Botany session 4

© Copyright PCNM 2011
Botany and Pharmacognosy
Session 4

Review
Last week we covered:
Leaves

This Session
During this session we
will cover:
Flowers &
Fertilisation

Flowers
• The main function of flowers are:
• Sexual reproduction
• Attraction of pollinators

Flowers
• Flowers are the reproductive structures produced by the flowering
plants, the angiosperms.
• A flower is made up of four concentric rings of structures. There is
an outer ring of modified leaves, the sepals. All the sepals are
collectively called the calyx Inside the sepals is another ring of
modified leaves, the petals. These are often brightly coloured. This
layer is called the corolla.
• Within the corolla are one or more stamens. Each stamen is made
up of paired anthers on a filament or stalk. The anthers are sacs
containing pollen. Each grain of pollen contains the apparatus to
produce sperm cells.

Flowers
• In the very centre of the flower are the female reproductive organs.
The female parts consist of an ovary containing one or more ovules,
a style and a stigma. The ovary is situated at the base of the flower.
The tubular style extends from the ovary. On the top of the style is
the stigma which receives the pollen. Stigma can be many different
shapes but most are designed to trap pollen.
• Pollen from the anthers of one flower is transferred to the stigma of
another flower, either blown there by the wind or transferred by an
insect, bird or animal.

Flowers

Flowers
• There are many variations of this basic arrangement. Flowers do not
always have the two outer layers of calyx and corolla. These two
layers are most noticeable in flowers pollinated by insects. The
corolla, or petals, is often brightly coloured and has markings
attractive to insects. Often the flowers are scented. White, highly
perfumed flowers are often pollinated by night-flying insects such as
moths.
• Flowers which are pollinated by insects or some types of birds often
contain nectaries which secrete sweet nectar. As the animal
searches for the nectar the pollen grains may be caught on their
bodies. This then may brush off onto the stigma of the next flower
visited.

Flowers
• Many flowers have evolved very specific associations with certain
birds or insects to ensure their pollination. The West Australian red
and green kangaroo paw has evolved to attract birds but not bees.
Bees cannot see the colour red but honeyeater birds are attracted to
red flowers. Kangaroo paws are unscented. Bees are attracted to
perfumes but flower-pollinating birds have a poor sense of smell.
The flowers produce large quantities of dilute nectar which is
suitable for birds. Bees require strong nectar. The shape of the
flower ensures that when a bird, or a tiny honey possum, drinks
nectar from the flower pollen is deposited on the creatureís head.
The shape of the kangaroo paw flower is not suitable for bees to
reach the nectar and come in contact with the pollen at the same
time.

Flowers
• The Lamiaceae family contains many of the herbs used medicinally.
Their flowers are adapted for insect pollination. They are often
coloured blue, purple white or yellow, which are the colours
preferred by insects. They contain nectaries which produce the
concentrated nectar required by bees and other insects. Birds do
visit these herbs but are considered opportunists; insects, especially
bees are the main pollinators of flowers which produce concentrated
nectar. Many bee-pollinated plants have ultraviolet ëlanding stripsí
which the bees can see. They land on these areas of the flower and
are guided towards the nectar.

Flowers
• The structure of flowers can be used in the classification of plants.
The types and organisation of the flower parts are key traits by
which angiosoperm families are determined. Flower structure can be
represented by a floral formula, using specific symbols to represent
the different flower parts, or diagrammatically by a floral diagram.

Flowers
Types of Flowers
• Flowers are described in terms of the presence or otherwise of the
four whorls – the calyx, corolla, stamens and carpels.
Complete and Incomplete
• Not all flower parts are present in all flowers.
• Flowers with all four parts are called complete; those which do not
have all four are incomplete.
• In some species either sepals, petals or both my be lacking
• Example: Members of the Euphorbiaceae family such as
poinsettias do not have petals or sepals. The colourful red ëetals
are in fact coloured leaves.

Types of flowers
• Many bird pollinated species do not have petals; for example,
grevillea have coloured sepals but no petals.
• Stamens and pistils may be present in different flowers.
• Example: stocks do not have properly developed stamens or
pistils and therefore are sterile.
Incomplete grevillea flower Complete hibiscus flower

Flowers
Bisexual or Perfect Flowers
• A flower with both stamens and pistil.
• Example: lamiaceae, lily, rose
Unisexual or Imperfect Flowers
• A flower that lacks either stamens or pistils. Petals or sepals may be
present or absent. These flowers are found on monoecious and
dioecious plants.
• Staminate flower (“male” flower) – Stamens are present and the
pistil is absent
• Pistillate flower (“female” flower) – The pistil is [resent and the
stamens are absent
• Example: melons, most palms, false unicorn root

Flowers
Regular and Irregular
• Actinomorphic or regular flower
• A flower that is radially symmetrical. Meaning it can be cut into two
balanced longitudinal halves in any position.
• Example: Lilium, hibiscus
• Zygomorphic or irregular flower
• A flower with one more whorls not alike. It therefore bilaterally
symmetrical and can only be cut into two equal longitudinal halves
one way.
• Example: orchid, pansy, sweet pea

Flowers
Regular hibiscus flower Irregular orchid flower

Flowers
Monoecious and Dioecious Plants
• Monoecious plant – staminate and pistillate flowers on the same
plant.
• The Cucurbitaceae family, the melon family, have monoecious
plants, that is the plants bear both male and female flowers.
• The female flowers are easily recognised by the swelling at the
base of the flower, the ovary.
• The staminate flower does not have this swelling but does have
a centre spire formed from stamens fused together.

Flowers
• Dioecious plant – staminate flowers on one plant and pistillate
flowers on another.
• Example: willow, holly, false unicorn root, most palms, kiwi fruit,
pawpaws.
Earlier plants, the gymnosperms, were monoecious or dioecious. As
the angiosperms evolved, flowers became more complex. The majority
of flowering plants have perfect flowers so are neither monoecious or
dioecious.

Checkpoint!

Fertilisation
• Pollen grains are formed in the anther sacs by the process of
meiosis. Meiosis is the process of cell division by which four haploid
cells or gametes are produced instead of two diploid cells as in
mitosis.
• Haploid cells have only one set of chromosomes whereas diploid
cells contain two sets of chromosomes. A gamete is a cell that fuses
with another gamete during the process of fertilisation found in
sexual reproduction.
• The result of this fusion is a diploid zygote from which a new
individual can develop. Each gamete supplies half the genetic
information of an individual.

Fertilisation
• The pollen grains are released from the anther to be transferred to
the stigma in the process known as pollination. This process may be
facilitated by insects, birds or wind, and sometimes by small animals
such as bats and honey possums.

Fertilisation
• Self-pollination is found in some species of plants. Self pollination is
when the pollen from an anther is transferred to the stigma of the
same flower.
• Cross pollination is the transfer of pollen from the anthers of one
flower to the stigma of a different flower. This becomes important in
commercial fruit production as many fruits, especially apples, plums
and avocado need to cross pollinate with a member of their species
with compatible pollen, As an example, not all apples can pollinate
each other.

Fertilisation
• Pollen grains germinate on the stigma. A pollen tube forms and
grows down the style of the flower. Each pollen grain contains two
sperm nuclei, essential for fertilisation.

Fertilisation
• The pistil is made up of the
stigma, style and ovary. Ovules
are produced in the ovary. These
are the female gametes produced
by meiosis. An ovule contains a
large embryo sac, protected by
layers of tissue called the nucellus
and integument. Within the
embryo sac are eight nuclei.
Three of these are involved in
fertilisation, two are called polar
nuclei and one is the egg nucleus.

Fertilisation
• When the germinating pollen grain sends its pollen tube down into
the ovary it grows into the ovule via the micropyle.
• The two sperm nuclei are released into the embryo sac where a
double fertilisation takes place.
• The term ‘double fertilisation’ is used because two sets of nuclei are
involved in fusion.
• One sperm nucleus fuses with the egg nucleus to produce the
zygote.
• The other sperm nucleus fuses with the two polar nuclei to produce
what is called the endosperm nucleus.

Fertilisation
• After fertilisation the zygote develops into an embryonic plant.
• While the fertilised polar bodies develop into the endosperm, which
is the food source for the developing embryo.
• The rest of the ovule develops into a seed and the ovary into a fruit.
• Therefore both the stamens and pistils are essential for seed
formation.

Fertilisation

Summary
Today we have covered;
Flowers & Fertilisation

Next Session
We will cover:
Fruit & Seeds
Fruit Dispersal
Inhibitors

Preparation
Brief Notes
• Don’t forget to log on to the LMS and download and print off your
brief notes and handouts for the next session.

Botany session 4

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Botany session 4