Plant Science
Introduction - Basic Plant Structure
Assessment Statement
9.1.1
9.1.2
9.1.3
9.1.4
9.1.5
9.1.6
9.1.7
Draw and label plan diagrams to show the distribution of tissues in the
stem and leaf of a dicotyledonous plant
Outline three differences between the structures of dicotyledonous and
monocotyledonous plants
Explain the relationship between the distribution of tissues in the leaf and
the functions of these tissues
Identify modifications of roots, stems, and leaves for different functions:
bulbs, tubers, storage roots and tendrils
State that dicotyledonous plants have apical and lateral meristems
Compare the growth due to apical and lateral meristems in dicotyledonous
plants
Explain the role of auxin in phototropism as an example of the control of
plant growth
Green plants make up one of the five kingdoms of living things. The kingdom is
Plantae.
Plants are different than other organisms because:

Each has a cell wall around each cell made of cellulose, a polysaccharide, which
is an extremely tough and protective material.

Cell organelles called chloroplasts are the site of photosynthesis, which the plant
takes carbon dioxide and water and produces organic molecules (carbohydrate)
and oxygen.
It is thought that plants evolved from aquatic, single-celled organisms called green algae.
Today, the dominant terrestrial plants are the angiospermophytes (flowering plants).
Fossil records indicate that the angiosperms took over about 100 mya.
Angiosperms, or flowering plants, produce seeds enclosed inside fruits. Angiosperm
comes from the word angerion – a container and sperma – a seed and phyton – a plant.
Angiosperms are divided into two large groups:
1.
Monocotyledons – include palms, gingers, lilies, irises, grasses, orchids, palms,
bananas, and others
2.
Dicotyledons – include most trees and shrubs, and many non-woody plants.
These names refer to the number of leaves contained in the embryo, called cotyledons.
The differences between the two are listed below:
Number of Cotyledons
Numbers of Organs in the
Flowers
Meristems
Leaf Attachment
Method of Formation of
new roots
Vascular Tissue in the Stem
Leaf Veins
Monocots
1
3 or 6
Dicots
2
4 or 5
Apical only – stems cannot
grow wider
Around the whole stem
circumference
By formation of roots from
the stem
Vascular Bundles spread
throughout
Parallel
Apical and lateral so stems
can widen
To one side of the stem by a
leaf stalk
By branching from other
roots
Branching to form a
network
Branching to form a
network
Draw the typical forms of each.
Basic Plant Structure and Growth
Whether the plant (angiosperm) is woody or not (herbaceous), the plant consists of a
stem, leaves and root.
Plants retain groups of stem cells throughout their lives, allowing them to continue to
grow indefinitely. These groups of cells are called ________. Cells in meristems are
small and go through the cell cycle repeatedly to produce more cells, by mitosis and
cytokinesis. These new cells absorb nutrients and water and increase in volume and
mass.
Primary meristems are found at the tips of stems and roots. They are called apical
meristems. The root apical meristem is responsible for the growth of the root. The
shoot apical meristem, at the tip of the stem, is more complex. It throws off the cells that
are needed for the growth of the stem and also produces the groups of cells that grow and
develop into leaves.
Meristems
Plants grow throughout their lifetime. This pattern of growth is called indeterminate.
Plants, of course, will die, based upon their life cycle. Annuals complete their life cycle
in one year, whereas perennials live many years and usually die due to a disease or
environmental factor.
Indeterminate growth is due to the meristematic tissue. In dicots, there are apical and
lateral, which have been mentioned before.
Apical Meristems
Sometimes referred to as primary meristems, these occur at the tips of roots and stems.
Primary tissue is produced and causes primary growth. In the root this allows the
structure to extend through the soil. In the stem, it allows the stem to grow longer and
increases the exposure to light and carbon dioxide. This type of growth results in
herbaceous, non-woody stems and roots.
Lateral Meristems
Lateral meristems allow growth in thickness of plants. This is referred to as secondary
growth. Most trees and shrubs have active lateral meristems. These plants have two
types of lateral meristems:

Vascular Cambium produces secondary vascular tissue. It lies between the xylem
and phloem in the vascular bundles. On the inside is produces secondary xylem,
which is a major component of wood. One the outside it produces secondary
phloem.

