Lesson Explainer: Structure of the Stem Biology • Second Year of Secondary School

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

Plant stems are vital to their survival. Stems are long, stalk-like structures that form the main body of a plant. They usually rise above ground, though some stems can be found underground too! The stem helps to support the plant and allows it to move to access light and to transport essential substances to the different plant organs.

Stems need to be well adapted for these functions, as some plants, such as the North American Hyperion tree in the image below, can have stems reaching as high as 115 metres (almost 380 feet)! In this explainer, we will be learning how the structure of plant stems can facilitate survival, including in these huge organisms.

A dicot, which is short for dicotyledonous, refers to a type of flowering plant that produces seeds with two food stores. These food stores, called cotyledons, provide nutrients for the developing plant embryo during germination and will eventually form the plant’s first leaves. In this explainer, we will be looking at the stem of dicots, but it is worth noting that monocots, which only have one seed leaf, will have a subtly different stem structure. Figure 2 shows the difference between monocot and dicot seeds.

A cross section is a cut that has been made at a right angle to the upward growth of a plant. A relatable example is shown in Figure 3 below, of a cross section from a tree trunk.

Let’s look at the structure of a typical dicot stem.

The cross-section of a dicot stem in Figure 4 shows some of these same structures that you have also seen in the tree trunk in Figure 3.

The epidermis, which is shown in Figure 4, is a single layer of cells that forms the outer covering of the plant stem. It acts as a tough “skin” for the plant. The epidermis is covered with a water-resistant waxy cuticle that protects the stem from mechanical damage or water loss and helps prevent the entrance of microorganisms that may cause infection. The epidermis will be present in other plant organs too, such as on leaves, roots, and even flowers.

Just within the stem’s epidermis, there is a layer of spongy cells that make up the cortex. The majority of the cortex is made up of parenchyma tissue, one of the three simple tissues found in plants. Parenchyma cells make up the soft, fleshy tissues inside various parts of a plant, such as the leaves, stem, and roots. Parenchyma tissue has plenty of intracellular spaces between each cell that provide aeration to promote gas exchange. Parenchyma cells contain chloroplasts to carry out photosynthesis.

Below the epidermis within the cortex of growing stems, there is a thin section of collenchyma tissue. Collenchyma cells are usually found below the epidermis of leaf veins and stems, particularly young stems, and are essential in the growing regions of a plant for providing structure. Collenchyma cells have thickened cellulose and pectin cell walls, and some contain chloroplasts to carry out photosynthesis.

Sclerenchyma tissues are the toughest of the three simple tissue types found in plants, and sclerenchyma cells are usually dead. The cortex of mature stems and leaves, the outer lining just below the epidermis, is usually full of sclerenchyma cells because they tend to be present in plant parts that do not require growth. If both sclerenchyma and collenchyma are present, sclerenchyma will usually be closer to the epidermis. They are tough, as their function is to provide mechanical strength to the plant.

The innermost layer of the cortex is called the endodermis. This is sometimes also called the starch sheath, as it is responsible for storing starch in addition to regulating the movement of water, ions, and plant hormones in the plants transport system.

Beneath the endodermis, there are several small structures called vascular bundles, the arrangement of which you can see in Figure 4. The vascular bundles make up the plant’s transport system, which moves essential materials around the plant to different organs that require them. Though vascular bundles are present in the roots and leaves as well as the stem, their arrangement differs depending on their location. Vascular bundles consist of phloem tissue, xylem tissue, and a layer called the cambium between them. Each vascular bundle is supported by a tough section of parenchyma or sclerenchyma tissue called the pericycle.

Example 1: Identifying the Structures in a Plant Stem

The diagram provided shows a simplified structure of a dicotyledonous plant stem. What structure is indicated by the question mark?

1. Epidermis
2. Cortex
3. Pith
4. Vascular bundle

Answer

The diagram shows a cross section of a dicot stem, and we need to identify one of the structures in it. To do this, let’s look at the different structures in a dicot stem and their functions.

The epidermis is a single layer of cells that forms the outer covering of the plant stem. It acts as a tough “skin” for the plant. The epidermis surface contains a waterproof waxy cuticle that protects the stem from mechanical damage or water loss and helps prevent the entrance of microorganisms that may cause infection.

Just within the stem’s epidermis, there is a layer of spongy cells that make up the cortex. The majority of the cortex is made up of parenchyma and collenchyma tissues, two of the three simple tissues found in plants. Parenchyma cells make up the soft, fleshy tissues inside various parts of a plant, while collenchyma cells provide structure.

The innermost layer of the cortex is called the endodermis. This is sometimes also called the starch sheath, as it is responsible for storing starch in addition to regulating the movement of water, ions, and plant hormones in a plant’s transport system. Pith is the spongy tissue in the center of stems, also made up of parenchyma cells.

Beneath the endodermis, there are several small structures called vascular bundles. The vascular bundles make up the plant’s transport system that moves essential materials around a plant to different organs that require them. Vascular bundles consist of a tough section of parenchyma or sclerenchyma tissue that supports the vascular bundle, called the pericycle, as well as phloem tissue, xylem tissue, and a layer called the cambium between them.

Therefore, the structure marked by a question mark is the vascular bundle.

Between each vascular bundle is a region of parenchyma tissue called the medullary rays, which you can see in Figure 5. You can also see that the bulk of the middle of the plant stem is made up of pith. Pith is the spongy tissue in the center of stems, also made up of parenchyma cells. Both the pith and medullary rays mainly function as storage tissues, though the medullary rays also transport materials from the vascular bundles to the pith for storage. You can see the composition of simple tissues in each part of the stem in Figure 5.

Example 2: Describing the Function of Pith

The diagram provided shows a simplified structure of a dicotyledonous plant stem. The pith occupies the center of the stem and is comprised of parenchyma cells. What is the main purpose of the pith?

1. To break down or destroy dead plant cells
2. To dissolve excess carbon dioxide
3. To act as the site of respiration
4. To store and transport nutrients
5. To provide mechanical support to the stem

Answer

You can see that the bulk of the middle of the plant stem is made up of a substance called pith. Pith is the spongy tissue in the center of stems, made up of parenchyma cells. Parenchyma cells make up the soft, fleshy tissues of various plant organs.

Beneath the endodermis, there are several small structures called vascular bundles that make up the plant’s transport system. This involves phloem tissue, xylem tissue, and a layer called the cambium between them. The phloem and xylem transport materials around the plant, some of which cannot be used immediately and need to be stored.

The pith mainly functions as a storage tissue and also transports materials from the vascular bundles to the pith cells for storage.

Therefore, the main purpose of the pith is to store and transport nutrients.

Let’s look at the composition of a vascular bundle in the stem in more detail.

Vascular systems are essential in any multicellular organism. Single-celled organisms, like an amoeba, can usually diffuse the materials they need across their surface and into their cells. Multicellular organisms, such as plants and animals, are unable to do this. They possess too many cells to acquire all of the materials they need from their environment by simple diffusion across their surface, as it would take far too long for materials to diffuse all the way into the innermost cells. Therefore, multicellular organisms have vascular systems specifically adapted to transport the materials they need to every cell in the body.

A daisy plant, for example, is multicellular and needs a transport system to move the water and minerals absorbed through the roots to the stem, flower, and leaves. It also needs a transport system to move the sugars and amino acids made in the leaves and stem to the other parts of the plant. The amoeba can simply diffuse substances like water and sugars across their cell surface membrane.

In plants, this transport system consists of the vascular bundle, which moves from the roots, through the stem, to the leaves, and to other organs of a plant. You can see the main two components of the vascular bundle, the xylem and phloem, in Figure 6.

The vascular bundle contains xylem tissues.

Xylem tissue consists of two main types of cells: xylem vessels, sometimes known as structurally similar tracheids, and xylem fibers. Xylem vessels are made of sclerenchyma cells that are lignified and, therefore, dead. These cells are stacked end to end, with their end walls broken down to form a hollow tube to allow water and dissolved minerals to flow through it like a straw.

The manner by which lignin is deposited into xylem-vessel cell walls makes them appear different. For example, when this thickening occurs in annular forms, it appears as discrete rings, while spiral thickening appears as a continuous helix of lignin running down the length of the vessel. Lignin waterproofs the xylem vessels and provides extra structural support to prevent them from collapsing. Both xylem vessels and tracheids are pitted to allow water and minerals to pass out of them, and tracheids are tapered and closed. Xylem fibers are also lignified, and their main role is to provide mechanical support.

The main function of xylem vessels is to transport water from the roots, where water is absorbed from the soil, to the parts of the plant that require it. Water is a key reactant in the process of photosynthesis and is therefore required in the photosynthesizing parts of the plant such as the leaves. Water is also a key medium for transport among its other functions in a plant, such as filling vacuoles and maintaining cell shape.

In addition to carrying water, the xylem transports dissolved mineral ions that have been absorbed by the roots from soil. These minerals will be transported up the plant, dissolved in the water that the xylem carries to different cells for functions such as building amino acids for growth and support.

The phloem is the other main vascular tissue found within the vascular bundle. Phloem tissue consists of four main types of cells: sieve tube members (or sieve tube elements), companion cells (which are examples of specialized parenchyma cells), fibers, and sclereids.

The role of the phloem is to transport sugars and other substances such as amino acids around the plant. Sugars, such as glucose, are mostly made by the plant leaves in photosynthesis, as these are the sections of the plant that are exposed to sunlight. These sugars are needed by all parts of the plant, however, in order to carry out cellular respiration to release energy for the plant’s various functions. Therefore, the sugars are transported from the leaves to the rest of the plant via the phloem.

The cambium, which sits between the xylem and phloem in the vascular bundle, is a region of actively dividing unspecialized cells called meristem cells. These meristem cells can form secondary xylem or secondary phloem. This means that, even following initial growth, the xylem and phloem can become as large as the plant requires as growth continues.

The pericycle is filled with parenchyma cells or sclerenchyma fiber cells that surround the vascular bundles and support them by holding the xylem and phloem “tubes” upright, allowing them to continue functioning efficiently as the plant grows.

Let’s recap some of the key points we have covered in this explainer.