Leaf



In botany, a leaf is an above-ground 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.

The comparable structures of ferns are referred to as fronds.

A structurally complete leaf of an angiosperm consists typically of a petiole (leaf stem), a lamina (leaf blade), and stipules (small processes located to either side of the base of the petiole).

The point at which the petiole attaches to the plant stem is called the leaf axil.

Not all species produce leaves with all of these parts. In some species, stipules are not obvious or present, or the petiole may be absent. The blade is not always laminar (flattened).

A leaf has a dorsiventral anatomy, in other words the upper surface and the underside have a different construction and different functions.

The external leaf characteristics (such as shape, margin, hairs, etc.) are important for identifying plant species.

A leaf typically consists of the following tissues:

1. An epidermis that covers the upper and lower surfaces

2. An interior chlorenchyma called the mesophyll

3. A number of veins (the vascular tissue).

Epidermis

The epidermis is the outer multi-layered group of cells covering the leaf blade. It is the boundary between the plant and its surroundings. The structure of the epidermis of the upper surface and the underside of the leaf may be different.

The epidermis has several functions: absorption of water, protection against water loss, gas exchange through photosynthesis, secretion of metabolic compounds, and protection against outside influences.

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 contains differentiated cell types; epidermal cells, guard cells, subsidiary cells and the epidermal hairs (trichomes).

The epidermal cells are the largest and the least specialized group. They are elongated in the leaves of monocots.

The epidermis is covered with pores called stomata (sing., stoma). It is part of the stoma complex: the stoma, enclosed at each side by chloroplast-containing guard cells, and two to four subsidiary cells (lacking chloroplast). These stomata enable oxygen and carbon dioxide to move in and out of the leaf. There is a substomatal chamber (an air space) below the stoma. These pores are more numerous over the abaxial (lower) epidermis than the (adaxial) upper epidermis in most leaves. Water vapour also passes out of the stomata during transpiration.

To conserve water, the paired guard cells at each side of the stomata may force the opening or the closing of the stomata. This happens through a differential thickening of the walls of the guard cells. When they become inflated in a humid surrounding, the internal water pressure (turgor pressure) opens up the stoma. The inverse happens, as the guard cells lose water pressure on a warm day, regulating transpiration and the flow of carbon dioxide.

The opening and closing of the stomata is also triggered by light. They are open during daytime and closed at night. The reverse happens with some plants from arid regions, such as some of the sedum family (Crassulaceae); to prevent extensive water loss, they open their stomata during the night.

Stomata are not found in some underwater plants.

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(ground tissue) or chlorenchyma tissue called the mesophyll (= middle leaf). This assimilation tissue is made of chloroplast and is the primary photosynthetic tissue of the plant. The products of this photosynthesis are called assimilates. There are also a few plants with mesophyll which lack chloroplasts, and therefore also lack assimilation.

In ferns and most flowering plants it 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 optimal advantage of light. The slight separation of the cells provides maximal absorption of carbon dioxide. This separation must be minimal to afford capillary action for water distribution. In order to adapt to their different environment (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 (substomatal chambers). These cells contain less chloroplasts.

The pores or stomata of the epidermis open into substomatal chambers, the 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 homogenous 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.

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. 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.