Cork cambium occurs within the bark of a plant and produces the cork cells of the
outer bark.
Make a table to compare Apical and Lateral Meristems
Stems
The stem supports the leaves in the sunlight, and transports organic materials (such as
sugar and amino acids), ions and water between the roots and leaves. As the plant
grows, the stem has to grow larger. Terrestrial plants support themselves:

With turgid cells, which are almost rigid because of their high pressure

With cells that have thickened cellulose walls

With xylem tissue, which has cells walls impregnated with lignin, making it
woody and hard.
The stem is surrounded or contained by a layer called the epidermis. Inside is vascular
tissue, which is composed of a system of veins or vascular bundles. The bundles are
made of xylem that transports water and phloem that transports organic solutes. In the
stem, the vascular bundles are arranged in a ring, towards the outside of the stem. The
vascular the tissue extends into the leaf with some specialized leaf tissues.
Besides the apical meristems at the tips, the stems of dicots can grow wider and the plant
grows taller. This is done by a lateral meristem, which develops in a complete circle
around the stem. The lateral meristem develops between two tissues called xylem and
phloem, and the cells produced develop into both of these tissues. Xylem develops on
the inner side of the lateral meristem, gradually adding to the width of the stem.
The plants that produce the largest amount of xylem can grow the tallest and compete for
light the most. Naturally, we refer to these as trees.
Monocots have stems with an apical meristem at the tip of the stem, but the tissues of the
stem differentiate in a different arrangement from those of dicots.
Draw the Plan Diagram of the Stem
Leaf Structure
A leaf consists of a leaf blade connected to the stem by a leaf stalk. It grows from the
apical meristem at the tips of the stem. Leaves must have a large surface area and a small
space between the upper and lower surfaces. The leaf is an organ specialized for
photosynthesis.
The leaf is composed of:
1.
The outer structure - Epidermis
It is a tough, transparent layer. The upper epidermis has an external waxy cuticle
on its surface. The cuticle is impervious and effectively reduces water vapour
loss from the leaf surface. The lower epidermis has a thinner cuticle, with
specialized epidermal cells called guard cells and pores called stomata (stoma).
They are the sites of inward diffusion of carbon dioxide. Within the inner
structures are air spaces that enhance the diffusion of carbon dioxide to the cells.
2.
Inner part of the Leaf
The inner part of the cell consists of mesophyll cells that are spread out over a
wide area, so the amount of light absorbed is maximized.
In the upper layer, the mesophyll cells are called palisade mesophyll cells and are
tightly packed close to the upper surface of the epidermis. They are packed with
chloroplasts.
In the lower half, there are spongy mesophyll cells. These are loosely packed in
the lower portion of the leaf, with large air spaces in between and fewer
chloroplasts.
Between the mesophyll cells lies a network of vascular bundles or the veins.
The vascular bundles contain xylem and phloem. This brings water (xylem) to
the cells and transports the sugars made (phloem).
The network of bundles has another function; to support the leaf. Mesophyll
tissue is supported by the turgidity of all its cells, contained in the non-elastic
epidermis and reinforced by the network of bundles.
Draw the Plan Diagram of a Leaf
Root Systems
One of the earliest stages in the development of an embryo plant is the formation of a
root. When seeds germinate, the embryonic root grows out of the seed coat and
downwards into the soil. The root anchors the plant and is the site of absorption of water
and ions from the soil. These roots go deep into the soil, and are called tap roots. When
the roots branch out to the side, to search for water outside their canopy, these are called
lateral roots.
The seedling root develops branches in some plants and hairs, which absorb more water
in dry soil. The branching helps with absorption by increasing the surface area. The
water flows in by osmosis.
Roots have an epidermis that forms a protective outer layer. The inner layer is called the
cortex. This is used for conducting water and can be modified to store nutrients. There is
an endodermis that surrounds the vascular layer that contains xylem and phloem.
Draw Patterns of Root Branching and tissue plan diagram
Modifications and Adaptations of roots, stems and leaves.
The structures of the root, leaf and stem are very similar, with some differences. As the
principle plant organs, they are used to build the structures that they need for water and
mineral absorption, support and photosynthesis.
Other functions can be carried out if the principle structures are modified.
Roots
Roots can be modified, based upon the needs of the plant. Besides tap roots and lateral
(fibrous) roots there are:

Prop Roots – thick adventitious roots that grow from the lower part of the stem
and brace the plant. Ex. Corn

Storage Roots – specialized cells within the roots store large quantities of
carbohydrates and water. Ex. Carrots and Beets.

