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You can determine the age of a tree by looking at its rings. In this lesson, we will look at the basic structures of stems and explore what causes the rings in a tree trunk.

The Shoot System

Within plants, there are two systems of structures: shoot and root. For now, we will focus on the basic structures of the shoot system, but in the future we will look at the growth of the shoot system as well as structures and growth of the root system.The shoot system includes the aboveground structures of plants. That means it includes the leaves, buds, stems, flowers, and fruits of plants. Because of all these different parts, the shoot system is rather complex.

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Even with this complexity, there are a few basic structures of the stem that we will focus on in this lesson, in addition to differences in the structures of stems based on the classification of the plant.

Basic Stem Structures

Cross section of a plant stem
Cross Section of Plant Stem

As we just went over, there are many different components of the shoot system. In this lesson, we will focus on the stem. The plant stem is best identified as the part of the plant above ground that provides support for other structures.

Some of these other structures include leaves and flowers. Within the stem, there are several types of tissue. Before we look at the specialized structures of vascular tissue, let’s cover a few other forms of tissue.The meristem is the tissue of the stem capable of cell division. The meristem is where the stem growth occurs. We will look at this more in a future lesson. The next tissue is similar to our skin.

We have skin to protect us from outside factors and to keep our insides inside. Plants also need something to protect their inner structures from outside elements. On the stem, this is the epidermis, which contains wax-coated cells for protection.

Vascular and Ground Tissue

The majority of tissue in a plant stem is called ground tissue and basically fills the space around the vascular tissue. We will look at vascular tissue after going over the three types of ground tissue: parenchyma, collenchyma, and sclerenchyma. While these words sound very unfamiliar, you may have noticed that they all end with -enchyma. This root word means ‘cellular tissue.

‘ Parenchyma is the most common form of tissue in plants and provides a variety of functions, including the storage of food and water. Collenchyma tissue helps support young stems and roots. Lastly, sclerenchyma tissue provides rigid support and protection for the plant stem. The Greek term sclera means ‘hard.’ This can help you remember that sclerenchyma is hard tissue that provides rigid support.Let’s now look at some specialized types of vascular tissue found in plant stems that is surrounded by the ground tissue.

Previously, the concept of vascular tissue was addressed. Remember that vascular tissue is the tissue used to transport water and nutrients throughout a plant. It acts like roads and plumbing, moving around nutrients and water needed by the plant. There are two types of vascular tissue: xylem and phloem. Xylem transports water and dissolved minerals, while phloem transports food. Previously, it was pointed out that an easy way to remember which vascular tissue is which is that ‘phloem’ and ‘food’ both start with the same sound.

This may help you remember that phloem moves food and that the other tissue, xylem, moves water.

Illustration of a monocot stem
Monocot Stem Illustration

Now that we have the basic concepts of xylem and phloem, let’s look at their structures a little more in depth. We will first look at xylem. Xylem is made of tracheids, which are non-living, elongated cells to allow for the transport of fluids. The xylem can sometimes help in the support of the stem.

The movement of fluids in plants is generally from the roots up through the stem to the leaves. While xylem is made of non-living cells, phloem is always made of living cells and transports nutrients from the leaves down through the stem. Phloem is made of sieve elements, including sieve cells, plates, and tubes that are specialized for the movement of food in plants.

Arrangement of Vascular Tissue

Botanists use the arrangement of vascular tissue in plant stems in order to help classify plants. We will focus on the differences seen between monocots and dicots. First, let’s review these terms. In a previous lesson on plant classification, we talked about the two types of angiosperms, or flowering plants, based on the number of seed leaves. Remember that the term that scientists use for seed leaf is cotyledon.

Flowering plants are either considered to be monocots or dicots. Monocot is short for monocotyledon, meaning one seed leaf, and dicot is short for dicotyledon, meaning two seed leaves. Monocots are simple flowering plants, such as grasses, corn and palm trees, while dicots include roses, sunflowers, cacti, and apple trees. We talked about the differences in seed leaves, flower arrangements, and leaf structures previously. However, we will now look at the differences in the arrangement of vascular tissue.

In monocots, the vascular tissue is found in paired bundles of both xylem and phloem. These bundles are dispersed throughout the stem.The structure of the vascular tissue in dicots is probably more familiar to you. In order to figure out the age of a tree, you count the number of rings. For example, a tree that is 30 years old will have 30 different rings. You can even tell when a year had a significant amount of rain versus a year when the tree may have been through a drought based on the spacing of these rings. This ability to determine the age of a tree is due to the vascular tissue, specifically the xylem.

In dicots, the xylem and phloem are found in rings – the xylem on the interior and the phloem on the exterior.

Cross section of a tree showing the xylem and phloem
Dicot Tree Rings Photo

The xylem must be made every year as the stem – or, in the case of trees, the trunk – continues to grow. The rings that you see in a tree trunk are the xylem that is made each year. The phloem are found closer to the exterior of the trunk, and can be used to get sap.

You may have seen trees being tapped for the collection of maple syrup. When people do this, they are basically breaking into the phloem of the tree in order to get the sugars that are transported from the leaves down the stem.

Lesson Summary

The shoot system of a plant consists of the parts that are above ground. We specifically looked at the stem in this lesson, as it provides support for other parts of the plant and a place for transit, similar to a highway that allows goods to be moved from one place to another. We looked at the core tissues found in the stem, specifically focusing on the ground tissue and the vascular tissue.Remember that there are three types of ground tissue: parenchyma, which is the most common form of tissue and stores water and food; collenchyma , which helps support young stems and roots; and sclerenchyma, which is hard tissue that provides support and protection for the stem.

There are two types of vascular tissue: xylem, which moves water, and phloem, which moves food. These structures are arranged differently in monocots and dicots. In monocots, which are flowering plants with only one seed leaf, the xylem and phloem are found in paired bundles throughout the stem. In dicots, which are flowering plants with two seed leaves, the xylem and phloem are arranged in rings, with the xylem on the inside and the phloem on the outside. Phloem can be tapped to collect sugar, such as maple syrup, and xylem is responsible for tree rings. While the structures of the stem have a variety of names and purposes, all of the structures of the plant stem are vital to the survival of the plant itself.

Learning Outcomes

After watching this lesson, you should be able to:

  • Identify the basic structures of plant stems
  • Classify different types of tissue and explain each
  • Determine how xylem helps reveal a tree’s age

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