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23–4 Leaves
Leaf Structure
The structure of a leaf is optimized for absorbing light and carrying out photosynthesis.
Photo Credit: Getty Images
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Leaf Structure
To collect sunlight, most leaves have thin, flattened sections called blades.
The blade is attached to the stem by a thin stalk called a petiole.
Blade
Most of a leaf consists of a blade attached to the stem by a petiole. The blade of a simple leaf (left) can be different shapes. In a compound leaf (right), the blade is divided into many separate leaflets.
Simple leaf
Leaflet
Petiole
Bud
Compound leaf
Stem
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Leaf Structure
Simple leaves have only one blade and one petiole.
Compound leaves have several blades, or leaflets, that are joined together and to the stem by several petioles.
Blade
Most of a leaf consists of a blade attached to the stem by a petiole. The blade of a simple leaf (left) can be different shapes. In a compound leaf (right), the blade is divided into many separate leaflets.
Simple leaf
Leaflet
Petiole
Bud
Compound leaf
Stem
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Leaf Structure
Leaves are covered on the top and bottom by epidermis made of a layer of tough, irregularly shaped cells.
Epidermis
Leaves absorb light and carry out most of the photosynthesis in a plant. Some of the most important manufacturing sites on Earth are found in the leaves of plants. The cells in plant leaves are able to use light energy to make carbohydrates.
Epidermis
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Leaf Structure
The epidermis of many leaves is covered by the cuticle.
The cuticle and epidermal cells form a waterproof barrier that protects tissues inside the leaf and limits the loss of water through evaporation.
Cuticle
Epidermis
Leaves absorb light and carry out most of the photosynthesis in a plant. Some of the most important manufacturing sites on Earth are found in the leaves of plants. The cells in plant leaves are able to use light energy to make carbohydrates.
Epidermis
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Leaf Structure
The vascular tissues of leaves are connected directly to the vascular tissues of stems.
In leaves, xylem and phloem tissues are gathered together into bundles that run from the stem into the petiole.
In the leaf blade, the vascular bundles are surrounded by parenchyma and sclerenchyma cells.
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Leaf Structure
All these tissues form the veins of a leaf.
Xylem
Vein
Phloem
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Leaf Functions
Leaf Functions
Most leaves consist of a specialized ground tissue known as mesophyll.
Photosynthesis in most plants occurs in the mesophyll.
Carbohydrates produced move into phloem and are carried to the rest of the plant.
Palisade mesophyll
Leaves absorb light and carry out most of the photosynthesis in a plant. Some of the most important manufacturing sites on Earth are found in the leaves of plants. The cells in plant leaves are able to use light energy to make carbohydrates.
Spongy mesophyll
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Leaf Functions
The layer of mesophyll cells found directly under the epidermis is called the palisade mesophyll.
These closely-packed cells absorb light that enters the leaf.
Palisade mesophyll
Leaves absorb light and carry out most of the photosynthesis in a plant. Some of the most important manufacturing sites on Earth are found in the leaves of plants. The cells in plant leaves are able to use light energy to make carbohydrates.
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Leaf Functions
Beneath the palisade mesophyll is the spongy mesophyll, a loose tissue with many air spaces between its cells.
Leaves absorb light and carry out most of the photosynthesis in a plant. Some of the most important manufacturing sites on Earth are found in the leaves of plants. The cells in plant leaves are able to use light energy to make carbohydrates.
Spongy mesophyll
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Leaf Functions
The air spaces connect with the exterior through stomata.
Stomata are porelike openings in the underside of the leaf that allow carbon dioxide and oxygen to diffuse into and out of the leaf.
Leaves absorb light and carry out most of the photosynthesis in a plant. Some of the most important manufacturing sites on Earth are found in the leaves of plants. The cells in plant leaves are able to use light energy to make carbohydrates.
Stoma
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Leaf Functions
Each stoma consists of two guard cells.
Guard cells are specialized cells that control the opening and closing of stomata by responding to changes in water pressure.
Leaves absorb light and carry out most of the photosynthesis in a plant. Some of the most important manufacturing sites on Earth are found in the leaves of plants. The cells in plant leaves are able to use light energy to make carbohydrates.
Guard cells
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Leaf Functions
Transpiration
The surfaces of spongy mesophyll cells are kept moist so gases can enter and leave the cells easily.
Water evaporates from these surfaces and is lost to the atmosphere.
Transpiration is the loss of water through its leaves. This lost water is replaced by water drawn into the leaf through xylem vessels in the vascular tissue.
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Leaf Functions
Gas Exchange
Plant leaves allow gas exchange between air spaces in the spongy mesophyll and the exterior by opening their stomata.
Plants keep their stomata open just enough to allow photosynthesis to take place but not so much that they lose an excessive amount of water.
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Leaf Functions
When water pressure within guard cells is high, the stoma open.
Plants regulate the opening and closing of their stomata to balance water loss with rates of photosynthesis. A stoma opens or closes in response to the changes in pressure within the guard cells that surround the opening. When the guard cells are swollen with water the stoma is open.
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Leaf Functions
When water pressure within guard cells decreases, the stoma closes.
Plants regulate the opening and closing of their stomata to balance water loss with rates of photosynthesis. A stoma opens or closes in response to the changes in pressure within the guard cells that surround the opening.
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Leaf Functions
Plants regulate the opening and closing of their stomata to balance water loss with rates of photosynthesis.
Stomata are open in daytime, when photosynthesis is active, and closed at night, to prevent water loss.
In hot, dry conditions stomata may close even in bright sunlight, to conserve water.
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