Alloy: Mixture of two or more metals

The Spongy Mesophyll: The Leaf's Airy Exchange Hub

The Anatomy of a Leaf: Where is the Spongy Mesophyll?

To understand the spongy mesophyll, we must first take a microscopic journey inside a typical plant leaf. If you were to slice a leaf very thinly and look at it under a microscope, you would see it is made up of several layers, like a layered cake. Each layer has a specific job.

The top and bottom of the leaf are covered by a clear, waxy layer called the epidermis, which acts like a plant's skin, protecting it from water loss and disease. On the underside of the leaf, the epidermis is dotted with tiny pores called stomata (singular: stoma). Each stoma is flanked by two guard cells that act like doors, opening and closing the pore.

Inside the leaf, between the upper and lower epidermis, lies the mesophyll. This is where the magic of photosynthesis primarily happens. The mesophyll itself is divided into two distinct layers:

• Palisade Mesophyll: This is the top layer, located just below the upper epidermis. Its cells are tall, tightly packed, and arranged vertically like pillars or a picket fence. They are jam-packed with chloroplasts^[1], the organelles that capture sunlight, making this layer the main site for photosynthesis.
• Spongy Mesophyll: This layer sits below the palisade layer and above the lower epidermis. Its cells are more irregular and spherical in shape. Crucially, they are loosely packed, with large air spaces between them. These cells also contain chloroplasts, but fewer than the palisade cells.

These air spaces are the key to the spongy mesophyll's function. They form a vast, interconnected network that is flooded with gases, creating an internal atmosphere within the leaf.

The Mechanics of Gas Exchange

The spongy mesophyll is the central hub for the leaf's gas exchange system. This process is a carefully coordinated dance of molecules entering and exiting the leaf. Here’s how it works, step-by-step:

1. The Stomata Open: When light is available and water pressure inside the plant is high, the guard cells swell with water and bend, opening the stoma pore.
2. Carbon Dioxide Enters: Carbon dioxide (CO$_2$) from the outside air diffuses^[2] through the open stoma.
3. Travel Through Air Spaces: The CO$_2$ molecules move through the labyrinth of air spaces within the spongy mesophyll.
4. Diffusion into Cells: The CO$_2$ dissolves in the thin layer of moisture that coats the cells of the spongy and palisade mesophyll and then diffuses into these cells.
5. Photosynthesis: Inside the chloroplasts, CO$_2$ is combined with water using light energy to produce sugar (glucose) for the plant and oxygen (O$_2$) as a waste product.
6. Oxygen Exits: The oxygen produced diffuses out of the cells, into the air spaces of the spongy mesophyll, and finally exits the leaf through the same stomata.

Simultaneously, water vapor from inside the leaf also diffuses out through the stomata in a process called transpiration. The loose packing of the spongy mesophyll maximizes the surface area of cells exposed to these internal air spaces, making this exchange of gases incredibly efficient.

Adaptations in Different Environments

Not all plants live in ideal, sunny, and moist conditions. Plants have evolved different leaf structures to survive in their specific environments, and the spongy mesophyll is a key part of these adaptations.

Desert Plants (e.g., Cacti): In hot, dry deserts, water conservation is the top priority. Many desert plants have leaves modified into spines. Photosynthesis happens in the thick, green stem. This stem may have a very reduced spongy mesophyll with extremely small air spaces to minimize water loss. Their stomata might also open only at night.

Water Plants (e.g., Water Lilies): Aquatic plants face the opposite challenge: getting enough carbon dioxide, which diffuses much slower in water than in air. The floating leaves of water lilies often have a very well-developed spongy mesophyll with gigantic air spaces (aerenchyma). This not only aids in flotation but also creates large internal reservoirs for storing CO$_2$ and O$_2$ for use when needed. Their stomata are only on the top surface, facing the air.

Shade-Tolerant Plants: Plants that grow on the forest floor, with less direct sunlight, often have leaves with a thinner palisade layer and a relatively larger spongy mesophyll. This maximizes the internal surface area for capturing every available CO$_2$ molecule to compensate for the lower light energy available for photosynthesis.

Observing the Spongy Mesophyll: A Simple Experiment

You can see the evidence of the spongy mesophyll's airy structure with a simple experiment you can do at home or in a classroom.

What you need: A clear glass bowl, water, a fresh leaf from a tree or shrub (e.g., a spinach leaf from the grocery store works perfectly).

What to do:

1. Fill the bowl with water.
2. Place the leaf gently on the surface of the water, with its top side facing up.
3. Push the leaf underwater with your finger. Observe what happens.
4. Now, flip the leaf over so its bottom side (where the stomata are) is facing up.
5. Push this side underwater. Observe again.

What you'll see: When the top side is up, the leaf will likely sink. When the bottom side is up, you will see tiny silver bubbles forming on the surface of the leaf. This is the air being forced out of the air spaces within the spongy mesophyll! The leaf may then sink. This bubble effect is a direct demonstration of the spongy layer's porous, air-filled nature.

Common Mistakes and Important Questions

Footnote

^[1]Chloroplasts: Organelles found in plant cells that contain chlorophyll and are the site of photosynthesis.

^[2]Diffusion: The passive movement of molecules from an area of high concentration to an area of low concentration.