IB Bio HL II - Mrs Kate Ng

Ch 9 Plant Science

9.1 Dicotyledonous plant structure

9.2 Transport in plants

9.3 Sexual reproduction in plants

Dicotyledonous vs monocotyledonous Plant

Phloem

Stem

Root

Xylem

Phloem

Xylem

Dicot: Vascular bundles in the Root

C. Structure of root

Xylem

Phloem

Piliferous layer

Cortex

A young dicotyledonous root

Phloem

Xylem

T.S. of root

Piliferous layer

Cortex

Endodermis

Phloem

Xylem

Enlarged view of T.S. of root

Endodermis

Cortex

Types of root system• There are 2 types of root system in the flowering plants:

1. Fibrous root system, and

2. Tap root system

• Each root has many root hairs close to its root tip.• Root hairs are tiny hair-like projections of the epidermal

cells of the root.• They greatly increase the surface area of roots for

absorption of water and minerals.

Fibrous root system Tap root system

Fig.5 Root systems

Vascular bundles in the Stem

Phloem

Stem

Root

Xylem

Phloem

Xylem

D. Structure of stem

Xylem

Phloem

epidermis

Cambium

Cortex

pith

Lateral meristems = cambium

Fig 9.4 Generalized young dicotyledonous stem (T.S.)

Phloem

Xylem

T.S. of Stem

VascularBundle

Pith

Phloem

Xylem

Enlarged view of T.S. of Stem, showing one vascular bundle

VascularBundle

Cambium

Epidermis

Cortex

Vascular bundles in the Leaf

Phloem

Stem

Root

Xylem

Phloem

Xylem

E. Structure of leaf

Phloem

Xylem

Upper Side

Under Side

Fig 7.5 (pg 112)Structure of a leaf

Lower epidermis

Palisade mesophyll cells

Spongy palisade cells

Upper epidermis

Air spaces

Cuticle

Phloem

Xylem

T.S. of leaf, showing mid-rib & typical leaf regions

Phloem

Xylem

T.S. of leaf, showing typical leaf region

Palisade mesophyll

Upper epidermis

Spongy mesophyll

Xerophytes• Plants that live in dry environment• Adaptations:

Reduced spiky leaves

Water storage tissues

Grow near ground

Flower and grow in the wet season and produce seeds before dry season

Thicken waxy cuticle and lesser stomata

Modifications of leaf, stem and roots

13.2 Transport in Flowering Plants

• There are 2 functions of the transport system:

1. Transport tissues• Important for plants living on land because

tissues in any region of plant are unlikely to have access to all the materials that they need from their environment.

2. Supporting tissues• Large, land living plants may need

additional support which transport tissues provide.

Transport Tissues

• The 2 transport (vascular) tissues of flowering plants are the xylem and phloem.

• Xylem transports water and dissolved materials.– Remember ‘Water in the XYlem – WXY’

• Phloem transports food.– Remember ‘PHloem carries PHood’

Fig.1 Position of Xylem and Phloem in Plant

Xylem

• Xylem is made up of dead tissues consisting mainly of long, tube-like vessels with lignified walls.

• Lignified means that the cellulose cell walls have lignin added to them, this makes them hard and permeable to water. Gives support for the plant.

Xylem• Characteristics:

1. They are long and narrow cells,

2. They are hollow and are without protoplasm (therefore dead cells),

3. The cells are joined end to end, without any cross walls and extend as continuous tubes from roots to stems and into the leaves,

4. The cellulose cell wall is strengthened by lignin.

Xylem

• The xylem tissues have 2 functions:

1. Conducting water, with its dissolved mineral salts, from the roots to the stems and leaves.

2. Providing mechanical support within the plant.

Xylem vessels showing the different patterns of lignification

Phloem

• Phloem is a living tissue consisting mainly of long sieve with cellulose walls.

• The function of the long sieve tubes is to transport food substances.

Phloem

• Characteristics:

1. They have cytoplasm but no nucleus.

2. The cells are joined end to end, and their end walls are perforated (known as sieve plates).

3. Strands of cytoplasm extend through the pores of sieve plates into the next cell.

Some componentsof phloem tissue

Transport of water

• The complete process of transpiration occurs in 3 distinct places:

1. Absorption by roots

2. Movement up the plant

3. Evaporation by the leaves.

1. Absorption by roots

– Roots are important for anchoring the plant in the soil, as well as for

– Absorbing substances from the soil.

1. Absorption by roots

– The absorption of water is by osmosis, which is a passive process.

– The cells of the roots contain relatively concentrated solution compared with water in the soil.

– The result is a concentration gradient across the root, causing water to be drawn towards the central vascular bundle.

1. Absorption by roots

1. Absorption by roots– The absorption of mineral salts can

occur by diffusion if the minerals involved are present in relatively small amount within the plants.

– Active transport allows the plant to accumulate particular minerals above the concentration found in the surrounding soil;

– The energy is derived from respiration.

1. Absorption by roots

– The relatively high concentration of minerals within the root tissues results in root pressure*, which probably helps push water up the plant.

2. Movement up the plant

– Water enters the open-ended xylem vessels in the central part of the root.

– Xylem tissues form an almost continuous system of thin woody (lignified) tubes connecting the roots with the stem and leaves.

2. Movement up the plant

– There are 3 possible ways in which water is made to move up the plant:

i. Root pressure

ii. Capillary action

iii. Transpiration pull (Cohesion Tension Theory)

i. Root Pressure– This arises from active transport into the root

cells. – accounts for less than 25% of the force

necessary to move water through the plant.

ii. Capillary action

– What happens to the suction when the straw has a hole?

