Gas Exchange in the Ground Tissue

Why are the stomata located on the underside of the leaf?


A11.7 Gas exchange through the stomata
You may remember on the lower side of the leaf, the dermal tissue is next to the spongy tissue cells (also called spongy mesophyll tissue). You may also remember that there are lots of air pockets between these cells. The stomata are located just below this layer in the epidermis on the lower side of the leaf, so the gas that enters through the stomata can go straight into the air pockets within the spongy mesophyll tissue. This gives the carbon dioxide needed for photosynthesis a place to be stored before it is used by the palisade tissue cells that are directly above the spongy mesophyll tissue layer.

The spongy tissue cells will move oxygen toward the stomata so the oxygen can escape the leaf easily, and they will move carbon dioxide closer to the palisade tissue cells to be used for photosynthesis. This makes the diffusion of these gases very efficient within the leaves and the stem. The gases needed are able to travel through these air pockets, guided by the spongy tissue cells, around the leaf and down the stem to where ever they are needed.

  Read This

Please reread the section called “Ground Tissue” on pages 311 to 313 in your Science 10 textbook. Make sure you take notes on your readings to study from later. This time, you should focus on the spongy tissue cells and how the spongy mesophyll layer helps with the diffusion of oxygen and carbon dioxide. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. Why are the stomata located on the underside of the leaf?

The stomata are located on the underside of the leaf so the gases entering the leaf can go straight into the spongy tissue layer where there are many air pockets for the gases to be stored in. The palisade tissue cells have to be at the top of the leaf in order to catch as much sunlight as possible, so the spongy tissue layer with its air pockets have to be located under the palisade tissue layer.
  1. What is the purpose of the air pockets between the spongy tissue cells?
The air pockets are for gas storage. They also get the gases closer to the palisade tissue cells where the gases are needed for photosynthesis. This way, the plant does not have to transport the gases; they naturally move up toward the palisade cells.