The Structure of a Leaf

The Structure of a Leaf

Leaves produce the food necessary for a plant to survive. The structure of a leaf is very intricate, containing several different tissues, each with its own role. Come with us as we explain it in easy-to-understand terms.

External Structure of a Leaf

The leaf blade, or lamina, contains four key parts: the tip, midrib, veins, and margin. The tip is the end of the leaf blade. The petiole is found at the base of the leaf and extends to the stem. The leaf shape, vein pattern, and margins help identify plants.

Midrib

Most leaves have a midrib. The midrib, or center vein, runs the length of the leaf and branches off to either side, creating smaller veins.

Veins

The leaf’s veins comprise two types of cells: the xylem and the phloem. The xylem moves water from the stem to the rest of the leaf blade. The phloem takes sugars from the leaf blade to the rest of the plant.

The veins form patterns in leaves and go by a fancy word called venation patterns. There are two types of patterns found in leaves.

Veins run in parallel lines down the leaf blade. These are called monocot venation (vein) patterns. The other types are dicots. The veins in these leaves branch out or are net-like in structure.

To take it a step further, leaves with dicot venation (vein pattern) are broken down into two more categories: palmate and pinnate.

The palmate venation pattern does not have a midrib, main middle vein. Instead, the veins start at the base of the leaf and run throughout the blade.

The pinnate venation pattern is when the veins branch off from a midrib.

Margin

The margin of the leaf is just like it sounds. It is the edge of the leaf. Margins take on many different shapes and help with plant identification. The most common shapes are entire, toothed, and lobed.

A leaf that has an entire margin has smooth edges. Toothed margins are ribbed. Some are serrated like a knife, while others are more rounded. Leaves with lobed margins have indentions in the blade going towards the midrib.

There are other subsets of margins. For more information on those subsets, check out this website.

Petiole

The petiole is the stalk that attaches the leaf to the stem. Not all leaves have a petiole. Those leaves that don’t are called sessile leaves. The base of the leaf attaches directly to the stem.

The petiole helps support the leaf. It also contains xylem and phloem, moving water from the roots to the leaves and nutrients from the leaves to the rest of the plant.

Internal Structure of a Leaf

Inside the leaf is where the magic happens. Various cells within three layers of tissue perform specific and important tasks to the plant’s survival. The three layers of tissue are the epidermis (purple), mesophyll (red), and vascular tissue (orange).

Cuticle

The cuticle is the waxy layer on the upper and lower parts of the leaf. Its job is to waterproof the leaf and reduce water loss.

Epidermis

The epidermis is the outermost layer of the leaf. There is an upper epidermis and a lower epidermis; think of the bread on a sandwich. The epidermis aids in regulating the exchange of gases from the air to the leaf.

Stomata

Stomata are present in the epidermis. Most of these pores, or holes, are on the underside of the leaf. If you look closely with a magnifying glass, you see them.

These openings allow the leaf to breathe. The stoma release moisture and exchange gases, such as oxygen and carbon dioxide. They are surrounded by guard cells.

Guard Cells

Guard cells act as a valve. They determine when to open and close based on the water present. When ample moisture is present, the guard cells swell up like a sponge and open, allowing the exchange of gases. On the flip side, when moisture isn’t plentiful, they decrease and close, forbidding the release of water or gas.

Cells of the Palisade Layer

The cells of the palisade layer are under the epidermis and are part of the mesophyll tissue. They are oblong-shaped cells packed in side-by-side. Inside these cells live chloroplasts.

Chloroplasts

Chloroplasts contain chlorophyll and give leaves their green color. Photosynthesis, or food-making, happens in these cells.

Spongy Layer of Cells

After the palisade layer in the mesophyll tissue is the spongy layer of cells. This layer contains chloroplasts as well, just not as many or as organized. The cells in this layer are irregular in shape and are not closely packed together. Think of this layer as the leaf’s storage unit. These cells store the gases and sugars the palisade layer created.

Sheath Cells

The sheath cells are part of the vascular tissue. The sheath cells protect the veins, the roadways through the leaf.

Xylem and Phloem

Xylem and phloem are the roadways through the leaf. The xylem brings water from the roots to the leaf. The phloem transports sugar, or food, from the leaf to the rest of the plant.

Three Functions of a Leaf

A leaf has three main functions: transpiration, photosynthesis, and respiration.

Transpiration

Transpiration is the process of letting go of water vapor through the stomata, holes in the epidermis of the leaf. When the stomata open to let carbon dioxide in, water seeps out. This process is thought to help cool down the plant, like when we sweat. It also allows the intake of carbon dioxide and the movement of nutrients through the plant.

To see transpiration at work, place a sandwich bag around a leaf. Twist tie the bag around the stem, ensuring the leaf is inside the bag. Leave the bag for a day or so and come back. When you return, you will see water droplets inside the bag!

Photosynthesis

Photosynthesis is the making of food by plant cells, chloroplasts. The process of photosynthesis is the combination of carbon dioxide and water and the conversion of light by chlorophyll in the chloroplasts to create food, or sugar, and oxygen.

Respiration

Plant respiration takes in oxygen and releases carbon dioxide, opposite photosynthesis. In respiration, the plant converts the oxygen from the air and the nutrients from the soil into water and carbon dioxide. The water and carbon dioxide are then released into the air through the stomata in the leaves. This process occurs at all times, day and night.

The leaf contains three layers of tissues each, with its own job. These jobs include transpiration, photosynthesis, and respiration. As you learn about the leaf, you see how important it is!

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The Structure of Leaf