REVISION: STRUCTURE & SUPPORT IN PLANTS 12 JUNE 2013

Lesson Description

In this lesson we revise:

the internal and external structure of a dicotyledonous root, stem and leaf

how the leaf is adapted to photosynthesis, gaseous exchange and transport.

Key Concepts

The External Structure of a Dicotyledonous Root The main functions of the root is to:

o Anchor the plant into the ground

o Absorb water and mineral salts from the soil

The External Structure of a Dicotyledonous Root

The Internal Structure of a Dicotyledonous Root. The internal structure of the root can be divided into three distinct parts

o The epidermis

o The cortex

o Central stele/cylinder

Line diagram of a cross-section through a dicot root

A cross-section through a dicot root showing the various tissues

1. The Epidermis

A thin single layer of epidermal cells –that protect the surface

NO cuticle. Epidermal cells have specialized

outgrowths called root hairs, which increase the surface area for the absorption of water

2. The Cortex Consists mainly of large thin-walled

parenchyma cells. – they allow water and minerals to pass through to the xylem

Large intercellular spaces between the cells

The inner layer of the cortex consists of single cells called the endodermis – regulates the passage of water

These cells are covered in thickened cork strips to form the Casparian strip

They fill the spaces between the upper and lower epidermis

Endodermis with Casparian Strips

3. The Central Stele Just underneath the endodermis,

thin-walled cells called the pericycle. These cells give rise to the lateral roots

The central stele consists mainly of vascular tissue i.e. xylem and phloem.

Xylem transports water and mineral salts to the leaf.

Phloem transports the dissolved organic substances from photosynthesis down to the roots and the rest of the plant

The External Structure of a Dicotyledonous Stem

The main functions of the stem are to:

o Place the leaves in a favourable position for photosynthesis.

o Place the flowers in a favourable position for pollination.

o Transport of water and mineral salts from the root to the leaf

o Transport organic substances from the leaf to the rest of the plant

Leaves and side branches

develop

Has meristematic tissue

and forms new cells and

results in primary growth

of the stem

The External Structure of a Dicotyledonous Stem

The internal structure of the dicot stem can be divided into three regions

o The epidermis

o The cortex

o Central stele/cylinder

Line diagram of a cross section through a dicotyledonous stem

A cross section through a dicotyledonous stem showing the various tissues

1. The Epidermis A thin single layer of epidermal cells –that protect the surface

Waterproof cuticle present.

Can have stomata and trichomes (multicellular hair)

2. The Cortex Consists of:

o Collenchyma –strengthens stem

o Parenchyma – storage and gaseous exchange

o Endodermis – stores starch

3. The Central Stele Is the vascular tissue, which is arranged in a circle of vascular

bundles.

The vascular bundle consists of the following

o Schlerenchyma cap – protection and support

o Phloem – transport organic matter to the rest of the plant

o Cambium – meristematic tissue that allows for

secondary thickening

o Xylem – transport water and mineral salts to the leaves

Primary Growth

When the plant grows in length.

Occurs in the apical meristem of the stem and the root.

Causes elongation in the roots and the stem.

Secondary Growth

The Internal Structure of a Dicot stem (Revise)

(Line diagram of a cross section through a dicotyledonous stem)

Secondary thickening results in the stem of dicot plants to

become thicker as the plant grows older.

This is brought about by the cambium in the vascular bundles.

Vascular bundles are arranged in a circle and are open.

The cambium lies between the xylem and phloem and is called

fascicular cambium.

The xylem tissue forms the wood of the tree.

With the start of secondary thickening layers of parenchymal

cells of the medullary rays become meristematic , this is called

interfascicular cambium

Cambium

Secondary xylem (towards the centre) and secondary phloem ( towards the outside) are

formed by repeated division of the ring of cambium.

The layers or concentric rings of secondary xylem form the annual rings.

As more layers or rings are formed the stem increases in thickness.

The primary xylem and primary phloem are pushed further apart.

The cell walls of the xylem are strengthened with lignin which provides support to the stem.

The cambium continues to form xylem tissue towards the inside of the plant and phloem

tissue towards the outside, just beneath the bark

(Stem showing secondary thickening)

Annual Rings

The annual ring is a new layer of wood produced during one growth season.

The layers appear as concentric rings which includes two colours of wood:

o Spring wood

o Autumn wood

Spring Wood

Xylem is formed during spring, when conditions are favourable for growth – i.e. sufficient water,

nutrients and sunlight.

Spring wood is light in colour due to large xylem cells with thinner cell walls. The springwood

ring is wider due to rapid growth and many cells being formed

Autumn Wood

Formed during autumn and winter months

Due to cold and dry conditions the growth is limited.

The xylem cells are smaller and more compact.

The xylem tissue appears as a dark ring in the cross section of a tree trunk.

The age of the tree can be determined by counting the number of rings from the inside of the

tree trunk to the outside.

The broader the springwood layers the more favourable the climatic conditions of that season.

Phloem tissue is formed towards the outside of the stem every growing season.

These and the cells of the cortex form the sapwood.

In many stems a cork cambium develops from the outer layer of the cortex layer, which

becomes meristematic.

The cork cambium forms cork tissue or bark towards the outside.

The bark is waterproof and protects the inner tissues from mechanical damage.

The bark of trees has small holes called lenticels to allow for gaseous exchange.

