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Structure of Leaves: The Epidermis, Palisade and Spongy Layers

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  1. 0:04 Structure of Leaves
  2. 1:16 The Cuticle and Upper Epidermis
  3. 2:05 The Palisade Layer
  4. 2:45 The Spongy Layer
  5. 4:10 Stomata and the Lower Epidermis
  6. 6:12 Lesson Summary
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Taught by

Danielle Weber

Danielle teaches high school science and has an master's degree in science education.

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.

The outermost layer of a leaf is called the cuticle
Cuticle

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

The air pockets in the spongy layer allow for gas exchange
Spongy layer of leaves

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.

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