TOPIC 9: Plant Science
9.1 Plant Structure and Growth
9.1.1: Draw and label plan diagrams to show tissues in the stem and leaf of a dicotyledonous plant
9.1.2:Outline three differences between the structures of dicotyledonous and monocotyledonous
plants
Monocots
Dicots
Number of cotyledons
Leaf veins
Vascular bundles in stem
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Parallel
Spread throughout
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Branching
Arranged in a ring
9.1.3: Explain the relationship between the distribution and function of leaf tissues
Leaf Tissues
Function
The epidermis is an outer support tissue that holds the leaf together. The
Epidermis
upper epidermis is exposed to direct sunlight so it has a waxy coating
(called the waxy cuticle) to prevent water loss. The lower epidermis has
specialized guard cells. Guard cells form adjustable pores that control the
rate of transpiration (water loss) as well as gas exchange.
The center of a leaf is composed of mesophyll, which has 2 layers:
Palisade mesophyll
palisade mesophyll and spongy mesophyll. Palisade mesophyll is an upper
layer of elongated cells that contain many chloroplasts to absorb light and
carry out photosynthesis.
The spongy mesophyll is a lower layer containing loosely packed cells. The
Spongy mesophyll
loose arrangement of cells allows water, O2 and CO2 to diffuse easily.
Rapid diffusion is necessary for transporting CO2 for the light
independent reactions of photosynthesis.
Leaves contain vascular bundles in the spongy mesophyll layer. Vascular
Vascular tissue
bundles contain xylem tissue and phloem tissue. Water moves through
xylem tubes and dissolved glucose moves through phloem tubes. The
phloem transports the products of photosynthesis to other parts of the
plant.
9.1.4: Identify modifications of roots, stems and leaves for different functions
 Stem tubers are produced by the stems of some plants. They are used by plants to over-winter and
re-grow the next year and as a means of asexual reproduction. Example: ceropegia.
 A root tuber is an enlarged root that functions as a storage organ. Root tubers store nutrients over
periods when the plant can not actively grow, thus permitting survival from one year to the next.
Example: sweet potatoes.
 A bulb is an underground shoot that has modified leaves that are used as food storage organs by a
dormant plant. A bulb contains food reserves to enable the plant to survive adverse conditions.
Roots emerge from the underside of the base, and new stems and leaves from the upper side.
Example: Onions.
 A tendril is a specialized stem or leaf with a threadlike shape that is used by climbing plants for
support and attachment, generally by twining around whatever it touches. Example: garden peas.
9.1.5: State that dicotlydenous plants have apical and lateral meristems
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9.1.6: Compare growth due to apical and lateral meristems in dicotyledonous plants
 Differentiated plant cells cannot divide or produce cells of a different type.
 Therefore, plant growth results from ongoing mitosis in specialized regions of undifferentiated cells.
 The regions of undifferentiated cells in plants are known as meristems.
 There are two types of meristems in plants: apical meristems and lateral meristems.
Apical meristems
Lateral meristems
Occur in the tips of stems and roots
Occur between xylem and phloem in stems
Produces soft tissues
Produces hard xylem tissue: wood
Lengthens roots and stems
Widens stems to support the weight of tall plants
Allows plants to develop special structures like
Allows trees to grow tall, helping them to compete
leaves and flowers
effectively for light
Found in all phyla of plants
Absent in mosses and horsetails
9.1.7: Explain the role of auxin in phototropism as an example of the control of plant growth
 Plants use hormones to control the
growth of roots and stems. When a plant
releases a growth hormone in response
to an external stimulus we call the
resulting directional growth a tropism.
 One type of tropism is phototropism:
growth in response to light. Phototropism
may be either positive (towards the light)
or negative (away from the light).
 Phototropism requires the absorption of
light by proteins known as phototropins.
Phototropins change to a new
conformation (a new shape) when they
absorb certain wavelengths of light. The
new shape causes phototropins to act as
‘on switches’ for a gene that regulates the activity of auxins.
 Auxins cause cells to become longer. Therefore, by releasing auxins on one side of a stem but not on
the other side, a stem will bend because one side becomes longer than the other.
 When a stem detects directional light it moves auxins from its sunny side to its shady side, which
causes the shady side to bend toward the light. Bending toward light allows plants to absorb more
sunlight and be able to photosynthesize at a faster rate.
 Auxins cause cells to become larger in the following way: 1) they cause cells to actively transport
hydrogen ions out of the cell, making the outside acidic; 2) the acid outside the cell makes the cell
wall softer; 3) softer cell walls make the cells more stretchable; and 4) stretchy cells are bigger
because the internal pressure inside the cell causes the cell wall to bulge out.
