Stoma

Plants have tiny mouths on their leaves.

These are called stomata. Two special cells act like gates. They open and close the tiny holes.

When the gates open, the plant breathes. It takes in air to make food. The air also lets water out.

Most trees have these mouths on the bottom. Some plants keep them closed at night. This helps them save water. It is a smart way to live!

Plants have tiny pores on their leaves and stems. We call these pores stomata.

These pores help the plant breathe. They let in carbon dioxide for food. They also let out oxygen.

Two special cells border each pore. We call these guard cells. They act like gates to control the pore. When the guard cells fill with water, they swell up. This makes the pore open. When they lose water, the pore closes.

Plants must balance breathing with saving water. When stomata are open, water vapor escapes. This is called transpiration.

Some desert plants use a special way to live. These are CAM plants. They keep their stomata closed during the hot day. They open them at night to take in air. This helps them save water.

Most trees have stomata only on the bottom of their leaves. Some plants, like onions, have them on both sides.

Plants need to breathe just like we do. They use tiny openings called stomata to move gases in and out of their bodies.

These pores are found on leaves, stems, and other plant parts. They act like little doors for the plant. Through these doors, carbon dioxide enters to help with photosynthesis. This is the way plants make their own food. At the same time, oxygen can pass out through the pores.

Two special cells called guard cells border every stoma. These cells control how wide the opening stays. When the plant has plenty of water, the guard cells fill up. This makes them swell and bow outward. This movement creates an open pore called a stomatal aperture.

When the plant needs to save water, the guard cells lose water. They shrink and move closer together. This closes the pore to stop water from escaping. This way of moving water is called transpiration.

Scientists have studied how these pores work for a long time. We can see them using special tools like scanning electron microscopes. Some plants have very different ways of using their stomata. Most plants open their pores during the day. However, desert plants called C.A.M. plants do things differently.

They keep their pores closed during the hot day to save water. They only open them at night when it is cooler.

Stomata come in many different shapes and sizes. The length of a stoma can be between 10 and 80 micrometers. The width can be as small as a few micrometers or up to 50 micrometers.

Where they are located on a leaf also changes by plant type. Many trees have stomata only on the bottom of their leaves. These are called hypostomatous leaves. Some plants, like onions or maize, have them on both sides. These are called amphistomatous leaves.

Learning about stomata helps us understand how all land plants live. Most land plants have these pores, except for a few like liverworts. They likely appeared during the Silurian period. This was a long time ago in Earth's history. Stomata likely evolved alongside a waxy coating on plants. This coating helped early plants live on land without drying out. By studying these tiny pores, we learn how plants survive in different worlds.

A stoma, which is the plural form stomata, is a microscopic pore found in the epidermis of leaves, stems, and other plant organs. These tiny openings are essential for life because they control the rate of gas exchange between the atmosphere and the internal air spaces of a leaf.

The term usually refers to the entire stomatal complex. This complex includes the pore itself, known as the stomatal aperture, and a pair of specialized cells called guard cells. Through these pores, carbon dioxide enters the plant for photosynthesis. At the same time, oxygen produced during photosynthesis can exit.

Water vapour also moves through these pores into the air in a process called transpiration.

The movement of the stoma is driven by the guard cells that border the aperture. When environmental conditions like high light intensity and high humidity are favorable, a proton pump moves protons (H+) out of the guard cells. This makes the electrical potential inside the cells increasingly negative. This change triggers potassium voltage-gated channels to open, allowing potassium ions (K+) to enter the cells. To balance this, negative ions like chloride or organic malate also enter.

This increase in solute concentration lowers the water potential inside the cells. As a result, water enters the guard cells through osmosis. This increases the volume and turgor pressure of the cells. Because of cellulose microfibrils that restrict their width, the guard cells can only elongate. They bow apart, which opens the stomatal aperture.

Plants must carefully manage this process because they cannot gain carbon dioxide without losing water. Most plants use an enzyme called RuBisCO to fix carbon dioxide in mesophyll cells. However, RuBisCO has a low affinity for carbon dioxide and can also fix oxygen, which wastes energy through photorespiration. To get enough carbon dioxide, RuBisCO often requires wide stomatal apertures, which leads to high water loss. Some plants use an alternative enzyme called phosphoenolpyruvate carboxylase (PEPcase) to help. PEPcase has a higher affinity for carbon dioxide, which allows for narrower apertures and less water loss. This is especially useful when light is plentiful but water is limited.

Different plant groups have evolved unique strategies for managing their stomata. Most plants, often called C3 plants, open their stomata during the day. However, a group of mostly desert plants known as C.A.M. plants (crassulacean acid metabolism) use a different method.

C.A.M. plants open their stomata at night when water evaporates more slowly. They use PEPcase to fix carbon dioxide and store the products in large vacuoles. The next day, they close their stomata and release the stored carbon dioxide to RuBisCO. This method allows them to survive in environments where water is severely limited.

Stomata are also categorized by their location on the plant. Plants with stomata on both the upper and lower leaf surfaces are called amphistomatous. Those with stomata only on the upper surface are epistomatous or hyperstomatous. Most trees are hypostomatous, meaning they have stomata only on the lower leaf surface.

In vascular plants, the number and size of stomata vary widely. For example, dicotyledons usually have more stomata on the lower surface. Monocotyledons, such as maize or onion, may have a similar number on both sides. Floating leaves often have stomata only on the upper epidermis, while submerged leaves may lack them entirely.

From a developmental standpoint, stomata arise from protodermal cells through specific patterns. An asymmetrical cell division creates a small cell called a meristemoid. This meristemoid divides further to become a guard mother cell, which then undergoes a symmetrical division to produce the two guard cells.

This patterning is controlled by signaling components like EPF (Epidermal Patterning Factor) and the YODA protein. Mutations in these genes can change how stomata grow. For instance, a mutation in the TMM gene can cause a "Too Many Mouths" condition where stomata cluster together. Conversely, a mutation in the SPCH gene can prevent stomata from developing at all.

Evolutionary history suggests that stomata appeared in land plants by the middle of the Silurian period. They likely evolved alongside the waxy cuticle, a trait that helped early plants survive on land. While there is little evidence in the fossil record, scientists believe they may have evolved from the conceptacles of alga-like ancestors. Today, scientists use specialized instruments to measure how stomata respond to the environment. By calculating the transpiration rate and the humidity gradient, they can determine stomatal resistance. This helps us understand how plants manage their water use efficiency in a changing world.