Download Outcome 3. Understand the structure and function of flowers, fruits

RHS Level 2 Certificate
Principles of Plant Growth,
Propagation and Development
1: Plant Classification, Structure and Function
Courses for
Royal
Horticultural
Society
Qualifications
Outcome 3. Understand the structure and function of
flowers, fruits and seeds
How do plants reproduce? Flowers and pollination
This section looks at the forms and structures of flowers and how the
reproductive processes of the plant work:
•
•
•
Terms for the various parts of the flower and how they are arranged
Their roles in the process of pollination
Different types of pollination
Throughout many centuries horticulturalists have used their understanding of
plant reproduction to breed plants for better fruits or vegetables, bigger and
brighter flowers, new inter-species hybrids.
How plants arrange for pollen to be transported from one plant to another, and
why this ‘cross pollination’ between plants is beneficial, are topics we will cover
in this outcome.
How do plants reproduce? Seeds and germination
This section looks at some forms and structures of fruits and seeds, and at how
seeds germinate.
.
•
•
•
•
The different types of fruits which develop
How their structures are adapted for different types of distribution
The structure of dicot seeds
The difference between epigeal and hypogeal germination
Contents:
3.1 Describe the structure and state the functions of flowers.
3.2 Describe the structure and functions of fruits and seeds.
Tip for pdfs: you can move easily to each of these sections using the 'bookmarks'
feature at the side of the pdf, or go to individual pages using the thumbnails.
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3. Understand the structure and function of flowers, fruits and seeds
1
Level 2 Certificate:
Principles of Plant Growth,
Propagation and Development
1: Plant Classification, Structure and Function
Courses for
Royal
Horticultural
Society
Qualifications
Outcome 3. Understand the structure and function of
flowers, fruits and seeds
The development of the flower, where seed can develop in an enclosed ovary, is
the major evolutionary improvement from gymnosperms to angiosperms.
• Flowers need help with pollination, so they have developed in different ways to
arrange this most efficiently:
 either by attracting pollinators, and giving them rewards for visiting flowers
 or by using the wind to spread pollen widely.
3.1 Describe the structure and state the functions of flowers.
Flowers develop at the end of a vegetative shoot: this ceases to grow and puts all its energy
into forming flowers and, after pollination, seeds.
The main functions of flowers are to aid pollination, and give rise to seeds and fruits.
The importance of flower type and shape for classification
There are infinite variations in the shape and colour of flowers, especially the petals.
These may occur as separate units (e.g. pansy or poppy flowers) or be partly united into tubes
(foxgloves and penstemons), or in fanciful shapes like orchids.
The arrangement and number of petals and all the other parts of the flower allow botanists to
classify angiosperms into families, genera and species.
Monocotyledons have flower parts arranged in threes or multiples of threes;
Dicotyledons have their flower parts arranged in fours and fives, or multiples of these.
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3 Understand the structure and function of flowers , fruits and seeds
2
The parts of a typical dicot flower
Vertical section of
a dicot flower
showing
petal













style
receptacle
Receptacle,
sepal,
petal,
calyx (all
sepals)
corolla (all
petals)
nectary,
anther,
filament,
stamen,
stigma,
style,
ovary and
ovule.
(note- the number
of petals and
stamens may not
be obvious in a
diagram which
shows a cut
section!
pedicel
or flower stalk
The parts of a typical
monocot
flower
Vertical section of a
monocot flower showing