9.2 Transport in angiospermophytes
9.2.1: Outline how the root system provides a large surface area for mineral ion and water uptake
 The root is the organ of a plant that typically lies below the surface of the soil.
 The two major functions of roots are 1) absorption of water and nutrients and 2) anchoring the
plant to the ground.
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As roots grow they normally branch out. Repeated branching results in an extensive fibrous network
that greatly increases surface area.
Most roots also develop hairs that further increase surface area for absorption of water and
minerals.
Plants absorb water at the root hairs by osmosis and take up mineral ions by active transport.
9.2.2: List ways in which mineral ions in the soil move to the root
 Mineral ions move through soil to the roots of plants by one of three means: 1) diffusion
2) mass flow of water in the soil carrying ions
3) mutualistic association with hyphae of fungi
9.2.3: Explain the process of mineral ion absorption from the soil into roots by active transport
 Plants take up mineral ions by active transport.In active transport, mineral ions are moved against a
concentration gradient, which requires: 1) numerous mitochondria in root hair cells for ATP
production; 2) protein channels in the cell membrane for active transport; and 3) oxygen in the soil
that is absorbed by root hairs for cell respiration.
 In order for mineral ions to be pumped into the roots the mineral ions must make physical contact
with protein pumps on cell membranes of root hair cells.The mineral ions move into contact with
root hair proteins in one of two ways: 1) diffusion and 2) mass flow. Mass flow is when draining
water carries minerals.
 Diffusion and mass flow are slow processes because mineral ions bind to the surface of soil particles.
Therefore many plants evolved mutualistic relationships with fungi to improve the rate of mineral
absorption. The long thread-like hyphae of the fungus intertwine with the root hairs of the plant,
and extend into the soil. Hyphae are highly efficient at absorbing mineral ions from the soil, which
they share with the plant roots in exchange for sugars. The photo below shows the thread-like
hyphae of a fungus growing amongst a plant's root hairs
9.2.4: State that terrestrial plants support themselves with cellulose/cell turgor/lignified xylem
9.2.5: Define transpiration
Transpiration is the loss of water (by evaporation) from the leaves and stems of plants.
9.2.6: Explain how water is carried by the transpiration stream
 The xylem is a system of long hollow tubes responsible for replacing water lost during transpiration
and photosynthesis. The xylem is made of two kinds of cells: tracheids and vessels. Xylem cells die
before they are functional: after they die they become long, narrow tubes with pores at each end
that allow water to pass through them.
 The xylem sap moves from roots, through the stem, to the leaves without any energy being spent by
the plant.
 Three processes cause water to rise up the xylem tube:
 Root pressure: Water moves into roots by osmosis because the roots have high concentrations of
solute. This causes a positive pressure that forces sap up the xylem towards the leaves. Root
pressure is highest in the morning before the stomata open and allow transpiration to begin.
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Capillary action: The xylem is a long tube that is microscopically thin. When water molecules contact
the surface of the xylem there is adhesion. Adhesion tends to pull water molecules upward by a
process called capillary action.
Transpiration pull: When water molecules evaporate from leaves the water potential drops at the
stomata. The low pressure then pulls new water molecules towards the stomata from the xylem
vessels. As these water molecules move they pull on water molecules behind them due to cohesion
(caused by hydrogen bonding). The pull is transmitted from one water molecule to the next, all the
way to the roots.
9.2.7: State that guard cells can regulate transpiration by opening and closing stomata
9.2.8: State that the plant hormone abscisic acid causes the closing of the stomata
9.2.9: Explain how abiotic factors affect the rate of transpiration in a typical terrestrial plant
 Transpiration is the loss of water (by evaporation) from the leaves and stems of plants. In a typical
terrestrial mesophytic plant, the rate of transpiration:
 Decreases when humidity increases because at high humidity, the air is saturated with water so
evaporation stops.
 Increases when light intensity increases because more stomata open in strong light to maximize the
rate of photosynthesis.
 Increases when temperature increases because water molecules evaporate faster creating negative
pressure at the stomata.
 Increases as wind speed increases because air movement carries water vapor away from stomata
creating negative pressure at the stomata.
9.2.10: Outline four adaptations of xerophytes that help to reduce transpiration
 Xerophytes are plants that are adapted for growing in dry habitats. Xerophytes can have any of the
following adaptations:
 Leaves: reduced in size and surface area to reduce water loss – the spines of cacti are reduced
leaves.
 Cuticles: thick to prevent water loss by evaporation – cacti have thick waxy cuticles.
 Stomata: opened only at night to reduce the amount of water lost by transpiration. Cacti do this and
it requires absorbing CO2 at night and storing it as malic acid until sunlight returns.
 Roots – deep and/or spreading to maximize the absorption of underground water.
