palisade layer is the spongy layer. The cells of the spongy layer are more rounded and not so tightly packed. The pores or stomata of the epidermis open into the spaces between the spongy layer cells.

Leaves are normally green in color, which comes from chlorophyll found in plastids in the chlorenchyma. Leaves in Temperate, Boreal, and seasonally dry zones may be seasonally deciduous (falling off or dying for the inclement season). In cold autumns they sometimes turn yellow, bright orange or red as various accessory pigments (carotenoids and anthocyanins) are revealed when the tree responds to cold and reduced sunlight by curtailing chlorophyll production.

Veins

The veins are the vascular tissue of the leaf and are located in the spongy layer of the mesophyll. The veins are made up of xylem, which brings water from the stem into the leaf, and phloem, which usually moves sap out, the latter containing the glucose produced by photosynthesis in the leaf. The xylem typically lies over the phloem, and both are embedded in a dense parenchyma with usually some structural collenchyma tissue present.

The leaves on this plant are arranged in pairs opposite one another ("decussate") along the red stem. Note developing buds in the axils of these leaves.

Leaf Types, Arrangements, and Forms

Leaves may be classified in many different ways, and the type is usually characteristic of a species, although some species produce more than one type of leaf. The terminology associated with describing leaf morphology is presented (with illustrations) at Wikibooks (http://wikibooks.org/wiki/Botany:_Leaves_(forms)).

Basic leaf types:
- Ferns have fronds.
- Conifer leaves are typically needle-, awl-, or scale-shaped.
- Angiosperm (flowering plant) leaves: the standard form includes stipules, petiole, and lamina.
- Microphyll leaves.
- Sheath leaves (type found in most grasses).
- Specialized leaves.

Arrangement on the stem
- Alternate — leaf attachments singular at nodes, and alternate direction up the stem.
- Opposite — leaf attachments paired at each node; considered decussate if, as typical, each pair is rotated 90^o going along the stem; or distichous if not rotated, but two-ranked, all in the same plane.
- Whorled — three or more leaves attach at each point or node on the stem. Note: opposite leaves may appear whorled near the tip of the stem.
- Rosulate — leaves form a rosette ( = a cluster of leaves growing in crowded circles from a common center).

Leaves of the Norway Spruce (Picea abies)
are needle-shaped and the arrangement
is whorled

Divisions of the lamina (blade):
- Simple leaves have an undivided blade. The leaf shape may be one of deeply divided lobes, but the gaps between lobes do not reach to the vein.
- Compound leaves have divided blades, each leaflet separated along a main or secondary vein.
  - Palmately compound leaves have the leaflets radiating from the end of the petiole, like fingers off the palm.
  - Pinnately compound leaves have the leaflets arranged along the main or mid-vein (called a rachis in this case).
  - Bipinnately compound leaves are twice divided: the leaflets are arranged along a secondary vein that is one of several branching off the rachis.

Characteristics of the petiole:
- Petiolated leaves have a petiole.
  - In peltate leaves, the petiole attaches to the blade inside from the blade margin.
- Sessile leaves do not have a petiole; the blade attaches directly to the stem.

Arrangement of the veins (venation):
- Parallel-veined — veins run parallel most the length of the leaf.
- Pinnate-netted — leaf has usually one main vein (called the mid-vein), with smaller veins branching off, usually somewhat parallel to each other.
- Palmate-netted — several main veins diverge from near the leaf base where the petiole attaches.