tepal,
anther
stigma
filament
style
ovary and ovule not
visible
receptacle
stamen
separate petals and
sepals may be visible on
some monocot flowers
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3 Understand the structure and function of flowers , fruits and seeds
3
Flower parts and functions 1
Pedicel
Bract
Receptacle
Sepal
a flower stalk. The stalk of a flower head is a
peduncle.
a leaf-like structure at the base of a flower.
Bracts can sometimes be larger than the flower
itself.
the top of the pedicel that supports the rest of the
flower.
a leaf-like structure which protects the flower bud
before it opens.
Calyx : the collective name for all the sepals
Petal
a thin leaf-like structure that is often brightly
coloured. It is there to attract pollinating insects to
the plant and to protect the sexual organs.
Plants that rely on the wind for pollination (typically
monocotyledons) have much less ‘showy’ flower
parts.
Corolla: the collective name for all the petals.
The calyx plus the corolla are known as the perianth.
In some plants, especially monocots, the sepals and petals cannot be
distinguished easily as they are both brightly coloured ( e.g. Tulipa spp.)
and they are called tepals or perianth segments.
In clematis and tulips the petals and sepals are not easily distinguished, so they are often called
tepals or perianth segments.
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3 Understand the structure and function of flowers , fruits and seeds
4
Flower parts and functions 2
Stamen
Carpel
the male sexual organ made up of:

The anther, where pollen is produced.

The filament, the stalk which attaches the anther
to the receptacle.
the female sex organ, made up of:

Stigma – a sticky, receptive surface at the end
of the style. For pollination to take place, pollen
grains must adhere to the stigma.

Style – the organ that joins the stigma to the ovary

Ovary – the part of the carpel that contains the
ovules. The ovary has a thick wall that will
develop into a fruit after pollination occurs.

Ovule – the female sex cell. These cells will
develop into seeds if fertilisation takes place.

There may be more than one carpel within each flower although
one is common.

The stamens can vary in number but are usually arranged in a ring
around the carpels.

Nectaries are often situated at the base of petals as a ‘bait’ to
bring in pollinators.
Not all flowers will have all structures.

Many flowers don’t have bracts, or nectaries.

Insect-pollinated flowers will have larger, showier petals than wind-pollinated ones.

Flowers that have both male and female parts are called perfect, or entire, or
hermaphrodite.

