The leaf has evolved as the chief part of the plant for gathering light energy from the sun and conducting photosynthesis to transform that light energy into biochemical energy. Hence, its structure is adapted to that function.
Leaves are formed by a plant to manufacture food. Photosynthesis—a complicated chemical reaction in which carbon dioxide from the air and water from the soil, in the presence of light, produce sugar—is carried out in the chloroplasts found packed within the leaf cells.
Because energy is derived from light by chlorophyll, either the leaf must be thin enough for light to penetrate all the cell layers or, in the case of plants with succulent leaves, chloroplasts must be most concentrated near the surface of the leaves.
Orientation to Light
No matter how many leaves a plant has, each is arranged in respect to light. Although some plants that are adapted to hot, dry conditions may orient their leaves to minimize exposure to the sun, most arrange their leaves to maximize exposure to the sun. Some are exposed to direct rays of the sun; others may face only a portion of the sky.
A plant may be forced into various growing patterns to allow the leaves to be exposed to light. Some species germinate their seeds high above the ground, in the cracks of bark on tree trunks and branches. Plants that climb wind their way around larger plants until they gain a place in the sun. Therefore, leaf shape may be determined by where a plant species is best adapted to grow.
The leaves of plants that grow mostly in shade, called shade plants, have a larger surface area, tend to be thinner, and have a higher concentration of chlorophyll. The leaves of plants growing mostly in the sun, called sun plants, have a smaller surface area, tend to be thicker, and have a lower concentration of chlorophyll.
Even leaves on the same plant can vary in structure depending on whether they spend most of their time in the sun or the shade, showing similar traits as seen in sun and shade plants. Taxonomists use leaf pattern, leaf arrangement, and leaf shape to help identify and classify plants.
As leaves vary in size and shape, so do their edges, or margins.
Many plants, such as holly and thistles, have prickly margins; prickles are the outgrowths of the leaf’s vein endings. The prickly boundary acts as a defense against grazing animals. Some margins have razor-sharp, saw like teeth that cut anything that brushes across them. The margins of many tropical leaves terminate in finely pointed tips, called drip tips.
When rainwater accumulates on the leaf’s surface, the tip helps the water to drain off the leaf so that accumulated water will not weigh down the leaf to the point of breakage. Many leaves, however, have no distinctive margins; when the margin is even all around, it is called an “entire” margin.
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Vascular tissue that makes up leaf veins is composed of xylem, which brings water up from the roots, and phloem, which is responsible for transporting the products of photosynthesis. Xylem and phloem extend from the branch or stem into the leaf by a leaf trace.
This strand is continuous through the petiole into the leaf veins that intersect the entire leaf. The veins are the point of contact between root and chloroplasts, ensuring that water can be continually furnished during the photosynthetic process. The veins may also act as a structural support within the leaves.
If a blade has a midrib, it appears that the petiole extends onward to the tip of the leaf. Often, secondary veins branch off from this central vein, forming a reticulate pattern; other times, many of the same-sized veins branch out from the base of the blade in a fan-shaped pattern.
The upper surface of a leaf is covered by a continuous, transparent sheet of cells called the cuticle. The cuticle may perform three functions: to help prevent excess water loss, to protect against physical damage or damaging organisms, and to aid in reflecting intense sunlight.
Cuticle cells are generally thin-walled, except, perhaps, on the margin, where thicker cells reinforce the leaf and aid in preventing tearing of the leaf by wind currents.
The cell layer immediately beneath the cuticle layers is the epidermis. Cells between the upper and lower epidermis form mesophyll tissues (from the Greek mesos, “middle,” and phyll, “leaf”). Mesophyll in dicot leaves forms two observable layers; the upper layer is the palisade mesophyll, composed of cells that are columnar and closely packed; they are also rich in chloroplasts.
Below this layer is the spongy mesophyll, so named because of numerous air spaces surrounding the small, oval cells. The air spaces are important in circulating carbon dioxide and oxygen that enters and leaves from stomata, small openings, generally confined to the underside of the leaf, where gas exchange is regulated.
The stomata are bounded and controlled by two kidney-shaped cells called guard cells. Water evaporates from the leaf cells and goes into the air through these ventilation sites by a process referred to as transpiration. Normally, during light hours, the stomata are open (and losing water). At night time, the cells close, andwater is retained.
To open, potassiumions (K+) are pumped into the guard cells, and water follows by osmosis, which causes an increase in internal pressure, called turgor pressure. As pressure increases, the water pushes against and stretches the guard-cell walls, bowing the cells outward. The filling and stretching of the guard cells opens the stoma.
Stomata close when water stress (lack of water in the plant) occurs, which can result from in sufficient water in the soil or excessive transpiration rates. The most likely physiological mechanism for stomatal closing involves the hormone abscisic acid (ABA). The effects of water stress seem directly to trigger the release of ABA.
The exact mechanism is unclear, but in some way ABA causes K+ ions to move out of the guard cells and, again, water passively follows. When the guard cells lose their water, they become limp and close, sealing the stomatal opening, thus greatly reducing transpiration.
Transpiration of water at the leaf surface may be affected by several factors.Wind blowing across the surface carries off water molecules, leaving room for more water molecules to take their place; an increase in temperature does the same thing. Loss of water may be slowed by opposite conditions. In rainy or foggy conditions when the air is already saturated with water, water loss from leaves is lower.
Water loss also occurs slowly in cool conditions, such as those prevailing at night. An average sized birch tree will typically lose 17,260 liters of water through transpiration in a single growing season. One acre of grass lawn may lose 102,200 liters of water in a single week.
Water transpired into the air can affect rainfall patterns: It is likely that 50 percent of rainfall in the Brazilian rain forest originates from transpired water. Plants that grow in arid conditions have developed specialized leaves to decrease the amount of water lost by transpiration. Many of these plants have leaves that are small and thick, so that surface area is reduced.
The stomata may be housed in deep pits, away from wind’s evaporative force. During especially dry periods, some plants even shed their leaves to reduce water loss. Others carry on an alternate form of photosynthesis that allows the stomata to remain closed during all or part of the day.
In certain plants known as C4 plants, the leaves have adapted a particular way of fixing carbon; this has resulted in a ringlike arrangement of photosynthetic cells around the leaves’ veins, called Kranz anatomy.
This term (Kranz in German means “wreath”) refers to the fact that in C4 plants the cells that surround the water and carbohydrate-conducting system (known as the vascular system) are packed very tightly together and are called bundle sheath cells. Surrounding the bundle sheath is a densely packed layer ofmesophyll cells.
The densely packed mesophyll cells are in contact with air spaces in the leaf, and because of their dense packing they keep the bundle sheath cells from contact with air. This Kranz anatomy plays a major role in C4 photosynthesis.
In C4 plants the initial fixation of carbon dioxide from the atmosphere takes place in the densely packed mesophyll cells. After the carbon dioxide is fixed into a four-carbon organic acid, the malate is transferred through tiny tubes from these cells to the specialized bundle sheath cells.
Inside the bundle sheath cells, the malate is chemically broken down into a smaller organic molecule, and carbon dioxide is released. This carbon dioxide then enters the chloroplast of the bundle sheath cell and is fixed a second time with the enzyme Rubisco and continues, as in non-C4 plants, through the C3 pathway.