4.3 Plant tissues

Meristematic tissue (ESG66)

Meristematic tissue is undifferentiated tissue. Meristematic tissue contains actively dividing cells that result in formation of other tissue types (e.g. vascular, dermal or ground tissue). Apical meristematic tissue is found in buds and growing tips of plants. It generally makes plants grow taller or longer. Lateral meristematic tissue make the plant grow thicker. Lateral meristems occur in woody trees and plants. Examples of lateral meristematic tissue include the vascular cambium that results in the rings you see in trees, and cork cambium or 'bark' found on the outside of trees.

Diagram	Micrograph

The following table highlights how the structure of the meristematic tissue is suited to its function.

Meristematic tissue is found in root tips as this is where roots are growing and where dividing cells are produced. Figure 4.5 shows a micrograph image of a root tip.

Permanent tissues (ESG67)

The meristematic tissues give rise to cells that perform a specific function. Once cells develop to perform this particular function, they lose their ability to divide. The process of developing a particular structure suited to a specific function is known as cellular differentiation. We will examine two types of permanent tissue:

1. Simple permanent tissues

   • epidermis
   • parenchyma
   • collenchyma
   • sclerenchyma

2. Complex permanent tissues

   • xylem vessels (made up of tracheids and vessels)
   • phloem vessels (made up of sieve tubes and companion cells)

Epidermis tissue (ESG68)

The epidermis is a single layer of cells that covers plants' leaves, flowers, roots and stems. It is the outermost cell layer of the plant body and plays a protective role in the plant. The function of key structural features are listed in table:epidermaltissue.

Guard cells and Stomata (ESG69)

A stoma is a pore found in the leaf and stem epidermis that allows for gaseous exchange. The stoma is bordered on either side by a pair of specialised cells known as guard cells. Guard cells are bean shaped specialised epidermal cells, found mainly on the lower surface of leaves, which are responsible for regulating the size of the stoma opening. Together, the stoma and the guard cells are referred to as stomata.

The stomata in the epidermis allow oxygen, carbon dioxide and water vapour to enter and leave the leaf. The guard cells also contain chloroplasts for photosynthesis. Opening and closing of the guard cells is determined by the turgor pressure of the two guard cells. The turgor pressure is controlled by movements of large quantities of ions and sugar into the guard cells. When guard cells take up these solutes, the water potential decreases causing water to flow into the guard cells via osmosis. This leads to an increase in the swelling of the guard cells and the stomatal pores open.

Structure

Activity: Practical investigation of leaf epidermis

Learners to use microscope and slide preparation skills.

NOTES TO TEACHERS

• Tradescantia, a common SA plant with purple leaves, works particularly well for this practical since the epidermis rips off easily.

• Learners should be encouraged to rip the leaves quickly in order to get epidermal tissue.
• They must search the entire specimen on low power, in order to get the best part to magnify. There is little value in just enlarging the first part of the leaf they focus on – there will be many stomata that have air bubbles with thick black outlines over them. Learners must search carefully and enlarge the best stoma they can find.
• Learners must be encouraged to draw the guard cells as they see them, even if they are lying at an angle.

Questions

1. Describe the shape of the guard cells and normal epidermal cells.
2. Which epidermal cells have chloroplasts?
3. Describe the wall thickness around the guard cells and account for any visible differences.

Answers

1. Guard cells are bean shaped and normal epidermal cells are irregular, square-shaped or elongated (depending on leaf used.
2. Only the guard cells.
3. Guard cells have thick inner walls and thinner outer walls, as this helps them to open the pores for gaseous exchange.

We will now look at parenchyma, collenchyma and sclerenchyma cells. Together these tissue types are referred to as ground tissues. Ground tissues are located in the region between epidermal and vascular tissue.

Parenchyma tissue (ESG6B)

Parenchyma tissue forms the majority of stems and roots as well as soft fruit like tomatoes and grapes. It is the most common type of ground tissue. Parenchyma tissue is responsible for the storage of nutrients.

Collenchyma tissue (ESG6C)

Collenchyma is a simple, permanent tissue typically found in the shoots and leaves of plants. Collenchyma cells are thin-walled but the corners of the cell wall are thickened with cellulose. This tissue gives strength, particularly in growing shoots and leaves due to the thickened corners. The cells are tightly packed and have fewer inter-cellular spaces.

Collenchyma
Diagram	Micrograph

Sclerenchyma tissue (ESG6D)

Sclerenchyma is a simple, permanent tissue. It is the supporting tissue in plants, making the plants hard and stiff. Two types of sclerenchyma cells exist: fibres and sclereids.

Sclerenchyma fibres are long and narrow and have thick lignified cell walls. They provide mechanical strength to the plant and allow for the conduction of water.

Sclereids are specialised sclerenchyma cells with thickened, highly lignified walls with pits running through the walls. They support the soft tissues of pears and guavas and are found in the shells of some nuts.

Sclerenchyma
Diagram	Micrograph

Sclerenchyma tissues are important components in fabrics such as flax, jute and hemp. Fibres are important components of ropes and mattresses because of their ability to withstand high loads. Fibres found in jute are useful in processing textiles, given that their principal cell wall component is cellulose. Other important sources of fibres are grasses, sisal and agaves. Sclereid tissues are the important components of fruits such as cherries, plums or pears.

We will now examine the complex permanent tissues. Remember the difference between simple and complex permanent tissues is that simple permanent tissues are made up of cells of the same type whereas complex permanent tissues are made up of more than one cell type that combine to perform a particular function. We will examine the vascular tissues, xylem and phloem tissues next.

Xylem tissue (ESG6F)

Xylem has the dual function of supporting the plant and transporting water and dissolved mineral salts from the roots to the stems and leaves. It is made up of vessels, tracheids, fibres and parenchyma cells. The vessels and tracheids are non-living at maturity and are hollow to allow the transport of water. Both vessels and tracheids have lignin in their secondary walls, which provides additional strength and support.

Xylem vessels are composed of a long chain of straight, elongated, tough, dead cells known as vessel elements. The vessel elements are long and hollow (lack protoplasm) and they make a long tube because the cells are arranged end to end, and the point of contact between two cells is dissolved away. The role of xylem vessels is to transport water from roots to leaves. Xylem vessels often have patterns of thickening in their secondary walls. Secondary wall thickening can be in the form of spirals, rings or pits.

Tracheids have thick secondary cell walls and are tapered at the ends. The thick walls of the tracheids provide support and tracheids do not have end openings like the vessels. The tracheids' ends overlap with one another, with pairs of pits present which allow water to pass through horizontally from cell to cell.

Diagram	Micrograph

In addition to transporting water and mineral salts from roots to leaves, xylem also provides support to plants and trees because of its tough lignified vessel elements.

Phloem tissue (ESG6G)

Phloem tissue is the living tissue responsible for transporting organic nutrients produced during photosynthesis (mainly as the carbohydrate sucrose) to all parts of the plant where these are required. The phloem tissue is made up of the following major types of cells:

• sieve elements: these are conducting cells which transport sucrose.
• parenchyma cells: which store food for transport in phloem.
• companion cells: are associated with parenchyma cells and control the activities of sieve tube elements, since the latter have no nuclei. Companion cells are responsible for providing energy to the sieve elements to allow for the transport of sucrose. Companion cells play an important role in loading sieve tubes with sucrose produced during photosynthesis. Companion cells and sieve tube elements are connected via connecting strands of cytoplasm called plasmodesmata.
• fibres: unspecialised cells and supportive cells.

Diagram	Micrograph

In the table below, the key structural features of the phloem are related to their function.