Flowers with only male or female parts are imperfect.
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3 Understand the structure and function of flowers , fruits and seeds
5
Some plants are monoecious, with separate male and female flowers on the same
plant:
e.g. Corylus avellana, Juglans regia, many courgettes
Others are dioecious, with male and female flowers growing on separate plants.
e.g. Ilex, Skimmia, hops (Humulus spp).
Only a very small number of plants are monoecious or dioecious, perhaps ten per cent; the
great majority of plants have both male and female flower parts.
Corylus avellana, hazel, has male catkins
carrying lots of pollen and tiny red female
flowers, essentially a collection of stigmas.
These both grow on the same plant.
Corylus is wind-pollinated, as is Juglans, the
walnut, another monoecious plant.
Courgettes and other gourds have male and
female flowers (below) on the same plant.
Most hollies (Ilex) and skimmias
(below and right) are dioecious,
which means that to get berries
you need both male and female
plants.
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3 Understand the structure and function of flowers , fruits and seeds
6
Level 2 Certificate:
Principles of Plant Growth,
Propagation and Development
1: Plant Classification, Structure and Function
Courses for
Royal
Horticultural
Society
Qualifications
What is the difference between pollination and fertilisation?
These are two stages in the process of reproduction in ‘higher plants’ - the
angiosperms. This is different from reproduction in the simpler ‘naked seed’
plants (gymnosperms), and more primitive plant forms like mosses and liverworts
which need water to reproduce.
Pollination is the transfer of pollen (the male gamete or sex cell) from the
anthers where it is produced to the stigma which is designed to receive and
recognise it. How plants arrange for pollen to be transported from one plant to
another, and why this ‘cross pollination’ between plants is beneficial, are topics
we will cover in this outcome.
Fertilisation is the next process – the male gametes in the pollen grain travel
down the style and into the ovary to unite with the ovule (female gamete),
producing a zygote which will be an embryo of a new plant - the seed.
You only need to know the outline of fertilisation - this is covered in detail at
level 3 – but you will need to know the definition.
Pollination
Pollination is the transfer of pollen from the male parts of a plant (stamen) to the
female parts (carpel).
In practice this is a transfer from the anthers to the stigma.
•
Pollen grains manufactured in the anthers contain the male gametes;
•
They need to be transferred onto the stigma of a receptive plant, generally of the same
species, to be recognised there.
•
When a pollen grain is recognised, the male gametes are able to move down the style
to the ovary, where a male gamete can fuse with an individual female gamete within
the ovule.
Pollination and pollen transfer
Chemical recognition signals are exchanged between the pollen and the plant on which it
lands – whether carried by birds, bees, butterflies, beetles or the wind.
If full recognition takes place (lots of irrelevant pollen is going to be transferred accidentally)
then growth is chemically stimulated in the pollen grain. Normally, only male and female
gametes from the same species will fertilise successfully.
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3. Understand the structure and function of flowers, fruits and seeds
7
Cross pollination
This is pollen from one plant landing on the stigma of another from the same
species.
It is beneficial to the plant because it:
•
•
allows for genetic variation
and for improved selection - whether artificial or natural (evolution).
Self-pollination doesn’t bring in new genetic material, so isn’t as advantageous to the plant.
It can only happen with hermaphrodite or monoecious plants – where all flowers have both
male and female parts, or where both male and female flowers are on the same plant.
Dioecious plants have enforced cross pollination.
Encouraging cross-pollination
The structure of the flower can ensure that the flower only accepts the correct pollen avoiding self pollination. This is heterostyly. For example,
•
With pin and thrum eyed primroses, the pollen is different in size and shape
between the plants with high and low stamens.
Pin eyed
Thrum eyed
Insects visit the flowers in search of nectar, which is located at the bottom of the flower
tube.
This positioning means only long-tongued insects can actually reach the nectar in the base.
An insect such as a Brimstone butterfly visiting a pin-eyed flower, gets pollen stuck to the
middle of its proboscis from the anthers half-way down the flower tube. If it then goes to
visit a thrum-eyed flower, the pollen is perfectly positioned to be wiped off on the stigma, in
this case, positioned halfway down the flower tube.
The reverse is also true. If the butterfly first visits a thrum-eyed flower, pollen is wiped off
onto the top of its proboscis as it searches for nectar. This is then ideally placed to be
transferred onto the stigma of the next pin-eyed flower which it visits.