– Apply to plants…– Results from water molecules “climbing” the

narrow xylem vessels.– Adhesion occurs between the water and

the lining of the xylem tubes.– These “pulls” water up the plant, although

only for fairly short distant.

iii. Transpiration pull (Cohesion Tension Theory)

– The cells inside the leaf are covered with a film of water. When this water evaporates into the air spaces in the leaf, it diffuses through the stomata into the surrounding air.

– The loss of water from the mesophyll cells pulls in water from the neighbouring cells.

– The tension or pull is created as the column water in the xylem vessels.

iii. Transpiration pull (Cohesion Tension Theory)

– It extends all the way to the roots and pull water up to the top of the plant in a continuous column.

– The flow of water is known as transpiration pull.

– This account for most of the water movement up the plant.

3. Evaporation by the leaves

– Water evaporates from the surfaces of the mesophyll cells in the leaves.

– The water vapour produced escapes to the exterior through specialised pores called stomata.

3. Evaporation by the leaves

– Water evaporates from the surfaces of the mesophyll cells in the leaves.

– The water vapour produced escapes to the exterior through specialised pores called stomata.

– Abscisic acid stimulates guard cells to close in response to water loss

Factors affecting the Rate of Transpiration

• Transpiration is dependent upon evaporation.

• Any factor that affects the rate of evaporation of water will affect the rate of transpiration.

External factors that influence the rate of transpiration are:

1. Humidity of the air– The more humid the air, the slower the

rate of transpiration.

2. Temperature of the air– A rise in temperature of the surroundings

increases the rate of evaporation, thus the rate of transpiration is greater.

External factors that influence the rate of transpiration are:

3. Strong wind– The stronger the wind, the higher the rate of

transpiration.

4. Light– Light affects the size of stomata. Presence of

light, transpiration rate increases.

5. Water availability• Shortage of water will cause the closing of the

stomata

IB Bio HL II - Ms Kate Loke

13.3 Sexual Reproduction in Flowering Plants

• Involves fusion of gametes

• Takes place in sex organs of flowers

• Involves pollination and fertilization

• Flower Fruit Seed Reproduction of new plant

General Parts of a Flower

Parts of flower Function

Sepals

Petals

Stamens – composed of a filament & anther

Carpels – composed of ovary, style & stigma

Pollination

• Refers to the transfer of pollen from an anther to a stigma

• 2 types of pollination:– Self pollination– Cross pollination

Self-pollination

• Occurs when pollen from the anthers of a flower is transferred onto the stigma of the same flower or another flower on the same plant

Cross-pollination

• Occurs when the pollen from the anthers of a flower is transferred onto the stigma of another plant

Insect vs Wind pollination

Insect vs Wind-Pollinated flowers

Features

Insect-pollinated

Wind-pollinated

Petals Large, conspicuous, bright coloured

Small or absent

Nectar May be present Absent

Scent May be present Absent

Anther Small, enclosed within flower

Large, hanging outside flower

Insect vs Wind-Pollinated flowers

Features

Insect-pollinated

Wind-pollinated

Filament Short, rigid Long, flexible

Pollen grains

Sticky, rough & relatively large. Adhere to insect body

Smooth, light & relatively small. Produced in large quantities, to offset losses.

Stigma Relatively small, enclosed within flower

Feathery, large S.A. exposed on outside of flower to collect pollen

Advantages of insects as pollinators

Insects are carrier of pollen as they are:• Small sized to enable them to gain

entry into the flowers to obtain nectar.• Highly mobile – many are winged or

very industrious which enable them to visit a large number of flowers.

• Numerous in number.• In a large variety of species.

Dispersal of Fruits & Seeds

• What are the advantages of dispersal methods?

– Reduces the chances of inbreeding

– Prevents overcrowding and competition for light and water with parent plant

– Decrease vulnerability to epidemic attacks of diseases

– Ensure variability

– Enable plants to colonise new and favourable habitats

• There are 4 types of dispersal methods:

1.Dispersal by wind

2.Dispersal by water

3.Dispersal by animals

4.Dispersal by self explosive mechanism

Dispersal Methods

Seed vs Fruit• How to tell from a seed and a fruit?• A fruit has

– One or more seeds, and– It has 2 scars:

• one where it was attached to the plant & • one where the style and stigma were

attached to

• A seed has– Only 1 scar

• where it was joined onto the fruit.

Structure of a seed

Structure of a typical seed

Feature Functions

Testa

Hilum

Micropyle

Plumule

Radicle

Structure of a typical seed

Feature Functions

Cotyledon

Seed Germination

Conditions required during germination

External environmental conditions necessary for germination:

• Sufficient water– With water, cotyledons produce enzymes

• Suitable temperature– Required for enzyme activity

• Adequate oxygen supply– For respiration

Metabolic events in Germination

• What is Germination?– It is the onset of growth of the embryo.

Changes during germination:

1. Seeds absorb water and swells, 2. Seed coat burst3. Water activates gibberelline, a hormone needed

for breaking the dormancy of the seed.

4. Gibberelline activates amylase whic to digest stored food into

5. Soluble products of digestion (glucose & amino acids) are transported to the growth regions of the embryos (i.e. plumule and radicle).

4. Glucose is used for the synthesis of cellulose and other cell wall materials

5. Amino acids are used for protein synthesis as components of protoplasm

6. The dry mass decreases at first because of tissue respiration to provide energy for growth.

7. This loss will continue until the seedling produces green leaves and starts to photosynthesise and make its own food.

Changes during Germination