How leaves form:

Leaves are the most diverse of all plant organs-because they can be tubular, feathery, needlelike, cupped, smooth, sticky fragrant, or waxy. They range in size from pinhead-sized leaves to 20-meter across, and they range in number from millions to only one or two in the whole lifetime of the plant. Leaves are formed by the coordinated efforts of several meristems, each of which is named for its position. The earliest stage of leaf development is a small bulge at the shoot apex called a leaf buttress or primordium, which consists of 100 to 300 cells. It is formed by cellular divisions one to three cell layers below the overlying protoderm. More cellular divisions and expansions produce a radially symmetrical cone called an apical peg, which has an apical meristem and a procambial strand that forms the leaf's midrib. The adaxial surface of the apical peg elongates slower than the abaxial surface, and arches the leaf primordium over the shoot apical meristem.

The leaf then forms an adaxial meristem that thickens the leaf. It forms an upper leaf zone and a lower leaf zone. The upper zone contains marginal meristems that form the flattened blade and stalklike petiole that attaches the leaf to the stem. The lower leaf zone forms the leaf base.

Continued growth of a leaf involves cellular expansion and division. These divisions continue until the leaf is one-half to three-fourths grown. Cellular expansion forms most of the intercellular spaces in a leaf. Stomata differentiate soon after intercellular spaces form. Except for vascular tissues which differentiates from the base into the tip of the leaf, other tissues in leaves differentiate from the tip toward the base. Until it is 30%-40% of its final size, a growing leaf depends on the rest of the plant for its nutrition.

The Structure of Leaves:

External Structure:

There are four basic kinds of leaves: simple, compound, peltate, and perfoliate: 

• Simple leaves-have a flat, undivided blade that is supported by a stalk called a petiole. The petiole is typically supported by collenchyma and sclerenchyma fibers.
• Compound leaves-have blades divided into leaflets that form in one plane and lack axillary buds. Each compound leaf has a single bud at the base of its petiole. There are two kinds of compound leaves: pinnately compound and palmately compound leaves. Pinnately form in pairs along a central stalklike rachis, and palmately attach at the same point.
• Peltate leaves-have petioles that attach to the middle of the blade.
• Perfoliate leaves-are sessile leaves that surround and are pierced by the stems.

Internal Structure:

Leaves consists of epidermal, ground, and vascular tissues. 

• Epidermis-it is compact, transparent, and usually not photosynthetic. It also contains many stomata. In horizontally oriented leaves, there are usually more stomata on the protected lower side that the exposed upper side. Vertical leaves usually have similar numbers of stomata on both sides. Although stomata occupy 1% of the leaf surface, they lose huge amounts of water in to the atmosphere.
• Vascular-Xylem and phloem in leaves form in strands called veins. Xylem forms on teh upper side of a vein, and phloem forms on the lower side. Veins are supported by fibers and are usually surrounded by a layer of parenchyma cells called the bundle sheath, which extends to the epidermis of the leaf. These help support the veins and may conduct water to the epidermal cells. Most dicots and some nonflowering plants have netted venation, meaning they have one or a few prominent midveins, from which smaller minor veins branch into a meshed network. The leaves of monocots have parallel venation, meaning several prominent and parallel veins interconnect with smaller, inconspicuous veins. Several factors influence the formation of veins, including the presence of other veins.
• Ground-The ground tissue of leaves is called mesophyll. It contains many types of cells, including sclerenchyma, storage parenchyma, and chlorenchyma. In general, the arrangement of chlorenchyma is determined genetically, and is influenced by whether the leaf is oriented horizontally or vertically when it forms.

Modified Leaves:

Like other organs, leaves are often modified for functions other than photosynthesis. Below are a few examples: 

• Tendrils-of plants are leaves modified for support. In some plants the entire leaf is a tendril; photosynthesis in these plants is delegated to leaflike structures called stipules at the base of each leaf. Tendrils, of many plants may be up to 30 cm long, which makes them well suited for seeking support in the plant's nearby environment.
• Stipules-are small, leaflike structures at the base of petioles, and have a variety of functions. Some are photosynthetic, while others form protective spines.
• Spines-leaves modified for protection.
• Bud Scales-are tough, overlapping, waterproof leaves that protect buds from frost, desiccation, and pathogens. Bud scales form before teh onset of unfavorable growing seasons ie winter.
• Window leaves-are common in many desert plants, are shaped like tin ice-cream cones and grow mostly underground, with only a small transparent "window" tip protruding above the soil level.
• Bracts are floral leaves that form at the base of a flower or flower stalk. They are usually small and scalelike, and protect developing flowers.
• Storage leaves-flowerpot leaves are typically of rosette plants, and such epiphytes. They are packed tightly into a flowerpotlike structure that catches falling water and debris.
• Insect-trapping leaves-in carnivorous plants, leaves modified for attracting, trapping, and digesting animals. These adaptations range from sticky flypaper surfaces to vatlike leaves.
• Leaves modified for reproduction-form tiny plants at the edges of their leaves. These plants become new individuals when they are shed from parent leaves.
• Cøtyledons-are embryonic leaves. Monocots, usually have one cotyledon, while dicots have two.
• Prophylls-are the first leaves to form on axillary buds. Monocots have one usually, where dicots have two, suggesting that tiny leaves may be analogous to cotyledons. Prophylls protect axillary buds.

By: Becky Earley
Information provided by: http://wwwfac.wmdc.edu