9.2.11: Outline the role of phloem in active translocation of sugars (sucrose) and amino acids
 Translocation is the movement of substances (like amino acids and sugars) from one part of a plant
to another.
 Translocation is achieved by phloem tissue, which links plant parts that need sugars or amino acids
to other parts that have a surplus.
 Plant parts that provide the phloem with substances are called ‘sources’. Examples of sources are:
leaves and green stems that produce sugar during photosynthesis, and tubers that release sugars in
spring to provide energy for growing shoots.
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Plant parts that receive substances from the phloem are called ‘sinks’. Examples of sinks include:
developing fruits that receive amino acids for growth and sugars for ripening; developing seeds that
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receive sugars to build starch; leaves that receive amino acids for growth; and roots that receive
sugars to produce tubers.
Sometimes sinks turn into sources so the phloem needs to be able to move materials in both
directions. The phloem controls the flow of fluid by creating pressure gradients with active
transport.
9.3 Reproduction in angiospermophytes
9.3.1: Draw and label a diagram showing the structure of a dicotyledonous animal-pollinated flower
9.3.2: Distinguish between pollination, fertilization and seed dispersal
 Pollination is the attachment of a pollen grain on the stigma of a flower, by wind or by an animal.
After pollination, a pollen grain grows a long pollen tube that stretches down the style to the ovary.
The pollen tube enters the ovary through a small opening, the micropyle, and releases sperm to
fertilize the eggs.
 Fertilization is the fusion of an egg and sperm to form a zygote. A zygote develops into an embryo,
and in flowering plants the embryo is packaged in a seed coat with food reserves.
 Mature seeds typically spread out from their parent plant, a process called seed dispersal. Plants
have evolved diverse methods of seed dispersal: some seed pods are explosive, some seeds are
attached to sails that blow in the wind, some seeds spin like helicopter blades, and others rely on
birds to eat them and spread them in their droppings.
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9.3.3: Draw and label a diagram showing the external and internal structure of a named
dicotyledonous
seed
Phaseolus vulgaris
9.3.4: Explain the conditions needed for the germination of a typical seed
 Mature seeds are dormant with very few metabolic
processes going on.
 The resumption of growth in a seed is called germination.
 Seeds need oxygen and water to germinate. Without
water, enzymes are not activated and without oxygen,
cellular respiration isn't possible.
 Warmth is also important for germination because the
enzymes involved in growth are more active at warmer
temperatures.
 Germination is usually triggered by a change in the environment; e.g., warmer temperature, wetter
soil, erosion of the seed coat by fire, etc.
9.3.5: Outline the metabolic processes during germination of a starchy seed
 Mature seeds are dormant with very few metabolic processes going on; the resumption of growth in
a seed is called germination.
 Seeds need oxygen and water to germinate. Without water, enzymes are not activated and without
oxygen, cellular respiration isn't possible.
 Warmth is also important for germination because the enzymes involved in growth are more active
at warmer temperatures.
 The first step in the process of germination is the absorption of water, sometimes as much as the
mass of the seed itself.
 The presence of water activates the hormone gibberellin, which in turn stimulates the production of
amylase.
 Amylase catalyses the breakdown of starch to maltose, and the maltose diffuses to the embryo to
supply energy for ATP production.
9.3.6: Explain how flowering is controlled in long-day and short-day plants, including the role of
phytochrome
Phytochrome
 Phytochrome is a protein pigment found in most plants. It acts as a photoreceptor, which means it
detects light.
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When PR absorbs red light (660 nm) it gets converted into PFR; and when PFR absorbs far-red light
(730 nm) it switches back to PR.
Switching to PR from PFR is a rapid process; but changing from PFR to PR is a slower process. Thus,
in sunlight, PR is quickly converted into PFR; but at night PFR is slowly converted into PR.
This difference in conversion rates means that PFR levels are highest in plants at the peak of
summer, when day-length is greatest. Thus plants can use PFR levels like a calendar, to determine
the date of mid-summer.
Long-day plants
 Plants that start to flower in mid-summer are called long-day plants. Long-day plants use PFR to
trigger the flowering process.
 In mid-summer, nights are too short to convert all of the PFR into PR. This results in many PFR
proteins becoming bound to receptor proteins, which in turn ‘turn on’ genes that produce flowers.
Short-day plants
 Plants that start to flower in autumn are called short-day plants. Short-day plants use PFR to inhibit
the flowering process.
 Thus, near mid-summer, the protein receptors of short-day plants act to ‘turn off’ the flowerproducing genes. By autumn, day-length is short so PFR levels drop too low to inhibit flowering, thus
flowering begins.
Extension question
How and why would natural selection cause different plants species to flower at different times?
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