The two flower-shapes and the different pollen shapes mean that most clumps of primroses
will cross-pollinate rather than self-pollinate.
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3. Understand the structure and function of flowers, fruits and seeds
8
Characteristics of insect-pollinated plants
•
Flowers relatively large
•
Petals ( or bracts) often brightly coloured
•
Flowers may incorporate nectar guides , or
landing pads for larger insects
•
Often scented with nectaries
•
Anthers are smaller and remain inside the
flower
•
Small amounts of pollen are produced
•
Pollen is often larger, rough or sticky in
texture
•
Stigmas remain inside the flower, flat or
lobed
Above, Iris and Digitalis
Flowers guide insects inwards and provide landing
spaces in Iris, Digitalis and Viola.
The bracts of Euphorbia pulcherrima (left) and
Hydrangea macrophylla (below) attract pollinators
towards insignificant flowers.
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3. Understand the structure and function of flowers, fruits and seeds
9
Characteristics of wind-pollinated plants
•
•
•
•
•
Flowers small and
inconspicuous
Petals often green
Rarely scented,
nectaries
are absent
•
Anthers loosely
attached the slightest movement
initiates pollen release
•
Large amount of pollen
produced
•
Pollen grains smooth
and light
•
Large feathery stigmas
hang outside the plant
Above, male and female flowers on a
Walnut (Juglans), on Corylus (below)
are wind pollinated, as are grasses (right).
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3. Understand the structure and function of flowers, fruits and seeds
10
The horticultural importance of pollination
Apples and pears are divided into a number of ‘pollination groups’ depending on their time of
flowering: some new trees are self-fertile, but generally two trees are needed to produce a
good crop by cross-pollination – the tree’s own pollen is not recognised and accepted.
Some apple and pear cultivars are triploid – their pollen
does not fertilise other apples, so for a successful crop
three trees are needed:
Pollen from Apple A fertilises Triploid Apple T, and
Apple B, but pollen from Apple B is also needed to
pollinate Apple A .
A
B
T
Triploids have extra sets of chromosomes which make them very robust plants and worth
growing despite this disadvantage. Bramley’s Seedling and Jonagold are triploid cultivars.
Many fruit and vegetables rely on insect pollination - commercial orchards are reliant on bees,
and the loss of hedgerows means that often hives have to be imported as the trees blossom.
If the weather is bad and bees won’t fly if it is wet, the annual crop will be low.
Wild flowers have sometimes evolved to be dependent on one specific insect pollinator, which
can lead to the loss of the plant if the insect becomes extinct.
Commercial production of tomatoes also relies on bees – usually bumblebees, which are
brought into the glasshouses to pollinate the crop.
Not all vegetables need insect pollinators: French beans are self fertile, so start cropping
reliably earlier than runner beans which need good weather for their insect pollinators to fly.
Cross pollination is the basis of plant breeding to improve size and quality of flowers and fruit selecting plants with the best characteristics to cross pollinate, then selecting again from their
offspring.
Indicative content:
(the RHS lists what you should have covered for this assessment point)
state the functions of flowers - to aid pollination, and give rise to seeds and fruits.
Draw a vertical section of a monocotyledon flower (NOT grass) and a dicotyledon flower to
show where appropriate: receptacle, tepal, sepal, petal, calyx, corolla, nectary, anther, filament,
stamen, stigma, style, ovary and ovule.
State the meaning of the terms ‘monoecious’ and ‘dioecious’ in relation to plants. Know two
examples of each.
State the meaning of the term ‘pollination’.
Describe characteristics of wind and bee pollinated plants: variations in flower structure and
pollen.
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3. Understand the structure and function of flowers, fruits and seeds
11
Level 2 Certificate:
Principles of Plant Growth,
Propagation and Development
1: Plant Classification, Structure and Function
Courses for
Royal
Horticultural
Society
Qualifications
Outcome 3. Understand the structure and function of
flowers, fruits and seeds
Fruits and Seeds
• The seeds, and the fruits which contain them, complete the process of
reproduction in angiosperms and gymnosperms.
• In angiosperms, the flower is pollinated, the ovule within the ovary is fertilised
and an embryo plant develops. These ovules become the seeds. Parts of the
ovary become a hard outer coat which will protect the seed in its resting state.
• The flower has now completed its role and will normally shrivel and die, except
for the ovary that continues to grow and form the fruit.
• These seeds are not able to geminate while the fruit surrounds them and must
wait until it has gone – either eaten or decayed on the ground.
3.2 Describe the structure and functions of fruits and seeds.
Fertilisation
The RHS defines this as the fusion of a male gamete from pollen with a female gamete in the
ovule to produce the embryo.
When pollen grains land on the stigma of a flower of
the correct species they germinate.
A pollen tube grows through the tissues of the flower
until it reaches an ovule inside the ovary.
The nucleus of the pollen grain (the male gamete) then
passes along the pollen tube and joins with the nucleus
of the ovule (the female gamete). This process is called
fertilisation.
After fertilisation the female parts of the flower develop
into a fruit. The ovules become seeds and the ovary
wall becomes the rest of the fruit.
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3. Understand the structure and function of flowers, fruits and seeds
Fertilisation is the fusion of
a male gamete from pollen
with a female gamete in the
ovule to produce the
embryo.
A fruit is formed from the
ovary after fertilisation.
A seed is formed from the
ovule after fertilisation.
12
Images from
http://www.bbc.co.uk/bitesize/standard/biology/w
orld_of_plants/growing_plants/revision/3/
The male nucleus contains the two male
gametes:
• One male gamete travels down the pollen
tube and fuses with the female gamete to form a
zygote.
• Further development and cell division takes
place and the zygote becomes an embryo, the
basis of a new plant within the seed.
• In many plants the second gamete fuses with
other cells within the ovule to form a food store
for the developing seed, the endosperm.
One pollen grain fertilises one ovule, but this process may be taking place with many pollen
grains and ovules at the same time within the plant.
Fruits
Fruit: the ripened ovary or ovaries of a seed-bearing plant, together with
accessory parts, containing the seeds and occurring in a wide variety of forms.
Fruits develop as the seeds mature.
The ovules become seeds, and the ovary wall swells to accommodate them as they develop
– sometimes it becomes fleshy, sometime it hardens into an outer covering or seedcase.
In botanical terms all of these are fruits, whether they are dry or fleshy, edible or spiky.
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3. Understand the structure and function of flowers, fruits and seeds
13
Not all fruits are formed from the ovary: some develop from other parts of the flower, and
these are termed false fruits or pseudocarps. Fruits that develop from the ovary are called
true fruits.
The fruit is designed to help with seed dispersal.
Seeds which fall too close to the parent plant will be unwanted competition; nature tries to
ensure that seeds will be spread widely – whether it is by a dry seedcase such as love-in-a
mist (Nigella) scattering seeds on the wind, an explosive capsule such as gorse (Ulex)
hurling seeds away from the parent plant, or a fleshy fruit like a cherry designed to be eaten
by birds (or humans).
They can be divided into many different groups. The main types are:
-
Dry, divided into
-
-
Dehiscent (the seedcase splits)
Indehiscent /hard (the seedcase remains intact)
Fleshy or succulent
Succulent or fleshy fruits
These include most of what you might consider fruits (apples, pears, plums and cherries,
strawberries and raspberries, gooseberries, grapes) as well as tomatoes, cucumbers,
squashes etc which are generally eaten as vegetables.
Succulent fruits often contain chemicals that prevent seeds from germinating until all the fruit
pulp has been removed – for example, when an elderberry passes through the gut of a bird
and the pulp is digested. The seed is then excreted far from the original plant with its own
fertiliser package.
Indehiscent dry fruits
These include not only nuts such as hazel, sweet chestnut and acorns but also single dry
seeds like sunflower, with parachutes (clematis or dandelion), and winged samaras like
sycamore and ash.
Dehiscent dry fruits
Seedcases of delphiniums, love-in-a-mist and poppies, split and release dry seeds; with
legumes the seed may be propelled a long way by the drying and twisting action of the
seedpod.
What are the functions of fruits and seeds?
The function of the fruit is
• the distribution and protection of the seed;
• it may impose dormancy
The function of the seed is
• the distribution and protection of the embryo;
• it may impose dormancy;
• it gives rise to new plants.
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3. Understand the structure and function of flowers, fruits and seeds
14
Fruits and seeds : methods of dispersal
•
•
Wind
Small seeds can be dispensed by a
censer mechanism e.g. Papaver spp.
The surface area may be increased by
various means to allow the seed to be
carried long distances by the wind, like
the winged seeds of Tilia (lime), Acer or
Fraxinus (ash).
Other plants use parachutes or silky tufts to
carry them on the wind, for example
Taraxacum officinale (dandelion) and Clematis spp.
Water
Water dispersal is relatively rare. Coconuts (Cocos nucifera) float on ocean currents before
being washed up on a beach where they germinate.
Coconut palms are therefore only found naturally on island fringes.
•
The waterlily (Nymphaea spp)
has a spongy seed called an
aril that floats
•
The coconut has a fibrous outer
layer that helps it to float.
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3. Understand the structure and function of flowers, fruits and seeds
15
Animals (birds, mammals etc)
Succulent seeds and fruits such as cherries
(Prunus avium) and hawthorn (Crataeus monogyna),
Blackberries, raspberries etc are designed to attract birds.
Nuts such as acorns (Quercus robur), hazelnuts
(Corylus avellana) and other seeds enclosed in hard
inedible fruits are dispersed by rodents.
Oily seeds are dispersed by
ants (gorse, Cyclamen
hederifolium).
Other fruits and seeds are dispersed externally by animals – hooks on many seeds attach
themselves to the coats of animals and are transported over long distances. Galium aparine
(cleavers), agrimony and burdock are very commonly found in large quantities on the coats of
dogs.
Hooked fruits and seeds such as burdock (Arctium tomentosum) attach themselves to the
coats of animals – or gardeners.
Propulsion
Some fruits do not rely on outside agencies to
distribute seed but have evolved explosive
mechanisms to spread seeds widely.
•
The tensions set up by unequal drying of the
seedcase cause the pods of legumes (for
example Pisum sativum) to split along one
side;
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3. Understand the structure and function of flowers, fruits and seeds
16
•
In Impatiens spp (including both Himalayan Balsam and Busy Lizzies) the water
pressure inside the fruit causes it to split open violently and throw the seeds outwards,
particularly if the seed pod is touched.
Impatiens pods before and after
expelling seeds.
Seed development
Seeds are the major way in which flowering plants reproduce. The seed consists of
•
an embryo, or immature plant,
•
a foodstore to fuel growth,
•
a protective outer coat to keep the embryo safe and make sure it does not dry out before
conditions are right for germination.
Many seeds are very hardy and are able to withstand cold temperatures and germinate the
following spring. They can remain dormant for many years, even centuries in some cases, if
the right conditions for germination do not occur.
Germination is the growth of a plant from its seed to its appearance above ground.
Dormancy is a period of reduced metabolic activity in the plant.
The normal ‘resting period’ while a seed waits through winter for suitable growing
conditions is also called ‘quiescence’ by some botanists ,to distinguish it from when seed
does not grow even when it has suitable temperature, moisture etc.
Seed is sometimes not ready to grow when it is shed by a plant – the embryo may need to
mature and develop within the protective seed coat for some months.
Seed viability is whether or not the seed has the capacity to grow – whether its embryo is
live. Some plants produce huge quantities of seed, but not all of them will be capable of
germination.
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3. Understand the structure and function of flowers, fruits and seeds
17
Dicot seed
Monocot seed
Phaseolus vulgaris and Vicia faba are examples of dicot seeds. This
stylised bean could be either a French bean or a broad bean.
Image from
http://generalhorticulture.tamu.edu/HORT604/LectureSupplMex07/AnatomyMorphology.htm
Zea mays is generally
given as an example of
a monocot seed,
though in fact it is a
fruit containing the
seed.
Many monocot seeds
have a large
endosperm food
source which makes
them useful food crops.
You will not need to
know the details of
monocot seed
structure for level 2.
Diagram from
http://bioditrl.sunsite.ualberta.ca/det
ail/?P_MNO=5742
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3. Understand the structure and function of flowers, fruits and seeds
18
The parts of the seed
Testa
The protective seed coat. It can be very tough and
may need to be damaged before germination can
happen.
It is formed from the integuments surrounding the
ovule.
Cotyledon
The seed leaf that provides a store of food for the
germinating seed.
There may be one or two cotyledons.
Plumule
The part of the seed that will become the first
shoot.
Radicle
The part of the seed that will become the first root.
Hilum
The point where the seed was attached within the
ovary.
Micropyle
Hypocotyl
Epicotyl
A small hole in the testa where the pollen tube
entered the ovule at the time of fertilisation.
Sometimes water can enter here and trigger
germination
This connects the plumule to the radicle - it lies
below the cotyledons and gives rise to the root
Between the plumule and the radicle, this lies
above the cotyledons and gives rise to the stem
.
In monocots, as the seed germinates there are also
Coleoptile
The sheath that protects the plumule on its
emergence through the soil.
Coleorhiza
The sheath-like structure that protects the radicle
and connects to the coleoptile
Many seeds also contain an endosperm.
It is formed at fertilisation and is a source of food for the embryo as it grows to
become a seed. In other seeds, the cotyledons form the main food store.
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3. Understand the structure and function of flowers, fruits and seeds
19
Dormancy
Perennial and biennial plants cope with winter cold by slowing down their metabolisms.
Growth above ground dies back in herbaceous plants, and leaves are shed by deciduous