Pneumatophores – produce by plants that live in wet places, these roots extend
above the soil or water surface and facilitate oxygen uptake. Ex. Mangroves and
cypress trees.

Buttress Roots – large roots that develop near the bottom of trees to provide
stability. Ex. Fig
Stems
Stems are capable of great modification.

Bulbs – vertical, underground stems consisting of enlarged bases of leaves that
store food. Ex. Onions

Tubers – horizontally growing stems below ground that are modified as
carbohydrate- storage structures. Ex. Potatoes

Rhizomes – horizontal stems that grow just below the surface to allow the plant
to spread. Ex. Ginger plant

Stolons – horizontal stems growing above the ground that allow a plant to
reproduce asexually. Ex. Strawberry plant
Leaves
They still function as photosynthesizing factories, but can adapt.
Tendrils – structures that coil around objects to aid in support and climbing. Ex. Pea
plants
Reproductive Leaves – produce tiny plants along the leaf margins that fall to the ground
and take root in the soil. Ex. Kalanchoe plants
Bracts or floral leaves – coloured modified leaves that surround flowers and attract
insects for pollination. Ex. Poinsettia
Spines – reduce water loss, may be associated with modified stems that carry out
photosynthesis. Ex. Cacti
Know at least one example of each.
Growth and Phototropism
Plants use hormones to control the growth of stems and roots. Rate and direction are
controlled. Both light and gravity influence growth. In light they grow towards the
brightest source and in the dark, grow in the opposite direction to gravity. The responses
are called tropisms. The growth towards light is called phototropism.
The steps of phototropism are:



Photoreceptors (proteins called phototropins) absorb light.
Light of an appropriate wavelength is absorbed which stimulates the
photoreceptors to bind in the cells and transcribe glycoproteins which will aid in
the transport of a plant hormone where it is needed. The stimulus of the light will
also transcribe a protein hormone to be produced called auxin.
Auxin does many things in the plant:
o Auxin synthesized in the tips of growing stems and transported down to
stimulate growth.
o It promotes the elongation of cells in stems, by loosening the connections
between the cell walls and cellulose microfibrils
o If phototropins detect a greater light intensity on one side of the stem than
the other, auxin is transported to the shadier side. This promotes the stem
to grow more on the shadier side and go towards the light. This allows the
leaves on the sunny side to get more light and photosynthesize at a greater
rate.
Do the worksheet on Phototropism
Transport in Angiosperms
Assessment Statement
9.2.1
9.2.2
9.2.3
9.2.4
9.2.5
9.2.6
9.2.7
9.2.8
9.2.9
9.2.10
9.2.11
Outline how the root system provides a large surface area for mineral ion
and water uptake by means of branching and root hairs
List ways in which mineral ions in the soil move to the root
Explain the process of mineral ion absorption from the soil into rots by
active transport
State that terrestrial plants support themselves by means of thickened
cellulose, cell tugor and lignified xylem
Define transpiration
Explain how water is carried by the transpiration stream, including the
structure of xylem vessels, transpiration pull, cohesion, adhesion, and
evaporation
State that guard cell can regulate transpiration by opening and closing of
stomata
State that the plant hormone abscisic acid causes the closing of stomata
Explain how the abiotic factors light, temperature, wind and humidity,
affect the rate of transpiration in a typical terrestrial plant
Outline four adaptations of xerophytes that help to reduce transpiration
Outline the role of phloem in active translocation of sugars (sucrose) and
amino acids from source (photosynthetic tissue and storage organs) to sink
(fruit, seeds, roots)
Root system, absorption and uptake
The roots provide a huge surface area to draw up essential ions and water. Contact with
the soil is vastly increased by root hairs that occur just behind the growing tip of each
root. The water flows in due to osmosis. The uptake of minerals is done via active
transport.
1.
Water uptake
Water uptake occurs from the soil in contact with the root hairs by osmosis.
Uptake is largely by mass flow through the interconnecting free spaces in the
cellulose cell walls to the xylem. There are three possible routes of water
movement through the plant cells and tissues. The cortex structure of the root
also facilitates the water uptake.

Apoplast Pathway (Mass Flow)

Water does not enter the cell. It moves through the cell walls until it
reached the endodermis. Cells of the endodermis have a Casparian Strip
around them that is impermeable to water. The water is diverted to the
cytoplasm of cells (momentarily), and then eventually to the xylem. The
Casparian Strip is thought to be a measure of control so water cannot
move directly into the xylem, so the cell can divert it out of the cell, if it is
contaminated.

Symplast Pathway
o Water enters the cytoplasm but not the vacuole. It passes from cell
to cell via connections between cellular cytoplasm of adjacent
cells, called plasmodesmata. The organelles are packed together
in cells, and as a result, block significant progress of water. It is
not the major pathway for water. Minerals mainly move
through this pathway.

Vacuolar Pathway
o Water enters the cell and move into the vacuole. It then travels
through the cytoplasm and the cell wall to the next cell.
Once in the endodermis, water can move into the xylem and pulled via transpiration
forces.
2.
Uptake of Minerals
There are three ways that minerals can move from the soil to the root.
1.
2.
3.
Active Transport using protein pumps in the plasma membranes of root
cells.
Mass Flow, or the water carrying the ions goes into the root.
Fungal hyphae – fungus grows on the surface of roots and sometimes
into the cells of the roots. The hyphae grow into the soil and absorb
minerals ions and phosphate from the surface of soil particles. The ions
are supplied to the roots and allow the plant to grow in mineral poor soil.
Some plants supply sugars, and other nutrients to the fungus. This is an
example of a mutualistic relationship.
Transpiration
Find the definition of Transpiration
Transport up the stem
Water is taken up the stem, from the roots by the Mass Flow or Apoplast Pathway, into
the xylem.
In the root, the xylem is centrally located, in the stem, the xylem occurs in the ring of
vascular tissue. Xylem, therefore runs from the roots to stem and then to leaf.
Xylem begins as elongated cells with cellulose walls and living contents, connected end
to end. During development, the end walls dissolve away, and the mature xylem is a long
hollow tube. As the tubes extend, the tissue dies. Therefore, xylem is dead tissue, and is
composed of two elements:

Tracheids – narrow cells arranged in columns, overlapping at tapered ends giving
some support to the plant. The ends have pits for water to move rapidly from one
cell to the next. Due to their structure, they are less efficient that xylem vessels.

Xylem vessels – larger columns of cells. When the dead cell walls disappear,
they become wider and transport water more efficiently. Since they are so wide,
they are reinforced by ligin for support of the plant. This only occurs in
angiosperms.
Mechanism of water transport
Transpiration controls the flow of water. But, how does water move against gravity?
First, in the leaf are stomata. These open and close to control the amount of water
present in the leaf. Around each stomata are guard cells, which open and close,
depending on the tugor of the cells.
When the plant is well hydrated, the guard cells are swollen, causing them to open, due
to the pressure on the cells walls. When the plant dries out, the guard cells sag and the
stomata close. Water loss is stopped and gas exchange is halted.
Other external factors that affect the opening and closing of stomata are:



Light causes stomata to open
Low CO2 levels in the air spaces in the cause the stomata to open
Shortage of water causes the stomata to close
When leaves are deficient in water, they synthesize a hormone called abscisic acid. This
closes the stomata and overrides any external stimuli – the stomata close. This allows the
plant to avoid dehydration and death.
When the stomata are open, water evaporates out of the leaves, diffusing water vapour
out. This maintains a concentration gradient that requires more water.
The water in the stem, which is connected by xylem, moves up to replace the water lost
by transpiration. As a result, the water is pulled up the plant. Water is polar and is held
together by its cohesive forces. Therefore, the water molecules stick together and
flow up the stem together. This is called transpiration pull.
Factors affecting Transpiration
The rate of transpiration is the amount of water vapour that a plant loses from its leaves
and stems per unit of time. The rate depends on the:




Size of the plant
The thickness of the cuticle
How widely spaced the stomata are
Whether the stomata are open or closed
These are the biotic factors. These are the factors the plant can control, based upon the
species of plant. There are four abiotic factors that affect the rate of transpiration.
1.
Temperature – affects the rate at which water evaporates from the surfaces inside
the leaf. At higher temperatures, evaporation increases. The higher temperatures
also increase the rate of diffusion between the inside of the leaf and the outside.
The increase in temperature reduces the air’s ability to hold more moisture and
reduces the relative humidity of air outside the leaf. The concentration gradient
increases, doubling the rate for every 10oC increase in temperature
2.
Humidity – is the water vapour content of the air. It is measured as a percentage
of the maximum amount of water vapour the air can hold. The humidity inside
the leaf is always around 100%. The lower humidity outside the leaf causes the
evaporation to be higher, increasing transpiration. Evaporation is much higher in
dry air, so on hot / cold, dry days, transpiration increases dramatically
3.
Wind – air currents take water vapour away from the leaf surface, keeping the
concentration gradient large, and increasing the rate of transpiration. On calm
days, the humidity around the leaf increases, slowing down transpiration.
4.
Light – in light, photosynthesis increases and stoma open to allow CO2 in, but
also water out. Therefore, transpiration increases. In darkness, there is no need
to absorb CO2, and water can be conserved, as the stoma close and
photosynthesis decreases.
What else does water do?
The take up of water, allows plants to grow very tall.
Plants do not have a skeleton. Woody trees and shrubs have xylem to support them.
Herbaceous plants have xylem, but depend on water to provide turgor pressure. Turgor
Pressure is the swelling of cells with water. As the water is absorbed into the cells, from
the xylem, the vacuole takes in water. As a result, the vacuole swells until its membrane
presses against the cell wall. The pressure provides that stiffness, or crispness, that we
see in vegetables, like celery.
Moving Food in Plants
Find the definition of Translocation is the movement of manufactured food (sugars and
amino acids). This occurs in the phloem tissues of the vascular bundles.
Sugars are made in the leaves (in the light) by photosynthesis and transported as sucrose.
The first formed leaves transport sugars to sites of new growth (new stems, new leaves,
and new roots). In older plants, sucrose is increasingly transported to sites or storage,
such as the cortex of roots or stems, and in seeds and fruits.
Amino acids are mostly made in the root tips. Here is where the absorption of nitrates
takes place. After they are made, amino acids are transported to sites where protein
synthesis is occurring. These are mostly in buds, young leaves and young roots, and in
developing fruits.
Translocation is not just limited to organic compounds. Chemicals that are applied to
plants by spraying, and are then absorbed by the leaves may be carried all over the
organism. Pesticides are called systemic for this reason.
Phloem tissue consists of sieve tubes and companion cells.
Sieve tubes are narrow, elongated elements connected end to end to form tubes. The end
walls, know as sieve plates, are perforated by pores. The cytoplasm of mature sieve tubes
has no nuclei, or many of the other organelles in a cell. But each sieve tube is connected
to a companion cell by strands of cytoplasm passing through gaps (called pits) in the
walls. The companion cells are believed to service and maintain the cytoplasm of the
sieve tube, which has lost its nucleus.
Phloem is a living tissue, and has a relatively high rate of aerobic respiration during
transport. In fact, transport of manufactured food in the phloem is an active process,
using energy from metabolism.
Phloem transport may occur in either direction in stem leaves and roots, and is
believed to move by mass flow. How this works is:

Solutes are loaded into the phloem sieve tubes, requiring ATP and then the solutes
flow through the phloem from a region of high hydrostatic pressure to low
hydrostatic pressure.

Hydrostatic pressure is high around photosynthetic cells in the light (mesophyll of
the leaf), and in the phloem sieve tubes nearby. It is the presence of the sugars,
which concentrate the fluid and creates a high osmotic pressure. Water flows in,
raising the hydrostatic pressure further. This is called a source area.

Hydrostatic pressure is low in cells where sugar is converted to starch and stored.
Areas of storage are the cortex of the root, stem, seeds and in the nearby phloem
tissue. Here the removal of sugars lowers the osmotic pressure, and water flows
away. These storage areas are called sink areas.
Below summarizes the sources (areas where sugars and amino acids are loaded into the
phloem) and sinks (where the sugars and amino acids are unloaded and used).
Sources
Photosynthetic tissues:
 Mature green leaves
 Green stems
Storage organs that are unloading their
stores:
 Storage tissues in germinating seeds
 Tap roots or tubers at the start of
the growth season
Sinks
Roots that are growing or absorbing
mineral ions using energy from cell
respiration
Parts of the plant that are growing or
developing food stores:
 Developing fruit
 Developing seeds
 Growing leaves
Sometimes sinks turn into sources and visa versa, and therefore, phloem must be able to
transport in both directions. Unlike the vessels in animals, there are no valves or a central
pump. However, they are similar because in both, fluid flows inside tubes due to pressure
gradients. Energy is needed for both, and therefore, both are active processes. The
movement of substances in phloem is called active translocation for this reason.
Adaptations of xerophytes
Xerophytes are plants that have adapted to arid climates. Examples are cacti are an
example.
In order to adapt to dry climates, xerophytes must decrease water loss due to
transpiration. Therefore, the plants have adapted by:








Small, thick leaves reducing the water loss by deceasing surface area (needles or
green stems)
Reducing the number of stomata
Having the stomata located in crypts or pits on the leaf surface, which causes
higher humidity near the stomata
Having a thickened, waxy cuticle
Having hair-like cells on the surface to trap water vapour
Becoming dormant in the dry months
Storing water in the fleshy stems and restore the water in the rainy season
Using alternative photosynthetic processes called CAM photosynthesis
(Crassulacean acid metabolism) and C4 photosynthesis. CAM plants close
stomata during the day and incorporate carbon dioxide at night. C4 plants have
stomata open during the day, but take in carbon dioxide more rapidly than nonspecialized plants.
Reproduction in Flowering Plants
Assessment Statement
9.3.1
9.3.2
9.3.3
9.3.4
9.3.5
9.3.6
Draw and label a diagram showing the structure of a dicotyledonous
animal pollinated flower
Distinguish between pollination, fertilization, and seed dispersal
Draw and label a diagram showing the external and internal structure of a
named dicotyledonous seed
Explain the conditions needed for the germination of a typical
dicotyledonous seed
Outline the metabolic processes during the germination of a starchy seed
Explain how flowering is controlled in long-day and short-day plants,
including the role of phytochrome
Flowering plants contain their reproductive organs in the flower. Flowers are often
hermaphrodite structures, carrying both male and female parts.
The parts of flowers occur in rings or whorls, attached to the tip of the flower stalk,
called the receptacle.
Find the definition of the following parts of a typical dicotyledonous flower
Sepal (together with the receptacle, called the calyx) –
Petals (together called the corolla) –
Stamen – male part of the flower, which consists of:
Anthers –
Filament or stalk –
Carpels – female part of the flower, and they may be on their own or fused together.
Each carpel consists or:
Ovary –
Stigma –
Connecting style –
Draw the Structure of a Dicotyledonous Flower with annotated labels
Pollination and Fertilization
Find the definition of Pollination
The pollen may come from the anthers or the same flower or flowers of the same plant,
which is called self-pollination. When pollen comes from flowers on a different plant of
the same species, which is transferred, is called cross-pollination.
Transfer of pollen is usually by insects, wind, birds or bats. Flowers that attract insects
usually produce nectar.
Find the definition of Fertilization
The pollen produces a tube, which grows down between the cells of the style, and
through the ovule. The pollen tube delivers two male nuclei. One of these male nuclei
then fuses with the egg nucleus in the embryo sac, forming a diploid zygote. The other
fuses with the other nucleus, which triggers formation of the food store for the
developing embryo.
Seed formation and dispersal
The seed develops from the fertilized ovule and contains and embryo plant and a food
store. After fertilization:

The zygote grows by mitosis. The cells form the embryonic plant, consisting of
an embryo root and stem.

