Lesson Video: Structure of the Stem Biology

In this video, we will learn how to describe the basic structure of a plant stem and recall the functions of different parts of the stem.


Video Transcript

In this video, we will learn how to describe the basic structure of a dicot plant stem. We will see which simple and compound tissues the stem is composed of and the specific structures that provide the stem with its useful functions.

Plant stems are vital to their survival. Stems are usually long stalk-like structures that form the main body of a plant. They tend to rise above ground, though some stems can also be found underground, like in this potato plant. The stem can help steady and support a plant under harsh environmental conditions, allow a plant to move to access light, and even to transport essential substances to the different plant’s organs.

Stems need to be really well adapted for these functions, as some plants, like this North American Hyperion tree, which is sometimes called a redwood, can have stems reaching as high as 115 meters. To understand just how large this plant is, this is approximately how a tree of this size would compare to the typical size of a giraffe, the tallest animal on Earth.

In this video, we’ll be learning about how the structure of plant stems can facilitate survival, including in these huge organisms. A dicot, which is short for dicotyledon, is a type of plant that produces seeds that have two seed leaves. These embryonic leaves are called cotyledon, and they’ll form eventually the first leaves that we’ll see above ground. In this video, we’ll be looking at the structure of dicot stems. But it’s worth noting that monocots, which is short from monocotyledon and produce seeds with only one seed leaf, will have a slightly different stem structure. An easy way to remember how many cotyledons each of these flowering plant types have is that mono- means one while di- means two.

Let’s look at a cross section showing the typical structure of a dicot stem. The epidermis is a single layer of cells that covers a plant stem as well as its leaves, flowers, and roots and forms a boundary to the external environment. It effectively acts as a tough skin for a plant, and it can sometimes produce a waxy cuticle to protect it. The waxy cuticle helps to protect the stem, especially the cells sitting just below the epidermis, from mechanical damage or excess water loss. It can also help prevent the entrance of dangerous microorganisms, like pathogens, that might cause infection.

Moving inwards from the epidermis, there are several layers of spongy tissues that make up a region called the cortex. The cortex is an outer layer of tissue that’s found immediately below the epidermis of a stem or of a root. And it’s composed of several different types of simple tissues. A simple tissue is made up of cells that are structurally and functionally very similar to each other. The majority of the cortex is made up of parenchyma tissue, which is one example of a simple tissue. Parenchyma cells make up the soft fleshy tissues inside various parts of the plant, such as the leaves, stem, and roots. Parenchyma tissue has plenty of intracellular spaces between its cells that provides aeration to promote gas exchange. Parenchyma cells also contain chloroplasts to carry out photosynthesis.

Directly below the epidermis within the cortex of growing stems, there is another region of simple tissues called collenchyma tissue. Collenchyma cells are typically longer than parenchyma cells, and they have thickened cellulose and pectin cell walls. Like parenchyma cells, collenchyma cells usually contain chloroplasts to carry out photosynthesis. Collenchyma cells are usually found below the epidermis of leaf veins and stems, particularly young stems, and are essential in growing regions of the plant for providing structure and some flexibility. So this region of the stem highlighted in orange would have a high proportion of collenchyma cells just below the epidermis.

Sclerenchyma tissues are the toughest of the three simple tissue types found in plants. Sclerenchyma cells have thick cell walls with a chemical called lignin deposited in them. This provides structure and mechanical support, particularly to mature stems and leaves. Sclerenchyma cells are usually dead. So they’re often found in the cortex of more mature regions of stems and leaves, like this region in pink. The region in orange is less likely to contain sclerenchyma as it’s still developing.

As our plant is still fairly young, let’s focus on these regions that will contain more collenchyma and parenchyma in the cortex. The innermost layer of the cortex is called the endodermis, which can be identified in our diagram as this orange structure. The endodermis is sometimes called the starch sheath, as it’s responsible for storing starch, in addition to regulating the movement of water, ions, and plant hormones in the plant’s transport system.

This transport system consists of several structures called vascular bundles, one of which has been circled here. The vascular bundles are responsible for moving essential materials around the plant to the different organs that require them. Though vascular bundles will be present in the roots and leaves as well as here in the stem, their arrangement differs depending on their location. Each vascular bundle consists of phloem tissue, identified here in pink; xylem tissue, identified here in blue; and a layer called the cambium that sits between them. Each vascular bundle is supported by a tough section of sclerenchyma tissue called the pericycle, shown here in red.

Let’s take a closer look at the distribution of these simple tissues in the plant stem. We already know that the cortex of growing stems is made up primarily of collenchyma and parenchyma tissues, and in more mature regions sclerenchyma tissues, and that the pericycle contains sclerenchyma cells to provide structural support to the vascular bundles. Between each vascular bundle are regions of parenchyma tissue called the medullary rays. And the bulk of the middle of the stem is called pith. Pith is a spongy tissue in the middle of plant stems and is made up of parenchyma cells. Both the pith and the medullary rays mainly function as storage tissues, though the medullary rays also transport materials from the vascular bundles to the pith for storage.

Let’s look at the composition of a vascular bundle in the stem in more detail. Vascular systems are essential in most multicellular organisms. Single-celled organisms, like this amoeba, can usually obtain the substances they need, like sugars, water, or oxygen, directly across their surface by diffusion and can usually release waste products in the same way, as they are small organisms with a large surface area relative to their volume. As soon as organisms are made up of more than one cell, diffusion of these substances becomes much slower, as all of their many cells will require these substances. When organisms become as large as multicellular plants that can have millions of cells, if not more, this process becomes nearly impossible.

Most multicellular organisms, like plants and animals, are unable to transport the substances they need across their surface, as it would take far too long to transport them to their innermost cells. Therefore, most multicellular organisms have vascular systems that are specifically adapted to transport all of these substances that they need to every single cell in the body. This plant, for example, is multicellular, and it has vascular bundles to transport substances like sugars and water around its body. The vascular bundles not only transport water and mineral ions from the roots to the leaves, stem, and other parts of the plant that require them, but it also requires vascular bundles to move solutes like sugars and amino acids from the leaves, where the majority of them are made, both up and down the stem to the other plant organs that require them.

This diagram shows a close-up view of one vascular bundle in the stem. And you can see that it consists of two main regions, the xylem and the phloem. While the phloem is responsible for the transport of these sugars and amino acids, the xylem is responsible for transporting water and minerals. Let’s take a closer look at xylem tissues first. Xylem tissue consists of two main types of cell: xylem vessels and xylem fibers. Some parts of the xylem might include cells called tracheids, especially in nonflowering plants. These are structurally similar to xylem vessels, but they are tapered and closed and often longer and thinner. As they’re made up of cells that differ in their structure and function, xylem is often described as a compound tissue, or sometimes as a complex tissue.

Compound tissues differ from simple tissues as they’re composed of various types of cell that vary considerably in both their structure and their function. Xylem vessels are made of sclerenchyma cells. And when they’re mature, they’re usually dead. Like other sclerenchyma cells, xylem vessels have lignin in their walls, so they’re referred to as lignified. The cells in xylem vessels are stacked end to end, with their end walls broken down to form a hollow tube. These tubes are waterproof because of the lignin in their walls. This allows water molecules and dissolved mineral ions to move up through the xylem vessels without leaking out, as if being moved through a straw.

The lignin in the xylem vessel walls not only waterproofs them but also makes them stronger. The xylem fibers are also lignified, which helps to provide structural support to the xylem vessels and prevents them from collapsing. The main function of the xylem is to transport water and mineral ions from the roots, where they’re absorbed from the soil, to the other parts of the plant that require them.

Water is a key reactant in photosynthesis. So it’s needed in the photosynthesizing parts of the plant, like the leaves. Water is also a key medium for transport among its other functions in plants, such as filling vacuoles and maintaining cell shape. The mineral ions that are transported in the xylem also have many functions in the plant, for example, for building up amino acids that can then form proteins for growth and support or for forming the photosynthetic pigment chlorophyll.