trees and shrubs. In evergreens, growth slows.
Annual plants get through winter in many survival capsules – the seeds. Only a small
proportion of the seeds produced by a plant will survive to germinate - they will rot, end up in
unsuitable situations or be predated.
Growth and development processes are deferred - for seeds, until the conditions are right for
germination.
Germination
While the seed is quiescent/dormant, all its processes (e.g. respiration) as slowed down: the
embryo remains alive but food resources are not being wasted.
When conditions are right for germination:
• The seed starts to take in water
through the micropyle (imbibition);
• Respiration rate rises as cells
start to grow and divide.
At this point the seed needs
• water
• oxygen for respiration and
• a temperature high enough to
sustain growth.
• Once the cells have taken in water
the embryo starts to grow (by cell division).
First the root initials develop and the radicle is pushed through the seed coat, to anchor the
seedling as it develops and take in water and minerals from the soil.
Then the shoot initials develop – the plumule emerges from the seed as it splits.
Eventually the plumule will develop into a whole plant that is above the ground, but first it has
to get through several inches of soil, and to avoid damage at this stage many plants grow
unevenly - one side of the shoot grows faster than the other so that the stem forms into a
loop, which pushes through the soil dragging the tip behind it to protect it from damage.
Epigeal and hypogeal germination
There are two types of seed germination, depending on whether the cotyledons stay under the
ground as a food source, or if they emerge as seed leaves to begin feeding the seedling by
photosynthesis.
•
•
The part of the shoot that is below the cotyledons (seed leaves) is the hypocotyl.
The part above the seed leaves is known as the epicotyl.
In epigeal germination the hypocotyl below the cotyledons grows upward through the soil,
curving and elongating to pull the cotyledons and plumule to the surface.
Once it reaches the surface, the hypocotyl straightens and pulls the cotyledons and shoot tip
into the air, where the cotyledons unfurl as seed leaves and can begin to photosynthesise in
the light.
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3. Understand the structure and function of flowers, fruits and seeds
20
Hypogeal germination is where the epicotyl above the cotyledons elongates and pulls the
shoot tip, the plumule, to the surface where the first true leaves will then grow; the cotyledons
stay underground where they eventually decompose.
Hypogeal germination - e.g. a broad bean, Vicia faba
As the broad bean germinates the cotyledons can stay below the ground to provide food for
the plumule and radicle as they develop.
The cotyledons wither and the first leaves that emerge will be true leaves, resembling the
adult leaves.
Epigeal germination – e.g. a French bean, Phaseolus vulgaris
In many other plants the cotyledons are pushed above ground by the developing stem – with
the plumule, containing the initials for further growth, and sometimes the remains of the seed
coat.
The first leaves above ground are the seed leaves, (two in dicots) which are usually simple
and do not resemble the adult leaves of the plant.
As soon as the leaves – seed or true – emerge they are able to start photosynthesising and
the seedling is independent of its food reserves.
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3. Understand the structure and function of flowers, fruits and seeds
21
Epigeal and hypogeal germination in seedlings
Image from http://www.backyardnature.net/n/x/hypogeal.htm
Indicative content:
(the RHS lists what you should have covered for this assessment point)
State the meaning of the term ‘fertilisation’ - the fusion of a male gamete from pollen
with a female gamete in the ovule to produce the embryo.
State the meaning of the terms ‘fruit’ and ‘seed’: a fruit is formed from the ovary after
fertilisation; a seed is formed from the ovule after fertilisation.
State the function of fruits and seeds: the function of the fruit is the distribution and
protection of the seed; it may impose dormancy; the function of the seed is the distribution
and protection of the embryo; it may impose dormancy; it gives rise to new plants.
Describe the means by which seeds are dispersed: wind - wing, parachute and censer
(Papaver); water*; explosive; animals ( attachment, scatter hoarding and frugivory (= fruit
eating)
Two plant examples for EACH of the above are required except for water*, where
only one is required.
Describe the internal and external structure of the seed and state the function of the
various parts: testa, cotyledon, embryo, radicle, plumule, hypocotyl, epicotyl, endosperm,
hilum, micropyle.
Examples to be studied include French bean (Phaseolus vulgaris) and broad bean (Vicia
faba).
Describe ONE example of epigeal germination and ONE example of hypogeal
germination: germination of the French bean (Phaseolus vulgaris), and broad bean (Vicia
faba).
RHS Level 2 Certificate Plant Growth Unit 1
Outcome 3. Understand the structure and function of flowers, fruits and seeds
22