Leaves may look pretty in the fall when they are changing colors, but they also provide many necessary functions for plants. In this lesson, we will explore the structures and functions of leaves.
The Structure of Leaves
Leaves are vital to the survival of plants. They help plants in a variety of ways, including producing food and oxygen through photosynthesis, balancing water loss, regulating gas exchange, and transporting the products of photosynthesis. Because of the vast array of jobs that leaves perform, there are many specialized structures. We will look at these parts of the leaf and relate them to their functions.
Remember that the shoot system contains the above-ground parts of plants, including the stem, flowers, and leaves. We already talked about the function of the stem, which is to provide support and transport for the plant as well as the structures involved in these functions. We also looked at how the stem grows. It is important to know that leaves originate at the apical meristem and are a result of primary growth.
While leaves do come in a variety of forms depending on the type of plant, we will focus on the structures of dicot plant leaves, as these are the most complex. Before we get into the layers and functions of the dicot leaf, let’s first take a look at a diagram. We will work our way from the top of the leaf down to the bottom of the leaf and look back at this diagram as we cover each layer.
The Cuticle and Upper Epidermis
Just like our skin helps protect us, leaves have an outer layer that protects them. This outermost layer is called the cuticle. It is generally waxy to protect the leaf and prevent water loss.
When you touch a leaf, you may feel this waxy coating, and on some plants, such as holly, you may actually be able to see the waxy coat shine a bit.Below the cuticle is the epidermis. On the top of the leaf, this is known as the upper epidermis. This is a single layer of cells found directly below the cuticle. It helps protect the leaf by aiding in preventing water loss and providing an extra layer between the outside and inside of the leaf.Now that we have looked at the first two layers of the leaf, let’s add these structures on our diagram.
The top waxy layer here is the cuticle and the layer right below that is the upper epidermis.
The Palisade Layer
Below the upper epidermis is the palisade layer. This is one or a few layers of cylindrical cells that contain many chloroplasts. Chloroplasts are an important part of plant cells because they are cell structures that allow for photosynthesis.
You may remember that photosynthesis is the process by which autotrophs convert light energy into chemical energy. The palisade layer is therefore mainly responsible for producing food and oxygen for the plant through photosynthesis. Let’s go back to our leaf diagram and add in the palisade layer. Remember that it is right below the upper epidermis and contains cells that are cylindrical.
The Spongy Layer
Below the palisade layer is the spongy layer. This is an area with loosely packed cells with many air pockets. The cells are more ball-shaped than cylindrical-shaped like the cells in the palisade layer. The large air pockets allow for gas exchange between different areas of the leaf. The cells in this layer contain few chloroplasts and are therefore not generally responsible for photosynthesis.
This makes sense, as the layer is further into the leaf and will therefore not get as much sun as the palisade layer, which is closer to the leaf surface.Within the spongy layer are a few other components of the leaf. Along with the air pockets for the exchange of gasses are vascular bundles that contain xylem and phloem. These vascular bundles are also called veins in a leaf. You may be able to see the veins in a leaf such as a maple leaf. This is where the movement of water and food occurs.
Xylem moves water and dissolved minerals, while phloem moves food.Let’s go ahead and label the spongy layer and the vascular bundle on our diagram. The spongy layer is the more open space found directly below the palisade layer. Within the spongy layer you can see the air pockets as well as the vascular bundle containing the xylem and phloem.Collectively, the area that contains the palisade and spongy layers is known as the mesophyll.
We can also label this on our leaf diagram.
Stomata and the Lower Epidermis
Remember that the top of the leaf has the protective coating of the cuticle and then a layer of cells known as the upper epidermis. On the bottom of the leaf is another protective layer of cells. Because it is at the bottom of the leaf, this layer is known as the lower epidermis.Within this layer are very important structures known as stomata.
These are pores in the leaf that allow for gas exchange. A way to remember this is that the root word, stoma, means ‘mouth’ in Greek. This mouth can open and close to allow the exchange of gases.On either side of the stomata are guard cells that control the opening and closing of stomata. Gas exchange is needed for both photosynthesis and cellular respiration.
We already went over photosynthesis, which converts light energy into chemical energy, so let’s quickly go over cellular respiration. Cellular respiration is the process in which oxygen is used to convert organic molecules to carbon dioxide and water and provide energy for the cell.
In addition to regulating gases such as oxygen and carbon dioxide that are involved in cellular respiration and photosynthesis, the stomata also control the loss of water. When plants release water is it known as transpiration. Transpiration in plants is similar to perspiration in people. This release of water vapor in plants is regulated depending on the needs of the plant as well as the conditions of the surrounding environment.
The guard cells will open or close the stomata based on these needs. For example, if the weather is hot and the plant is likely to lose water, the guard cells will ensure that the stomata are closed. However, when the air is cooler and more humid, the guard cells will allow the stomata to open.Let’s go back and add these last few parts to our diagram.
We know that the bottom of the leaf is the lower epidermis. Within this layer we can see the stomata, which are the openings. These are surrounded by two guard cells that regulate the opening and closing of the stomata.
All parts of the leaf help support the life of the plant, which includes producing food and oxygen through photosynthesis, balancing water loss, regulating gas exchange, and transporting the products of photosynthesis.The cuticle and upper epidermis provide protection for the plant.
Below that is the palisade layer, which is the location of photosynthesis within the leaf. Below the palisade layer is the spongy layer, which contains cells that are more spread out, allowing for air pockets. Within the spongy layer are vascular bundles that contain xylem and phloem, allowing for the transport of products of photosynthesis. Together, the palisade and spongy layers are known as the mesophyll. The bottom of the leaf is protected by the lower epidermis.
Within this are the stomata, which are pores allowing for gas exchange that are regulated by the guard cells. All parts of the leaf must work together to allow for the plant to go through both photosynthesis and cellular respiration.
After watching this lesson, you should be able to identify the different structures of leaves and describe their functions for plants.