Under the microscope they really look like mouths, although more like a fish than a human: the stomata of plants. The ‘lips’ of each stomata are two banana-shaped guard cells on either side of a pore. You can’t see them with the naked eye, because they are only 0.01 to 0.08 millimeters in size. An average leaf has about five hundred per square millimeter. Almost all plants have them, from horsetails, ferns and grasses to daisies and giant sequoias. But what about aquatic plants?
Stomata help plants and trees with gas exchange. They are mainly found in the leaves. During the day, carbon dioxide enters the leaves through the stomata. With the help of water and sunlight, the plant makes sugars and oxygen: a process called photosynthesis. He uses the sugars for his own growth and energy storage. And the oxygen leaves through the stomata, together with water vapor.
Most land plants have stomata on the underside of the leaves. There is often a thicker waxy layer at the top, the cuticle, which prevents the leaf from drying out too much in the sun. For the same reason, the stomata are mainly located on the underside of the leaves: stomata in the shade lose less water vapor.
Hot and dry weather
To further limit the loss of water vapor, the plant can also close its stomata. Many plants do this at night, when there is no sunlight and therefore no carbon dioxide needs to enter for photosynthesis. And even when it is very hot and dry during the day, the stomata sometimes close. Then no gas exchange for a while.
The banana-shaped guard cells, which can swell or shrink, do this opening and closing. You think so nice microscope videos online. The cells respond to factors such as sunlight, humidity and CO2-content. Exactly how they ‘feel’ that is still a subject discussion. But in any case, the result is a cascade of molecular ‘pumps’ that go to work in the cell membrane of the guard cells. These increase or decrease the concentration of particles in the cell fluid. At higher concentrations, water enters the cells (‘osmosis’). The two guard cells swell and close the opening together. At lower concentrations they actually lose water to their environment. Then they shrink and the opening becomes free again.
And what about aquatic plants? Plants that are only sometimes submerged, for example in floodplains, simply have stomata, but they close as soon as they go under. This way they prevent the air-filled spaces in their leaves from filling up with water. If a plant is under water for longer and forms new leaves in the meantime, they are often thinner and they also have a thinner outer layer, without stomata. This allows them to exchange gases directly with the water. Just like frogs, actually. And just like ‘wet land plants’, such as peat moss and liverworts.
Water lilies and other plants with floating leaves do have stomata, but at the top of the leaves. They mainly exchange gases with the air, and hardly with water.
And then there are underwater plants that never rise above the surface, such as certain types of pondweed, algae and seagrass. Just like peat moss they have no stomata. The outer layer of their leaves and stems is so thin that gases can pass directly through it.