A leaf is a plant structure or 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.
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.
Leaves are normally colored green, which comes from the 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.
A leaf typically consists of the following tissues:
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 stomata may close up during the night.
Hairs grow out from the epidermis in many species.
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.
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 produce 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.
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.
Gymnosperm leaves: typically needle-, awl-, or scale-shaped.
Angiosperm (flowering plant) leaves: standard form includes stipules, petiole, and lamina.
Sheath leaves (type found in most grasses).
Arrangement on the stem
Alternate — leaf attachments singular at nodes, and alternate direction up the stem.
Opposite — leaf attachments paired at each node; decussate if, as typical, each pair is rotated 90o going along the stem.
Whorled — three or more leaves attach at each point or node on the stem. Note: opposite leaves appear whorled near tip of stem.
Rosulate — leaves form a rosette.
Divisions of the lamina (blade):
Simple leaves have an undivided blade. The leaf shape may have deeply divided lobes, but the gaps between lobes do not reach to the vein.
Compound leaves have divided blades, each leaflet separeted along a main or secondary vein.
Palmately compound leaves have the leaflets radiating from the end of the petiole.
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 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.
In order to survive in a harsh environment, leaves can adapt in the following ways:
Hairy leaf surface to lessen water loss
Waxy leaf surface to prevent water loss
Small, shiny leaves to deflect the sun's rays
Thicker leaves to store water
Spines instead of leaves (e.g. cactus)
Leaves to trap insects (e.g. pitcher plant)