Briefly explain what is The Leaf?

In a typical dicot, leaf primordia arise from the apical meristem, which is housed within the terminal bud on a shoot. The leaf cells are produced from meristematic tissues called nodes, and these tissues include the protoderm, procambium, and ground meristem.
The protoderm produces a surface layer of epidermal cells that elongate and migrate to cover the rest of the growing mass of tissues. Protodermal cells eventually mature into the upper and lower epidermis, which has highly specific functions. The epidermis cells do not contain photosynthetic chloroplasts. The epidermis produces a waxy coating of a substance called cutin, which acts like waterproofing to help the internal leaf tissues retain water. The cutin coating is also referred to as the cuticle.

Special lower epidermal cells called guard cells form pores to let carbon dioxide gas into the leaf, and to allow the release of oxygen gas produced by photosynthesis. The scientific term for these functional units that open and close, is stoma, or stomate.
Some leaf epidermal cells have adaptations that enable them to perform highly specialized functions. Some epidermal cells grow tiny projections, or hairs, which are thought to either discourage leaf eating insects, or perhaps interfere with wind currents in an effort to slow down evaporation. Some leaf hairs contain sticky glue-like substances to trap organisms, and some even have toxic chemicals to make themselves unappetizing to hungry bugs.

The developing leaf gets its supply of water, nutrients and minerals through a vascular system of primary xylem tissue that develops from the procambium. Food that is produced in the leaf travels to other parts of the plant through the primary phloem, which also develops from the procambium.

The ground meristem gives rise to storage and support cells such as parenchyma, collenchyma, and sclerenchyma. Two forms of parenchyma cells containing chloroplasts develop in the leaf: palisade parenchyma and spongy parenchyma. The palisade parenchyma are tall vertically positioned column-like cells that look like stakes in a palisade fence, and form a tightly packed upright cell layer just below the upper leaf epidermis. The spongy parenchyma consists of loosely packed cells with lots of air spaces between cells, and are located below the palisade parenchyma and above the lower epidermal layer of the leaf. The air spaces allow for diffusion of gases into and out of the spongy parenchyma cells through the stomates. Together, the palisade and spongy parenchyma are referred to as the mesophyll, or middle layer between the upper and lower epidermis.

The palisade parenchma and the spongy parenchyma both contain chloroplasts and therefore carry out photosynthesis. Water, minerals and nutrients are brought to the photosynthetic tissues by xylem, which is connected through branches and through the leaf stalk, otherwise known as the petiole. Photosynthetic products of sugar and carbohydrates are transported to the rest of the plant through the phloem. Both the xylem and phloem form vascular bundles known commonly as leaf "veins," and the main central vein is what we recognize as the leaf midrib. Vascular bundles branch into smaller bundles that feed the different regions of the leaf blade.

The petiole is attached to the twig or branch at a node, and there may be two little leaf-like scales called stipules that grow near the base. The petiole bears the leaf blade, which is the flattened structure of green photosynthetic tissue. The placement of the leaves on a twig can be arranged in alternating, opposite, or whorled (forming a ring around the twig) patterns.

Monocot leaves are somewhat different from the dicot leaf described above. Monocots like bamboo or corn do not have petioles that attach the leaves to the stem. Rather, the leaves sheath the stem in a tube and wrap around it. So monocot leaves are made in two parts: the sheath and the blade, which angles away from the stem above the wrap. The vascular bundles also differ from typical dicot patterns of venation, which are netted. Monocots typically have parallel vascular bundles that feed the leaf.