Lesson Explainer: Plant Tissues Biology

In this explainer, we will learn how to describe the structure and function of different tissues found in plants.

Have you ever wondered while eating a stick of celery why it is so crunchy? Or have you ever wondered while enjoying a pear why it feels a little gritty? This is because of the different types of cells found in specific plant tissues, as well as the chemical and structural components they contain.

A tissue is a group of specialized cells that carry out a particular function. In celery, many cells group together to make the tissues found in the stalk, which you can see tapering into leaves in the image below.

Fresh leaf celery
Figure 1

The pressure of water on the cell walls in this stalk tissue gives it a signature crunch when bitten into. Lignin is an organic polymer that deposits in the cell walls of some plant cells to provide mechanical support to resist various stresses, and it is what gives the pear fruit its gritty texture. Plants are fantastically diverse in their internal structure, but most of them have certain properties and tissues in common.

Definition: Tissue

A tissue is a group of specialized cells that work together to carry out specific functions.

There are two main types of tissues in plants: simple tissues and compound (or complex) tissues. The micrograph below shows part of a cross section of a pumpkin stem revealing some of the differences between simple and compound tissues.

Tissue cell- cross section cucurbits (pumpkin) stem
Figure 2

Simple tissues are generally composed of one type of cell. There are three different types of simple tissues, which are also known as ground tissues, shown in the micrograph above: sclerenchyma, collenchyma, and parenchyma. Within these simple tissues, the majority of cells have a similar structure and generally carry out similar functions. Simple tissues are located in every part of the plant and have many different roles. Let’s have a closer look at some of these different tissues in the micrograph above by working from the outer layers of the stem inward.

Key Term: Simple Tissues

Simple tissues are composed of cells that are both functionally and structurally similar.

The darkest-blue layer of cells around the exterior edge of the plant stem is called the epidermis.

The epidermis in plants is the outermost layer of cells that coat the plant stem, leaves, roots, and other plant parts.

Key Term: Epidermis

The epidermis in plants is the outermost layer of cells that coat the plant stem, leaves, roots, and other plant parts.

Just below the epidermis moving inward, there is a layer of bright-blue-ringed cells that look like elongated rectangles or polygons. These cells are collenchyma cells. Below this collenchyma layer is a layer of purplish-red-ringed cells called sclerenchyma. You may even be able to identify sclerenchyma cells in the micrograph by their slightly thicker cell walls.

The bottom left of the original micrograph has been cropped to form the image below, which shows a more focused view of a cross section of these three layers: the outermost epidermis (left), bright-blue collenchyma cells (middle), and purplish-red-ringed sclerenchyma cells (right).

epiderms cells
Figure 3

Moving inward from these layers, parenchyma cells are visible. Parenchyma cells are identifiable as generally oval-like purplish-blue-ringed cells making up most of the central region of the micrograph. Parenchyma cells vary in their sizes, but the majority of them, as you can see in this micrograph, are fairly large in comparison to the other cells. Some parenchyma cells are unspecialized in their structure and are able to differentiate to perform various functions. This micrograph has been cropped to show just parenchyma cells in the image below.

Parenchyma cells
Figure 4

The larger oval-like red-ringed shapes that are grouped into distinct regions surrounded by various other cells in the original micrograph are part of compound tissues in this plant stem. These compound tissues, the xylem and the phloem, make up the plant’s vascular bundles, which function as transport tissues. Compound tissues consist of multiple different types of cell, varying both structurally and functionally. You can see a vascular bundle cropped from the original micrograph in the image below.

vascular bundle
Figure 5

Sclerenchyma cells are also situated around the exterior of each vascular bundle, so you can see some in the cropped micrograph above surrounding the large red ovals.

Key Term: Compound Tissues (Complex Tissues)

Compound tissues are composed of various types of cells that perform various functions.

Key Term: Vascular Tissues

The vascular tissues of a plant contain the vascular bundle, which is the transport system in plants, consisting of xylem and phloem tissues.

Example 1: Describing the Differences between Simple and Compound Plant Tissues

What is the difference between a simple and a compound tissue in plants?

