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In botany, a leaf is an above-ground plant organ specialized for photosynthesis. For this purpose, a leaf is typically flat and thin, to expose the chloroplast containing cells  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. The comparable structures of ferns are correctly referred to as fronds.
As a stem grows, leaves tend to appear arranged around the stem in away that optimizes yield of light. In essence, leaves come off the stem in a spiral pattern, either clockwise or counterclockwise, with the same angle of divergence. 
Two basic forms of leaves can be described considering the way the blade is divided. A simple leaf has an undivided blade. However, the leaf shape may be one of lobes, but the gaps between lobes do not reach to the main vein. A compound leaf has a fully subdivided blade, each leaflet of the blade separated along a main or secondary vein. Because each leaflet can appear to be a "simple leaf", it is important to recognize where the petiole occurs to identify a compound leaf.

A leaf (plural leaves) is a flattened structure of a higher plant, typically green and blade like that is attached to a stem directly via a stalk (oxford dictionary). They exist in various types, shapes, and sizes.

Their structure
The outermost layer of a leaf is called the epidermis. This layer is protected by cellulite. These plant structures have both upper and lower epidermis. This layer is important for gaseous exchange process. The epidermis also has tiny opening known as stomata which are used in photosynthesis. 
They are covered by a waxy substance known as the cuticle. This layer helps to prevent excessive water loss from the surface.

Some also have small hairs on their surface. These are referred d to as trichomes. Their function is to reduce consumption by plants and insects by storing toxic and bad tasting compounds in them. Their general structure consists of a midrib, blade, and a margin. Its edge is what is referred to as the margin. Some are attached to their stem directly while others are attached using a petiole. 
They have veins running across and among them. Venation pattern is the arrangement of veins in the lamina. They exist in two types of venation; reticulate and parallel. Reticulate venation is characterized by their organization in a net-like pattern. This is found in di-cotyledons; rese, maple, hibiscus, and tulsi. 
Parallel venation involves veins running parallel to each other. This is found in monocotyledons; bananas, palm, onions, and grass.

Their arrangement on a plant
Phyllotaxy is the arrangement of leaves on a stem. Each botanical species has its own unique arrangement. It can be; alternate, spiral, opposite or whorled. Alternate and spiral are characterized by having only one of them per node. 
In the alternative, they are organized on each side of the stem in a flat plane. Two or more connected to a node makes a whorled pattern. 
Most arrangements enable them to optimize the yield of light.

The major classification is simple and compound. Simple ones contain blades that are undivided. They are not divided into smaller units. They have a toothed edge which can either be unlobed or lobed. Lobed ones contain gaps in between them but will not reach the midrib of leaves. 
Compound ones consist of leaflets which are attached to a rachis

This is the process by which green plants make their own food. They do this by transforming light energy into chemical energy. Light energy penetrates through the leaves and is used to convert water, minerals and carbon dioxide into oxygen. Therefore, oxygen is the by-product of this process. It is excreted through the stomata. A lot of water is usually lost during this process simply from the leaves.

Photosynthesis plays a key role in maintaining the creatures on earth. Without it, there probably wouldn’t be enough food or other organic matter. It helps in maintaining oxygen levels in the atmosphere. Most earthly organisms completely depend on oxygen for their survival. It is utilized in the human bodies to maintain healthy organs and also keep the heart beating.

The energy produced during photosynthesis is used in creating fossil fuels (coal, oil, gas). Fossil fuels are formed from remains of plants (leaves) organisms that eat them which were trapped under the earth’s crust. Because they were sheltered from any oxidation, fossil fuels slowly began to form. Once the fossil fuels were discovered, they have been utilized in several industries homes and forms of transportation. They are also highly utilized in the manufacture of synthetic materials and plastics.

Chlorophyll is the pigment that is responsible for their green color. It is beneficial to both botanic and other organisms. Consumption of extracted chlorophyll by a human being can cause; skin healing, stimulation of the immune system, detoxification, intestinal cleansing, increased energy levels and prevention of cancer.

Leaves may look like tiny organisms but they clearly pack a punch, meanwhile these leaves are functional and beneficial to most living organisms. On top of that, they look amazing and contribute to the aesthetics of our planet. Grassy forested parks mostly have visitors because of the magnificent color and appeal of these structures. Some are also a great source of vitamins when consumed.

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