Let’s take a closer look at the phloem next. Phloem tissue is another example of a compound tissue, as it’s made up of four main different types of cell: sieve tube members, which are otherwise known as sieve tube elements; companion cells, which can be identified here in pink; and fibers and sclereids, which have not been shown in this diagram as their main function is to provide mechanical support to the phloem. The role of the phloem is to transport sugars and amino acids primarily from the leaves and other photosynthetic parts of the plant to the other plant organs that require them.

Sugars like glucose are mainly synthesized in the parts of the plant that are exposed to light, as these parts will be carrying out photosynthesis. But these sugars are needed in all parts of the plant, as they will all be carrying out cellular respiration to release energy for the plant’s various processes. Therefore, these sugars are transported via the phloem to all the parts of the plant that might require them for respiration. The phloem is also responsible for transporting amino acids to the various plant organs.

A special feature of the phloem is that it can transport these solutes both up and down the stem depending on which direction they’re required in. This is different from the xylem, which can only transport water and minerals up the plant from the roots where they’re absorbed. To facilitate the movement of these substances, there are sieve plates between each sieve tube member. These sieve plates have large holes in them, much like a sieve, that allow certain substances to pass through them.

The companion cells, which are linked to the sieve tube members, contain many mitochondria to release the energy that’s needed for transport in the phloem. The companion cells and sieve tube members are linked to each other and pass substances through their walls through gaps called plasmodesmata. This forms a continuous flow of cytoplasm between the two types of cell.

Let’s see how much we can recall about the structure of the stem by having a go at a practice question.

The diagram provided shows a simplified structure of a dicotyledonous plant stem. What structure is indicated by the question mark? (A) Epidermis, (B) cortex, (C) pith, or (D) vascular bundle.

This diagram shows us a cross section of a dicot stem. And we need to identify one of the structures within it. To do this, let’s look at the dicot stem structures that are mentioned in these answer options.

The epidermis is a single layer of cells around the plant stem that forms a boundary to the external environment, acting as a tough skin for the plant. The epidermis often produces a waxy cuticle to coat it that can protect the stem from mechanical damage or water loss and may help prevent the entrance of dangerous microorganisms that may cause infection.

Just below the epidermis are several layers of cells that make up a region called the cortex. The cortex is a spongy region made up of many simple tissues, such as parenchyma tissues, and in growing stems collenchyma tissues, right below the epidermis. While parenchyma cells make up a lot of the soft fleshy tissues within various parts of the plant, collenchyma cells provide some structure and flexibility to growing stems.

Pith is a spongy tissue found in the center of dicot stems. Pith is also mainly made up of parenchyma cells, as its primary function is storage of substances like water and sugars.

The question mark is pointing to a structure called a vascular bundle. The vascular bundles are the plant’s transport systems, which move essential nutrients, like water and sugars, around the plant to the different organs that require them. Around the exterior of each vascular bundle is a tough region of sclerenchyma tissues, which helps to support the vascular bundle. The vascular bundle also includes phloem tissues, which transport sugars and amino acids around the plant, and has been shown in this diagram in blue.

Moving closer to the pith, the vascular bundle also contains xylem tissues. Xylem is responsible for transporting water and dissolved mineral ions from the roots of the plant to the other parts that require them. It’s shown in this diagram in red. Therefore, the structure that was marked with a question mark in this diagram was the vascular bundle.

Now that we’ve had a go at applying our knowledge, let’s review the key points that we’ve covered in this video. The typical structures in a dicot stem include the epidermis; the endodermis, which is sometimes known as a starch sheath; and the cortex, which is positioned between the two. There are also several vascular bundles, and between each vascular bundle are the medullary rays. The medullary rays transport materials from the vascular bundles to the pith in the center of the stem for storage. The vascular bundles are the transport system of the plant, and they include phloem tissues and xylem tissues. While the phloem transports sugars and amino acids from the photosynthetic parts of the plant to the other organs that require them, the xylem transports water and dissolved mineral ions from the roots to the rest of the plant.

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy.