The Structure and
Functions of Flowers
The Female Reproductive
Organ: The Carpel
The
female parts of a flower consist of an ovary,
which contains one or more ovules, a
style and the stigma. The ovary is at the base of the flower.
From
the ovary, extends a tubular
structure called the style and on
the top of the style is a surface receptive to pollen called the stigma.
The stigma can take many different forms,
most of them designed to help trap pollen. There are many variations on this
basic structural theme.
After
fertilization the ovule becomes the seed
and the ovary becomes the fruit.
The Male Reproductive
Organ: The Stamen
The male parts of a flower consist of one or more stamens. Each stamen is made up of paired anthers (sacs containing pollen) on a filament or stalk.
The anthers
are the orange/yellow structures often seen in the centre of a flower.
Pollen from
the anthers of one flower is transferred to the stigma of another usually
either by wind, or by animals, especially insects.
PETALS, COROLLA, SEPALS, CALYX
The reproductive structures in
higher plants are contained within flowers. Flowers have more than one petal,
and the flower petals are collectively
called the corolla. A flower bud is
protected by green leafy structures called sepals. Collectively, all of the
sepals form the calyx.
The corolla or petals are often brightly coloured with markings
attractive to insects. The flowers may also be scented. For instance,
Honeysuckle has showy, attractive flowers which attract insects by day.
However, in the dark, their colourful show is not much use, and their heady
scent then helps to attract night-flying moths.
In
insect-pollinated plants, there are also usually nectaries which secrete sugary
nectar, located within the flower. These provide an incentive to insects to
visit the flowers. In the search for nectar, the insects will often get pollen
grains caught on their bodies. This may then brush off onto the stigma of the
next flower visited and in this way the flowers are pollinated.
The receptacle is the place on the stem where floral organs originate
and attach.
Formation of Sex Cells
The
sex cells of the flowering plant are called
gametes. There are both male and female gametes thus the flower undergoes sexual reproduction.
Male
Gametes
Pollen Formation: Development Of A Pollen Grain Within The Pollen Sac Of
An Anther: A cross section of the developing anther
displays four chambers. These chambers are called pollen sacs (see upper
illustration). Each pollen sac is filled with cells containing large nuclei. As
the anther grows, each of these cells goes through two meiotic divisions,
forming a tetrad. These cells are called microspores. Each one of these
microspores eventually becomes a pollen grain. Each pollen sac is enclosed by a
protective epidermis and a fibrous layer. Inside the fibrous layer
is the tapetum. This is a food store
and will provide energy for future cell divisions.
Each pollen
grain is surrounded by a tough protective wall called an exine. This is a tough covering that allows the pollen grain to
survive harsh conditions for long periods of time. The intine is another thin protective coating.
First, each nucleus divides by mitosis to become
two nuclei. One is the tube nucleus. The other is a generative nucleus. The wall of the cell thickens to protect the
developing pollen grain. As the anther ripens, the wall between the paired
pollen sacs disappears. The pollen sacs burst open and the mature pollen grains
are ready for dispersal.
Development of the Embryo Sac
Each ovary contains one or more ovules. The green structure at the top
of the diagram is the ovule. The integuments are the 2 walls of the ovule.
There is a small opening in the walls called a micropyle. This is where the
pollen tube will enter. (Will be discussed later.) The nucellus is cells that
provide nutrition for the growth of the ovule. The embryo sac, also known as
the megaspore, divides by meiosis to form 4 haploid
cells. Three of these cells degenerate and one remains. Only one megaspore
survives in each ovule. This becomes the embryo sac. The haploid nucleus of the
surviving megaspore undergoes three mitotic divisions. Eight haploid nuclei are
now present. Within the swollen ‘megaspore cell’ six haploid cells and two
‘polar nuclei’ are formed. The entire structure is called the embryo sac. One
of the cells near to the micropyle end of the ovule is the haploid female
gamete (egg cell). 
The Carpel With a Mature Embryo Sac will appear as shown below:
Pollination
Pollination is the transfer of pollen (male gamete) from the
anther to a stigma. Cross-pollination: pollen
is transferred to the stigma of another plant. Increases genetic variation,
population more resistant to environmental change. Self-pollination: pollen
transferred to the stigma of the same flower or a flower of the same plant. Guarantees
reproduction if pollinating agent is absent or not efficient.
Pollination can be accomplished by the wind or by animals. Insects are the most common animals that will pollinate a carpel.
The most sophisticated relationships between plants and
insects are generally those involving bees. Bees collect pollen and nectar not
only for themselves but also to feed their young. For this reason bees have
developed a number of adaptations that make them particularly good pollen
carriers. Bees have special hairs that are arranged to form pollen 'baskets' on
their hindlegs and the underside of their abdomen. These adaptations allow them
to gather and carry large volumes of pollen. Bees are ideal pollinators because
they visit many flowers while carrying lots of pollen, before returning to
their nest. So the chance that a bee will transfer the pollen between flowers
of the same species is very high.
Many insects eat
pollen. In the process of eating they become covered in it. Pollination happens
when the pollen feeder transfers the pollen to the pollen receivers of the same
plant, or another plant of the same species, as the insect looks for more
pollen to eat.
Fertilisation
Fertilisation is the union of the male and female gametes to form a zygote.
Since the male and female gametes are haploid (n) when the two unite the
zygote is diploid (2n).
Fertilisation
starts when a pollen grain lands on the stigma. The pollen grain then grain
germinates forming a pollen tube. The tube nucleus controls the
growth of the pollen tube. The pollen tube is an example of chemotropism
since it is growing toward chemicals produced from the ovule. The generative
nucleus travels down the pollen tube. It undergoes mitosis forming two
haploid male gamete nuclei. The pollen tube enters the ovule by way of the
micropyle. The two male gamete nuclei are released into the embryo sac. The
tube nucleus disintegrates.
