THE LEAF : Animation description

During photosynthesis a leaf captures light energy, absorbs atmospheric gases and transports water and sugars in a system of veins. 

Upper epidermis: 
The leaf surface consists of compactly arranged epidermal cells covered by a layer called the cuticle. The cuticle consists of fatty material, or cutin, and may also contain waxes. The cuticle restricts water loss and is resistant to infection by pathogens. Its formation is affected by light intensity and availability of water. Without the cuticle the leaf would lose large amounts of water.

Mesophyll parenchyma: 
This is the photosynthetic ground tissue of the leaf and contains an interconnected system of air-filled spaces between the cells. In the leaves of some plants it contains cells of uniform size and structure. In the leaves of other plants mesophyll consists of an upper layer of columnar shaped cells called the palisade mesophyll. Chloroplasts in palisade cells are distributed along the vertical cell walls; this is the principle photosynthetic layer of the plant. The elongate shape of the cells serves to transmit light from the epidermis efficiently, eg to act as 'light pipes' transmitting light to the chloroplasts along its walls and also down to the lower layers of the leaf. The lower portion of the mesophyll often contains irregularly-shaped photosynthetic cells called spongy mesophyll.

The lower epidermis: 
The spongy mesophyll makes contact with the lower leaf epidermis. This contains epidermal cells but also numerous guard cells. Guard cells in most dicots are bean-shaped and usually have a thicker wall adjacent to the stomatal pore. Guard cells of grasses and sedges are elongated with swollen ends, much like dumbbells. Guard cells are associated with special epidermal cells called subsidiary cells that are believed to have a role in the functioning of the guard cells. Together these comprise the stomatal apparatus. Guard cells are capable of reversible changes in shape, moving apart to create a pore or stomum (pl, stomata) and moving together to close this pore. In most plants of temperate climates guard cells are more frequent on the lower leaf surface with an average of 180 stomata per mm2 being recorded. In plants adapted to dry climates guard cells are usually found only on the lower leaf surface and are often recessed below the surface.

Guard cells: 
Leaves absorb atmospheric gasses and regulate the loss of water. In many plants these functions take place on the lower leaf surface where numerous pairs of guard cells are located. Unlike the surrounding cells of the epidermis, guard cells contain chloroplasts. 

Turgor pressure: 
In some plants the opening of the stomata can be explained by the movement of potassium ions. In the morning when the sun comes up potassium ions enter the guard cells where they accumulate inside the vacuoles. Water then moves across the vacuole membrane by osmosis causing the vacuoles to expand and increase in size. The enlarging vacuoles press against the cell walls and pressure known as turgor pressure increases within the guard cells. Guard cell expansion is restricted by strengthening of guard cells walls with hoop-like deposits of cellulose. 

Stomatal pore opening: 
Increasing turgor pressure combined with the limitations on wall expansion eventually results in the guard cells, which are held together at their bases, 'bowing' apart along their length. This creates a space between them called the stoma. Atmospheric gasses are now able to enter the leaf but water vapour within the leaf is also able to escape. Although stomata may only occupy 1% of the leaf surface, they account for 90% of the water lost from a leaf.

Stomatal pore closing: 
In the evening, potassium ions followed by water move out of the guard cells and back into the epidermal cells. The resulting water loss from the guard cells results in the closure of the stoma. In this way gas exchange in leaves is regulated by the opening and closing of stomatal pores. In some succulent plants adapted to very dry climates, the stomata open in the evening when it is cool, capturing carbon dioxide and converting it to organic acids. The stomata of these plants close in the morning and the organic acids made during the night yield carbon dioxide used in photosynthesis during the heat of the day.

Carbon dioxide and water: 
When the stomatal pores are open carbon dioxide and other atmospheric gasses pass into the leaf. The carbon dioxide is then able to diffuse inside the extensive system of intercellular spaces of the spongy and palisade mesophyll parenchyma. The thin colourless epidermal cells on the upper suface allow sunlight to pass through to the mesophyll below. When carbon dioxide contacts a moist mesophyll cell it goes into solution and passes through the wall. Inside the cell the dissolved carbon dioxide passes into the chloroplasts along with water. Both are utilised in photosynthesis to produce organic compounds and oxygen which are released into the surrounding cytoplasm.

Loading of minor vein phloem: 
Mesophyll cells release oxygen and organic compounds. The organic compounds, mostly sucrose, are transferred across the bundle sheath and into the minor vein where they accumulate in the phloem cells. Sucrose, a disaccharide, is water soluble and is the major transport sugar in plants. Starch, a long chain polymer of glucose, is not water soluble and is the major storage polysaccharide in plants. An increase in sucrose concentration inside the phloem causes water in the xylem to move into the phloem by osmosis. In some plants minor veins comprise 95% of the total vein length providing an extensive interface with the mesophyll cells. 

Transport in veins: 
As water continues to accumulate inside phloem cells the increasing pressure drives the water and the sugars along the strand of phloem and out of the leaf. Water also moves from the xylem and into the mesophyll cells. This water is used in photosynthesis and also to replace water that is lost to the intercellular spaces. The veins therefore transport water into the leaf and sugars out of the leaf. The water and oxygen in the intercellular spaces of the leaf are lost through the open stomata. The transport of sucrose and other compounds is at rates of 50 to 100 centimeters per hour.

Simple leaves: 
The blade (lamina) of the leaf is undivided , e.g., simple. The blade is supported by a stalk called a petiole which attaches it to the stem. An axillary bud is located above the junction of petiole and stem (the leaf axil).

Compound leaves: 
The blade is divided into leaflets, e.g., it is 'compounded' of many parts. If the leaflets are arranged along a linear axis (rachis) it is called a pinnately compound leaf. If the leaflets are all attached at one point, e.g., at the end of the petiole, it is called a palmately compound leaf.

A plant adapted to a dry environment

Plants that grow in dry habitats usually have leaves with a reduced surface area. 

Oleander (Nerium oleander), upper epidermis 
Water loss from the upper surface of oleander leaves is reduced by the absence of guard cells and a thick cuticle layer.

Oleander, lower epidermis: 
The lower epidermis forms pockets that project into the spongy mesophyll cells. These appear as cavities in the lower surface. Numerous hairs line these cavities. At the base of each cavity there are several stomata and such cavities are therefore called a stomatal crypts. The cavities reduce the rate of water loss from open guard cells by restricting the drying effects of winds.

A plant adapted to an aquatic environment

Plants that grow in water often have large floating leaves. 

Water lily (Nymphaea sp.), upper epidermis: 
There are numerous guard cells on the upper epidermis which is also covered by a water-repellent cuticle.

Water lily, mesophyll: 
The spongy mesophyll is unusual as the cells are arranged in sheets forming the walls of box-like structures each containing a large air space. These pockets of air allow the large leaf to float on the water surface. Large, thick-walled cells called sclereids are found in the mesophyll which contribute to structural support.

Lower epidermis: 
Guard cells are absent.