Plant tissue systemsCompare the structures, position in the stem and function of sclerenchyma fibres and xylem vessels.

Review of cell walls

Plant cells are strong because they are wrapped in a protective layer of cellulose which forms the cell wall

Cellulose is a polysaccharide made from β glucose monomers. Alternate glucose molecules “flip over” in the chain, forming hydrogen bonds between adjacent cellulose chains.

Because cellulose has no side branches the chains can be packed closely which increases the strength of the hydrogen bonds further.

Cellulose

Individual cellulose chains are packaged together into microfibrils. The microfibrils wind around each other forming cellulose fibres.

The cell wall is build from layers of these fibres. The two structure you need to be aware of are the:

Primary cell wall: first to form, cellulose fibres laid down in the same direction

Secondary cell wall: forms later, cellulose fibres laid down at right angles to those in the primary wall. Provides much greater strength.

Introduction to plant tissues

Like other organisms, plant cells are grouped together into various tissue.

There are three types of tissue systems in plants:Dermal tissueVascular tissueGround tissue

? What are the primary functions of a stem?

Dermal tissue

Dermal tissue consists of the epidermis and the periderm.

The epidermis is generally a single layer of closely packed cells. It both covers and protects the plant. It can be though of as the plant’s skin.

Depending on the part of the plant that it covers, the dermal tissue can be specialized to a certain extent.

? Refer to p 206 and give two ways in which the epidermis may specialized.

The periderm, also called bark, replaces the epidermis in plants that undergo secondary growth. The periderm is multilayered as opposed to the single layered epidermis.

Ground tissue The ground tissue system synthesizes organic

compounds, supports the plant and provides storage for the plant. It is made up of three types of cells: Packing tissue called parenchyma - which holds

everything in place. Strengthening tissues that help support the plant:

Collenchyma - this tissue can stretch as the plant grows Sclerenchyma - provides more strength in mature plants.

Ground tissue is mostly made up of parenchyma cells but can also include some collenchyma and sclerenchyma cells as well. Parenchyma cells are unspecialized cells that can synthesize and store organic products in a plant.

? Refer to p 207 and give two ways in which parenchyma cells may specialized.

Ground tissue

Collenchyma cells have a support function in plants, particularly in young plants. These cells help to support plants while not restraining growth due to their lack of secondary walls and the absence of a hardening agent in their primary walls.

? Where can collenchyma cells be found?

? Give an example of collenchyma tissue.

Ground tissue

Sclerenchyma cells are also formed from parenchyma cells and also have a support function in plants as a plant gets larger.

Unlike collenchyma cells, they have a hardening agent called lignin and are much more rigid.

? Where is sclerenchyma tissue found?

? What makes sclerenchyma so strong and flexible?

? What name is given to sclerenchyma cells that are completely filled with lignin? Where are these found?

? Answer the two questions on p 207

Homework:Vascular tissue

Xylem and phloem throughout the plant make up the vascular tissue system. They allow water and other nutrients to be transported throughout the plant.

? Refer to p 208. What are the primary functions of xylem and phloem?

? What is cambium?

? Summarise how the structure of xylem is formed using a diagram. (Refer to fig. 4.1.12)

? Answer the questions on p 209.