  1. Simple tissues only carry out the basic functions of cell support, whereas compound tissues carry out complex functions like respiration and photosynthesis.
  2. Simple tissues are comprised of one type of cell only, whereas compound tissues are comprised of more than one type of cell.
  3. Simple tissues are only found in the stem of plants, but compound tissues are found in all parts of the plants.
  4. Simple tissues are formed of unspecialized cells, but compound tissues are formed of fully differentiated cells.

Answer

Simple tissues in plants are composed of cells that are very similar in their structure and function. Many simple tissues are made up of only one type of cell, but these cells can carry out a number of different functions, such as support, photosynthesis, or storage, depending on the tissue type. Simple tissues are usually found all over the plant. While some simple tissues are undifferentiated and unspecialized, such as some parenchyma cells, others are specialized. Compound tissues are more complex, as they consist of multiple different types of cell that differ greatly in their structure and function.

Our correct answer is, therefore, the following: simple tissues are comprised of one type of cell only, whereas compound tissues are comprised of more than one type of cell.

We will now go into more detail regarding the structure and function of the three simple tissues that we have seen in these micrographs: parenchyma, collenchyma, and sclerenchyma. A simple diagram of their structures is shown in Figure 6 below.

Figure 6: A diagram showing a cross section and a longitudinal view of the three main types of simple tissues in plants.

Let’s start with parenchyma tissues.

Parenchyma tissues are composed of cells that make up most of the soft, fleshy tissues inside various parts of a plant, such as the leaves, stem, and roots. Potatoes are an example of tubers, which grow from underground portions of the stem in some plants, and are fleshy because they are composed mostly of parenchyma cells storing starchy sugars. Some parenchyma cells vary in their shape, such as squares or rectangles, when they are present in certain xylem tissues, but most parenchyma cells are round or oval in shape, as you can see in the micrograph below. Each of the blue oval-like shapes is an individual parenchyma cell.

Cross sections of plant stem
Figure 7

Many parenchyma cells are not yet specialized, so they have the potential to differentiate into any type of cell. These cells share similar basic characteristics. For example, most parenchyma cells have a large number of chloroplasts and thin cell walls made primarily of cellulose. Parenchyma cells also have large vacuoles filled with water, starch, and minerals.

Parenchyma cells are living cells that carry out a number of functions: photosynthesis, storage of nutrients and water, secretion of sap, and assistance with aeration. This final function is made possible by the large intercellular spaces that can form between some specialized parenchyma cells so that gases can diffuse through these spaces easily and be exchanged with the atmosphere for photosynthesis.

Key Term: Parenchyma Cells

Parenchyma cells are thin-walled cells that make up the bulk of the inside of soft plant structures, such as leaves, stems, and roots.

Example 2: Describing the Major Functions of Parenchyma Tissues

Parenchyma cells are involved in the metabolic functions of the plant. Which of the following is not a major function associated with parenchyma tissue?

  1. Thermoregulation
  2. Photosynthesis
  3. Storing water
  4. Storing nutrients

Answer

Parenchyma cells are found in simple parenchyma tissues. They make up most of the fleshier parts of plant stems, leaves, and roots and have a wide range of metabolic functions. They contain many chloroplasts to carry out photosynthesis. They also have a thin cellulose cell wall so that water can pass into the cells easily. Their functions also include storing water and nutrients, among other various roles, such as gas exchange. Thermoregulation, the process of maintaining the internal temperature of an organism at a normal level, may be achieved in plants through simple water loss but is not a primary function carried out by parenchyma tissues.

Therefore, the only option that is not a major function associated with parenchyma tissues is thermoregulation.

A great example of where you can encounter lots of collenchyma cells is in a stick of celery, which is actually the stalk of the growing plant that tapers into leaves. Celery is crunchy, as the main role of collenchyma is to provide mechanical support and flexibility to these growing regions.

Collenchyma tissues are composed of cells that are longer than parenchyma cells. They are usually found below the epidermis of leaf veins and stems, particularly young stems, and are essential in the growing regions of the plant. The micrograph below shows a cross section of some collenchyma cells in a plant stem.