Important Terms

Annular rings Cork Secondary thickening Vascular bundles Dicotyledonous Lenticels Cambium Secondary Xylem Spring wood Secondary phloem Autumn wood Apical meristem

The External Structure of a Leaf The typical leaf consists of a flat lamina (leaf blade) at is attached to the stem by means of a

petiole

The veins of the leaf blade consist of xylem and phloem that are continuous with the xylem

and phloem of the root and stem

Important Terms:

Lamina Cuticle Petiole Vascular bundles Dicotyledonous Stomata Epidermis Transpiration Palisade mesophyll Photosynthesis Spongy mesophyll

Lamina

The Internal Structure of a Leaf Diagram below is a cross section of a dicotyledonous leaf that shows all the different tissues

that make up the leaf.

The leaf consists of three main sections:

o The epidermis

o The mesophyll

o The vascular bundles

4. The Epidermis An upper and lower surfaces of the leaf are covered

by a thin transparent epidermis

The epidermis is thin and transparent so that the light can pass through for photosynthesis

A thin waxy cuticle covers the epidermis and also ensures that water is not lost through the epidermis.

The lower surfaces of the epidermis have specialized epidermal cells called stomata.

The stomata consist of a kidney-shaped guard cell that open and close the stomatal pore.

The stomata are responsible for gases entering and exiting the leaf (gaseous exchange) for photosynthesis and respiration. Water vapour is also lost through the stomata, this is transpiration

5. The Mesophyll There are two different types of mesophyll cells i.e.

palisade and spongy mesophyll.

They fill the spaces between the upper and lower epidermis

The palisade mesophyll cells are:

Thin-walled parenchyma cells, they are elongated and are situated just below the upper epidermis.

Contain large numbers of chloroplasts to absorb sunlight for photosynthesis.

The spongy mesophyll cells are:

Round parenchyma cells that have many intercellular spaces and air chambers to allow gases to diffuse easily.

They have less chloroplasts than the palisade mesophyll as they store food that is made by the palisade.

6. The Vascular Bundles The veins consist of xylem and phloem.

Water is carried from the roots up the stem and into the leaf where it is needed in the mesophyll tissues for photosynthesis.

The phloem then carries the food that is made by photosynthesis down to the other parts of the plant

How is the leaf adapted to perform its functions i.e. Photosynthesis, gaseous exchange and transport of water and food.

The lamina is large – providing a large surface area to absorb as much sunlight as possible.

Thin lamina – allows gases to move in and out easily by diffusion.

Cuticle is impermeable to water and transparent - prevents the loss of water and allows

sunlight through to the palisade mesophyll cells.

Epidermal cells are transparent – allow sunlight through.

The lower surface has many stomata – ensures the gaseous exchange occurs.

The palisade tissue is directly below the epidermis and arranged longitudinally – increase the

surface area exposed to the sunlight.

The palisades have many chloroplasts – to absorb the maximum amount of sunlight.

The palisade has thin walls – to allow osmosis and the diffusion of gasses into and out of the

cells.

The spongy mesophyll cells have large intercellular air spaces – gaseous exchange

The spongy mesophyll have thin cell walls - to allow osmosis and the diffusion of gasses into

and out of the cells.

The vascular bundles have xylem – to transport water and minerals from the root, up the stem

to the mesophyll for photosynthesis.

The vascular bundles have phloem – to transport the dissolved nutrients made during

photosynthesis, to other parts of the plant.

Questions

Question 1

Study the diagram of the external structure of a dicotyledonous root and answer the questions that

follow

a) Identify the parts labelled 1-7.

(7)

b) Name the functions of parts numbered 1 and 6

(2)

c) How does the tissue in region 4 differ with the

tissue in region numbered 2 (3)

d) Name the main functions of the root.

(2)

Question 2

Different plant structures are built from specialised groups of plant cells called tissues. Each plant structure is made up of tissues designed to carry out specialised functions or processes. The diagram below shows a cross-section of the stem of a typical plant. Each year the stem will grow thicker by secondary thickening.

Explain how the thickness of the stem increases over a number of years. In your answer, you should:

Identify the tissue in the stem where secondary thickening occurs

Explain how the changes to those tissues in the stem allow the stem of the plant to grow thicker.

7

2

4 6 5 3

Question 3

Study the following diagram of the cross section through a dicot leaf and answer the questions that

follow

a.) Give the main function of this plant organ. (1)

b.) Provide labels for parts 1, 2, 9, 10, 12. (5)

c.) State the number of the region of this plant organ where you find diffusion of gasses? (2)

d.) In which numbered part does photosynthesis mainly take place? (1)

e.) How is the part mentioned in question d structurally adapted to the process of

photosynthesis? (3x2)

f.) Question f refers to gasses. Which gas moves

i. in at number 12 during the day.

ii. out at number 12 during the day (2)

g.) Label parts 3 and 4 and state what is transported by each one. (4)

h.) The part numbered 12 is involved in gaseous exchange. Name THREE gases that are

exchanged through the opening at 12. (3)

Links

Worksheets: http://freepdfdb.com/ppt/plant-organs-worksheet

Worksheets and Diagrams : http://www.biologycorner.com/worksheets/leaf_coloring.html

Diagrams : www.enchantedlearning.com/themes/leaf.shtml