A seed leaf or cotyledon forms. The seed leaf has two forms, as angiosperms
have two classes.
o Monocotyledons – have a single seed leaf
o Dicotyledons – have two seed leaves

The formation of stored food reserves is triggered. In many seeds the food store
is absorbed into the cotyledons.
As the seed matures, the outer layers of the ovule become the protective seed coat, or
testa. The micropyle is a small hole through the testa, where it was attached to the parent
plant. The whole ovary develops into the fruit. The water content decreases and the seed
moves into a dormancy period. Water only makes up 10 –15% or the seed weight.
Dispersal of the seed is needed to make sure there are not many seeds close together, as
they will compete for resources. Therefore they travel long distances from then parent
plant. The type of seed dispersal depends on the structure of the fruit; dry and explosive,
fleshy and attractive for animals to eat, leathery and winged to catch the wind, or covered
in hooks to catch the coats of animals.
Draw the External and Internal Structure of a Bean Seed
Seed Germination
Seeds are in suspended animation and there is very little metabolic activity occurring.
When the metabolic activity starts, this is the start of germination.
The seed stays dormant due to:
1.
2.
3.
Incomplete seed development
Presence of a plant growth regulator – abscisic acid
Impervious seed coat
The conditions that are essential for germination are:
1.
2.
3.
Water – hydrates plant and activates amylase and removes the abscisic acid
Oxygen – for Cellular respiration
Period of warm temperatures as this is important for enzyme production.
Outline the metabolic processes during the germination of a seed.
The seedling develops and functional leaves appear. Photosynthesis takes over and
becomes less dependent on the maltose. The seed will eventually become a mature plant
and produce seeds of its own, starting the whole process over again.
Control of Flowering in Angiosperms
Light is required for photosynthesis and controls aspects of growth and development.
Plants are able to detect the presence of light, its direction, wavelength and intensity.
Photoperiodism is the plant’s response to light involving the lengths of day and night. It
is the length of day and night that controls flowers.
When the plant is young and structures, such as leaves, stem and roots grow, this is called
the vegetative phase. When the plant is ready to reproduce, this is called the reproductive
phase and flowers form. As mentioned before, flowers are very important, as they attract
pollinators. There are three categories of plant in relation to light and flowering.
Plant Type
Long-day plants
Short-day plants
Day-neutral plants
Flowering and Light
Bloom when days are
longest and nights are
shortest (midsummer)
Bloom in spring, late
summer and autumn when
days are shorter
Flower without regard to
day length
Examples
Radishes, spinach and
lettuce
Poinsettias,
chrysanthemums and asters
Roses, dandelions, and
tomatoes
It is actually the length of night that controls the flowering process in the long-day and
short-day types. The control by light is brought about by a special blue-green pigment
called phytochrome. Phytochrome is a large protein that is not a plant growth hormone,
but a photoreceptor pigment.
There are two forms of phytochrome:


inactive form Pr
active form Pfr
When red light (600 nm) is present in available light, the Pr is converted to Pfr. It is done
rapidly. The active phytochrome can absorb far-red light (730 nm). In darkness, the
active form (Pfr) slowly converts back to Pr. The slow conversion allows the plant to time
the dark period and controls the flowering in short-day and long-day plants.
What does this all mean?
In long-day plants, the remaining Pfr at the end of a short night, stimulates the plant to
flower. It acts as a promoter in these plants. The long day, long period of daylight,
causes the accumulation of Pfr.
In short-day plants, enough Pfr acts as an inhibitor. It has been converted to Pr, to allow
flowering to occur. In other words, the very long nights required by short-day plants
allow the concentration of Pfr to fall to a low level, removing the inhibition.