Structure of Collenchyma Cells
Figure 8

Collenchyma tissues are composed of cells that are living and are easily recognizable by their elongated, rectangular, or polygonal shape and their thick, irregular cell wall made of cellulose, hemicellulose, and pectin. Collenchyma tissues provide not only some mechanical strength, but also elasticity and flexibility to the growing regions of the plant. You may have noticed that the cells are not regular in their arrangement in the micrograph, as they may be cut either across their center or along their length.

Key Term: Collenchyma Cells

Collenchyma cells are elongated cells with thick cell walls that are found under the epidermis and provide structural support to the plant.

Sclerenchyma tissues are the toughest of the three simple tissue types. The cortex of the stems and leaves, which is the outer lining just below the epidermis and collenchyma layers, is usually full of sclerenchyma cells. Sclerenchyma cells also tend to reinforce the strength of the vascular bundles in plant tissues, so they are often found in the xylem and phloem tissues. Sclerenchyma cells can also be found in various other plant parts, such as in the fruit. Sclerenchyma cells mature with the surrounding tissues and tend to provide more permanent support than that provided by collenchyma cells.

The micrograph below shows a closer view of a cross section of sclerenchyma cells in a plant stem.

Structure of Sclerenchyma
Figure 9

The top layers of orange-red cells in the micrograph above are the epidermal and collenchyma cells. Just below this layer are many red-ringed sclerenchyma cells.

Sclerenchyma tissues are composed of cells that are tough, as their function is to provide great mechanical strength to the plant. Mature sclerenchyma cells are usually technically dead, and they provide such mechanical strength by having very thick cell walls made of cellulose, hemicelluloses, and a chemical called lignin, which makes the cells waterproof. It is also this lignin that adds the gritty texture to the pear fruit.

Sclerenchyma cells are typically recognizable by their thick cell walls, especially compared to parenchyma cells. Collenchyma cells also have thick walls, but they may be thinner and more irregular than the cell walls of sclerenchyma cells. You can also spot some blue-stained collenchyma cells just beneath the sclerenchyma tissue layer. The stains applied to tissues during microscopy are attracted to different polymers. The most commonly used stains (which have been used to produce the micrograph above) stain the lignin in the cell walls of sclerenchyma with a reddish-pink color and the cellulose in the cell walls of collenchyma and parenchyma with a bluish-green color.

Key Term: Sclerenchyma Cells

Sclerenchyma cells are thick-walled, lignified cells that provide mechanical support in stems, leaves, and roots.

Table 1 summarizes the locations, structures, and functions of the three simple tissues that we have covered.

Table 1: Summary of the three simple tissues: parenchyma, collenchyma, and sclerenchyma.
Tissue NameTypical LocationMain FunctionsCell WallOther Structural IdentifiersLiving or Nonliving?
ParenchymaInside soft plant structures, such as stems, leaves, and rootsPhotosynthesis, nutrient and water storage, sap secretion, and assistance with gas exchangeThin cell walls made primarily of celluloseCells are generally round but can also have other shapes. They contain many chloroplasts and a large vacuole and can differentiate to perform various functions.Living
CollenchymaUnder the epidermis of young stems and leaf veinsAdditional support, structure, and flexibility, especially in growing regionsThick, irregular cell walls made of cellulose and pectinCells are elongated, rectangular, or polygonal.Living
SclerenchymaIn the cortex of stems and leaves, vascular bundles (xylem and phloem), and other plant parts (e.g., fruits)Mechanical strengthThick, layered cell walls made of cellulose, hemicelluloses, and ligninCells vary in size, shape, and structure.Usually nonliving when mature

We will now go into more detail regarding the structure and function of the two compound tissues that we saw in the first micrograph we looked at: the xylem and the phloem.

Figure 10 below shows some information about these two tissues in more detail. The xylem and phloem are both vascular tissues. This means that they are involved in the transport of substances around the plant. The vascular system consists of vascular bundles containing these xylem and phloem tissues. They run up through the roots, stem, and leaves to transport nutrients, water, and dissolved minerals to various parts of the plant where they are needed.