Double Fertilisation
Since there are 2 sperm nuclei that have
reached the embryo sac both nuclei will fuse with female gametes. One sperm
nuclei will fuse with the egg cell to form the zygote (2n) while the other sperm nucleus fuses with the 2 polar nuclei
in the embryo sac to form an endosperm
nucleus (3n).
Click
here to view an animation of double fertilisation
Seed Formation
The
fertilized becomes the seed. The integuments become the wall of the seed
called the testa. The micropyle closes. The endosperm nucleus leads to
the formation of triploid endosperm, a food tissue. The diploid
zygote, by mitosis, develops into a plant
embryo. The
developing embryo draws nourishment from the endosperm. The
embryo ceases development and goes dormant. The ovule becomes a seed, which
contains a dormant plant embryo, food reserve, and the protective coat called
the testa.
The Embryo
The embryo is made up of the radicle or future root and the plumule or future shoot. The endosperm cells divide many times and absorb the nucellus. This is the nutrition (mainly fats, oils and
starch) for the embryo.
There are 2 types of seeds. Some are endospermic
while others are non-endospermic. In
endospermic seeds the food reserve is the endosperm, which is outside the
plant embryo. Examples of this type of seed are maize and wheat.
Non-endospermic seeds have food reserve within the cotyledon(s) of the plant
embryo. This occurs in broad beans.
Monocots
and Dicots
Monocots have one cotyledon in the seed
while dicots have two cotyledons. The cotyledons are food reserves for the
young plant after it germinates from the soil. It uses these food reserves
until it is capable of making its own food. In monocots the food is absorbed
from the endosperm while in dicots the food is stored in the cotyledons.
Monocot Dicot
Fruit Development
The ovary becomes a fruit. The wall of the
ovary becomes the wall of the fruit called the pericarp. The fruit
protects the developing seeds and plays an important role in seed dispersal.
This process is controlled by auxins produced by the seeds. Once the
fruit forms the rest of the flower parts die and fall away.
Fruit
and Seed Dispersal
Seed dispersal is the scattering of
offspring away from each other and from the parent plant. As a result of
dispersal there is an improved chance of success by reducing competition and
overcrowding. Dispersal also enables colonisation of new suitable habitats and thus, there is an increased chance of
species survival.
Methods of Seed Dispersal
Wind: The seeds of wind-dispersed plants are
lightweight seeds. They have a high air resistance so they can be carried far away from
the mother plant.
Water Dispersal
Fruits which float such as those of the water lily and the coconut palm are carried by water. Coconuts can travel for thousands of kilometres across seas and oceans.
The original coconut palms on South Sea Islands grew from
fruits, which were carried there from the mainland by ocean currents.
Mangroves in the swamp regions of countries such as Thailand are another example.
Animal Dispersal
Some plants have juicy
fruit that animals like to eat.
The animal eats the fruit
but only the juicy part is digested.
The stones and pips pass
through the animal's digestive system and are excreted to form new plants. This
can be far away from the parent plant.
Blackberry, cherry and
apple seeds are dispersed in this way.
Birds also like to eat
fruit and they help to disperse seeds to other areas through their droppings.
Mistletoe has sticky
fruits that are attractive to birds. The sticky seeds stick to the bird's beak.
They then rub their beaks clean on the bark of trees. The sticky seeds are left
on the bark to grow into new mistletoe plants - mistletoe is a parasitic plant.
Squirrels collect nuts
like acorns and bury them for winter food, but they often forget where they
have buried them and these grow into new trees.
Some fruits like that of
the burdock plant have seeds with hooks.
These catch on the fur of
animals and are carried away.
Self-Dispersal: Some plants have pods that explode when ripe and shoot out the seeds. Lupins, gorse and broom scatter their seeds in this way. Pea and bean plants also keep their seeds in a pod. When the seeds are ripe and the pod has dried, the pod bursts open and the peas and beans are scattered.
Dormancy
Dormancy is a period of inactivity.
There is very little cellular activity and no growth. One or many of the
following reasons bring about dormancy:
Auxins that inhibit growth- Growth Inhibitors
The testa is impermeable to water and oxygen- The
testa will eventually break down and allow water and oxygen into the seed.
The testa may be too hard for the embryo to
germinate.
An Auxin (Growth Regulator) may be absent until suitable environmental conditions develop.
Germination
The embryo will germinate from the seed if the proper environmental conditions are
present. When this occurs the embryo resumes its growth. In order for
germination to occur the following conditions must be present:
- Water must be
present. This allows the seed to swell and enzymes to function.
- Oxygen must be
present in the soil.
- The temperature
must be suitable for the species of plant. Suitable temperatures are
usually between 5-30 degrees Celsius depending on the species.
- The dormancy period
must be complete.
- Some seeds need
light and others need darkness.
Events
of Germination
When germination
begins the first thing that happens is water is absorbed by the seed through
the micropyle and through the testa. Enzymes in the soil now digest the foods stored in the seeds:
·
Oils become fatty acids and glycerol
·
Starch becomes glucose
·
Protein becomes amino acids
·
These foods now are absorbed by the embryo.
·
The glucose and amino acids make new structures such as cell walls and enzymes.
·
The fats and glucose are used in cellular respiration to produce energy.
·
The stored food of the seed is being used up as the embryo
grows larger.
·
The radicle grows larger and breaks through the testa. It
becomes the roots of the new plant.
·
The plumule grows larger and emerges above the ground.
·
Leaves form.
Germination occurs differently in
different plants. In some plants the cotyledon remains underground while in
other plants the cotyledon emerges above ground. The diagrams below show these
2 methods of germination.
Click here to see an animation of: