Plant Leaves and Leaf Anatomy

Plant leaves help to sustain life on earth as they generate food for both plant and animal life. The leaf is the site of photosynthesis in plants. Photosynthesis is the process of absorbing energy from sunlight and using it to produce food in the form of sugars. Leaves make it possible for plants to fulfill their role as primary producers in food chains. Not only do leaves make food, but they also generate oxygen during photosynthesis and are major contributors to the cycle of carbon and oxygen in the environment. Leaves are a part of the plant shoot system, which also includes stems and flowers.

Leaf Anatomy

Leaf Anatomy
Basic Leaf Anatomy of Flowering Plants. Credit: Evelyn Bailey

Leaves can be found in a variety of shapes and sizes. Most leaves are broad, flat and typically green in color. Some plants, such as conifers, have leaves that are shaped like needles or scales. Leaf shape is adapted to best suit the plant's habitat and maximize photosynthesis. Basic leaf features in angiosperms (flowering plants) include the leaf blade, petiole and stipules.

Blade - broad portion of a leaf.

  • Apex - leaf tip.
  • Margin - leaf edge boundary area. Margins can be smooth, jagged (toothed), lobed, or parted.
  • Veins - vascular tissue bundles that support the leaf and transport nutrients.
  • Midrib - central main vein arising from secondary veins.
  • Base - area of the leaf that connects the blade to the petiole.

Petiole - thin stalk that attaches the leaf to a stem.

Stipules - leaf-like structures at the leaf base.

Leaf shape, margin, and venation (vein formation) are the main features used in plant identification.

Leaf Tissues

Leaf Cross Section
Leaf Cross Section Showing Tissues and Cells. Credit: Evelyn Bailey

Leaf tissues are composed of layers of plant cells. Different plant cell types form three main tissues found in leaves. These tissues include a mesophyll tissue layer that is sandwiched between two layers of epidermis. Leaf vascular tissue is located within the mesophyll layer.

Epidermis

The outer leaf layer is known as the epidermis. The epidermis secretes a waxy coating called the cuticle that helps the plant retain water. The epidermis in plant leaves also contains special cells called guard cells that regulate gas exchange between the plant and the environment. Guard cells control the size of pores called stomata (singular stoma) in the epidermis. Opening and closing the stomata allows plants to release or retain gases including water vapor, oxygen, and carbon dioxide as needed.

Mesophyll

The middle mesophyll leaf layer is composed of a palisade mesophyll region and a spongy mesophyll region. Palisade mesophyll contains columnar cells with spaces between the cells. Most plant chloroplasts are found in palisade mesophyll. Chloroplasts are organelles that contain chlorophyll, a green pigment that absorbs energy from sunlight for photosynthesis. Spongy mesophyll is located below palisade mesophyll and is composed of irregular shaped cells. Leaf vascular tissue is found in spongy mesophyll.

Vascular Tissue

Leaf veins are composed of vascular tissue. Vascular tissue consists of tube-shaped structures called xylem and phloem that provide pathways for water and nutrients to flow throughout the leaves and plant.

Specialized Leaves

Venus Flytrap
The leaves of the Venus flytrap are highly modified with a trigger mechanism to trap insects. Credit: Adam Gault/OJO Images/Getty Images

Some plants have leaves that are specialized to perform functions in addition to photosynthesis. For example, carnivorous plants have developed specialized leaves that work to lure and trap insects. These plants must supplement their diet with nutrients gained from digesting animals because they inhabit areas where the soil quality is poor. The Venus flytrap has mouth-like leaves, which close like a trap to snare insects inside. Enzymes are then released in the leaves to digest the prey.

The leaves of pitcher plants are shaped like pitchers and brightly colored to attract insects. The inside walls of the leaves are covered with waxy scales that make them very slippery. Insects landing on the leaves may slip into the bottom of the pitcher-shaped leaves and be digested by enzymes.

Leaf Imposters

Amazonian Horned Frog
It is difficult to detect this Amazonian Horned Frog among the leaf litter of the forest due to its coloration. Robert Oelman/Moment Open/Getty Images

Some animals mimic leaves in order to avoid detection. They camouflage themselves as leaves as a defense mechanism to escape predators. Other animals appear as leaves to capture prey. Fallen foliage from plants that lose their leaves in the fall makes a perfect cover for animals that have adapted to resemble leaves and leaf litter. Examples of animals that mimic leaves include the Amazonian horned frog, leaf insects, and the Indian leafwing butterfly.

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Bailey, Regina. "Plant Leaves and Leaf Anatomy." ThoughtCo, Aug. 25, 2016, thoughtco.com/plant-leaves-and-leaf-anatomy-373618. Bailey, Regina. (2016, August 25). Plant Leaves and Leaf Anatomy. Retrieved from https://www.thoughtco.com/plant-leaves-and-leaf-anatomy-373618 Bailey, Regina. "Plant Leaves and Leaf Anatomy." ThoughtCo. https://www.thoughtco.com/plant-leaves-and-leaf-anatomy-373618 (accessed September 26, 2017).