Figure 10: A diagram showing the basic structure of xylem and phloem vessels and their respective directions of flow.

As you can see in Figure 10, the role of the xylem tissues is to transport water and dissolved mineral ions. These substances are moved from the roots, where they are absorbed from the soil, to the other parts of the plant that need them. For example, the leaves need water to photosynthesize. For this reason, the flow in the xylem is in one direction only, up from the roots to the rest of the plant.

Mature xylem tissues consist of three main types of cells: xylem parenchyma, xylem vessels, and xylem fibers, the latter two of which can be seen in Figure 11 below. The xylem vessels are made of sclerenchyma cells, which are lignified and dead. The lignin in their cell walls waterproofs them and provides extra structural support to the plant. These cells are stacked end to end, with their end walls broken down to form a hollow tube. This tube-like structure allows water and dissolved minerals to flow through it continuously like a straw. The xylem fibers are also lignified, and their main role is to provide mechanical support.

Figure 11: A diagram showing a longitudinal section of the structure of the xylem tissue, including xylem vessels and xylem fibers.

Key Term: Xylem

The xylem is the tissue in plants that transports water and dissolved mineral ions in one direction from the roots to the other parts of the plant.

As you can see in Figure 12, the role of the phloem tissues is to transport the products of photosynthesis. These are organic solutes in the form of dissolved sugars and amino acids, which are needed in various regions of the plant. These substances are moved primarily from the leaves, as this is where most of the photosynthetic process occurs, but as sugars and amino acids are needed everywhere in the plant, the phloem can transport substances both up and down the plant.

The phloem consists of sieve tube members (or sieve tube elements), companion cells (which are a type of parenchyma tissue), and fibers and sclereids (which are both types of sclerenchyma tissues).

As you can see in Figure 12, the sieve tube members are long, hollow columns of cells fused end to end, but unlike xylem vessels, their end walls are not entirely broken down. There are sieve tube plates between each adjacent sieve tube member, which, much like a sieve, have pores to allow the solutes to pass through. In order to make the sieve tube members hollow, the majority of their organelles break down, and mature sieve tube members have no nucleus.

Companion cells are linked to the sieve tube members by pores in their cell walls called plasmodesmata, which link together the cytoplasm, plasma membranes, and endoplasmic reticula of the two cells, as shown in Figure 12. Companion cells are examples of specialized parenchyma cells. Fibers and sclereids, which are examples of sclerenchyma cells, have thick cell walls and primarily provide structural support to the phloem vessels.

Figure 12: A diagram showing a longitudinal section of the structure of the phloem tissue, including sieve tube members linked to companion cells by plasmodesmata.

Key Term: Phloem

The phloem is the tissue in plants that transports the products of photosynthesis to the cells of the plants in two directions: up and down.

Example 3: Describing the Composition of the Vascular System

Plant tissues can be organized into systems. What two structures is the vascular system comprised of?

  1. Xylem and stem
  2. Stomata and phloem
  3. Xylem and phloem
  4. Stomata and stem

Answer

The plant’s vascular system is its transport system. This consists of the vascular bundle, containing both xylem and phloem tissues. Xylem vessels are responsible for transporting water and dissolved mineral ions from the roots to the other parts of the plant. The phloem is responsible for transporting dissolved organic solutes, in the form of sugars and amino acids, from their site of production to the other parts of the plant where they are needed. These are usually produced in the leaves through photosynthesis and then transported along the stem. The stomata are involved in gas exchange, taking carbon dioxide into the plant for photosynthesis and releasing oxygen.

Therefore, the correct structures that the vascular system is comprised of are the xylem and phloem.

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

Key Points

  • Simple tissues are composed of cells with similar structures and functions, whereas compound tissues are composed of different cells with various functions.
  • Simple tissues in plants are parenchyma, collenchyma, and sclerenchyma.
  • Compound tissues in plants are the xylem and phloem, which make up the plant’s vascular system.
  • The xylem transports water and mineral ions, whereas the phloem transports sugars and amino acids.

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