INTRODUCTION

Plants are typically made up of roots, stems and leaves. Plant tissues can be broadly categorized into dividing meristematic tissue or non-dividing, permanent tissue. Permanent tissue is made up of simple and complex tissues. There are over 200 000 types of plant species in the world. Green plants provide the Earth’s oxygen, and also directly or indirectly provide food for all animals because of their ability to photosynthesise. Plants also provide the source of most of our drugs and medicines. The scientific study of plants is known as botany.

The chart below provides an overview of the types of plant tissues being studied in this chapter.

It is important that for each tissue type you understand:

Where it is located
What its key structural features are and how these relate to function
How each tissue type looks under the microscope
How to draw biological diagrams of each structure

Meristematic tissue

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. Meristematic tissue is found in root tips as this is where roots are growing and where dividing cells are produced. 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.

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

Structural adaptation	Function
Cells are small, spherical or polygonal in shape.	This allows for close packing of a large number of cells.
Vacuoles are very small or completely absent.	Vacuoles provide rigidity to cells thus preventing rapid division.
Large amount of cytoplasm and a large nucleus.	The lack of organelles is a feature of an undifferentiated cell. Large amount of nuclear material contains the DNA necessary for division and differentiation.

Permanent tissues

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. There are two types of permanent tissue:

Simple permanent tissues

Epidermis
Parenchyma
Collenchyma
Sclerenchyma

Complex permanent tissues

Xylem vessels (made up of tracheids and vessels)
Phloem vessels (made up of sieve tubes and companion cells)

Epidermis tissue

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 of epidermal tissue are listed in table below

Structure	Function
Layer of cells covering surface of entire plant.	Acts as a barrier to fungi and other microorganisms and pathogens.
Layer is thin and transparent.	Allow for light to pass through, thereby allowing for photosynthesis in the tissues below.
Epidermal tissues have abundant trichomes which are tiny hairs projecting from surface of epidermis. Trichomes are abundant in some plant leaves.	Leaf trichomes trap water in the area above the stomata and prevent water loss.
Root hairs are elongations of epidermal cells in the root.	Root hairs maximise the surface area over which absorption of water from the soil can occur.
Epidermal tissues in leaves are covered with a waxy cuticle.	The waxy outer layer on the epidermis prevents water loss from leaves.
Epidermal tissues contain guard cells containing chloroplasts.	Guard cells control the opening and closing of the pores known as stomata thus controlling water loss in plants.
Some plant epidermal cells can secrete poisonous or bad-tasting substances.	The bitter taste of the substances deter browsing and grazing by animals.

Guard cells and Stomata

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 specialized 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.

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

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.

Parenchyma
Structure	Function
Thin-walled cells.	Thin walls allow for close packing and rapid diffusion between cells.
Intercellular spaces are present between cells.	Intercellular spaces allow diffusion of gases to occur.
Parenchyma cells have large central vacuoles.	This allows the cells to store and regulate ions, waste products and water. Also function in providing support.
Specialised parenchyma cells known as chlorenchyma found in plant leaves contain chloroplasts.	This allows them to perform a photosynthetic function and responsible for storage of starch.
Some parenchyma cells retain the ability to divide.	Allows replacement of damaged cells.

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.

Collenchyma tissue

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
Structure	Function
Cells are spherical, oval or polygonal in shape with no intercellular spaces.	This allows for close packing to provide structural support.
Corners of cell wall are thickened, with cellulose and pectin deposits.	Provides mechanical strength.
Cells are thin-walled on most sides.	Provides flexibility, allowing plant to bend in the wind.

Sclerenchyma tissue

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 specialized 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
Structure	Function
Cells are dead and have lignified secondary cell walls.	This provides mechanical strength and structural support. The lignin provides a ‘wire-like’ strength to prevent from tearing too easily.
Sclereids have strong walls which fill nearly the entire volume of the cell.	Provide the hardness of fruits like pears. These structures are used to protect other cells.

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.

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