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REPRODUCTIVE SYSTEMS IN FLOWERING PLANTS
The flower is the reproductive structure of flowering plants
...

Flowers come in all sizes, shapes, colours, patterns and arrangements
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They are usually arranged in concentric rings, one above the other, on the receptacle or
thalamus (the swollen tip of the pedicel)
...
The Non-Essential Floral Parts and
B
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The Non-essential floral parts
They are non-essential floral parts because they are not directly concerned with sexual reproduction
...
They include
a
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 They are usually small and green
 They range from three to five in dicots and they occur in multiples of three, in monocots
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g
...
g
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They can be modified to form:
A
Largely, brightly coloured petal-like Petaloid structure e
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g Tridax
Functions of Calyx
1
...

2
...

3
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Epicalyx: They are additional floral leaves occurring below the calyx as in Hibiscus rosasinesis
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2
...

4
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Petals:
 The petals of a flower make up the corolla which is the second whorl to the calyx
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 They are brightly coloured and scented, especially in those flowers pollinated by insects and birds
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But when joined to form a corolla tube,
it is called Gamopetalous as in Allamanda
...

Petals also offer protection to stamens and carpels
The colour and scent attract pollen-transferring animals (pollinators) such as insects and birds
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The calyx and corolla are together referred to as the non-essential organs of a flower
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Perianth: is the word commonly used to describe alike calyx and corolla of a flower
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g
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Each unit of the perianth is called Tepal
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The parts of the corona may be fused to form a cuplike structure e
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Spider lily or free from one another e
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Passionflower and Oleander
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b
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They produce the gametes required for sexual reproduction
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Stamens
The whorl just inside the petals is a group of stamens collectively known as Androecium
...

It consists of a long slender stalk, the filament and a swollen end called the anther which is attached
either to the receptacle as in the pride of Barbados or to the petals as in Allamanda
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In some flowers, the filaments are united while the anthers are free
...

In sunflowers, the anthers are fused while the filaments are free
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Anther
2

 It is a 2-4 lobed structure
 Pollen grains are produced in these lobes within the regions called Pollen sacs
...

When the anther is mature, it dehisces (splits) to liberate the pollen grains which are seen as
yellowish powdery substances at the surface
...

Pollen grains: Each pollen grain is composed of:
A)
two coats: a tough protective outer coat called exine and a thin inner coat called intine of
cellulose
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B
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It is the central whorl of a flower
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Flowers may have
A)
just one carpel (monocarpous) e
...
g Rose, Periwinkle and Bryophyllum
C)
two or more carpels fused to form a single structure (syncarpous) e
...
Hibiscus
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Most pistils have
A)
an ovary, the enlarged bottom part of the pistil
B)
one or more styles, slender structures arising from the top of the ovary and
C)
one or more stigmas, the apical part of the styles
Ovary:
It contains one or more ovoid structures called ovules
...
After fertilization, the ovary develops into a fruit while the ovules develop into seeds which
contain the embryos
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The fruit also helps
to disperse seeds
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It is structurally modified in various ways to receive
pollen grains
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Stigmatic surfaces may be smooth, rough or hairy
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They become sticky so that pollen grains can adhere to them
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Style:
It supports the stigma and holds it in the best position for receiving pollen grains
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Position of Ovary and Type of Flower
A flower may be described as Hypogynous, Perigynous and Epigynous according to the
position of the ovary on the receptacle to the other flower parts
...
g
...
The
flower having this type of superior ovary is described as a Hypogynous flower e
...
Allamanda,
Crotalaria, Delonix regia etc
...
g
...
The
flower having this type of half-inferior ovary is described as a Perigynous flower
...
E
...
The
flower having this type of inferior ovary is described as an Epigynous flower
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In most flowers, the embryo sac consists of
an egg cell and two other cells (synergid cells) at the end near the micropyle
two polar nuclei in the centre that usually fuse at some stage to give a diploid nucleus, and
three cells (antipodal cells) at the end farthest from the micropyle
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 Placentae are outgrowths of the ovary wall
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 The ovules are attached to the placentae by short stalks called funicles
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 The compartment in which the ovules lie is called the loculus
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 Syncarpous pistils may have one or several loculi separated by partitions or septa
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Types of Placentation
The various kinds of placentation are:
Marginal placentation: The ovules are attached to the placentation on one margin of the ovary e
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flowers here have a monocarpous pistil e
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Beans, Cowpea, Pride of Barbados, Flamboyant,
Cassava, Crotalaria
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g
...
g
...
g
...

Axile placentation: The carpels of a syncarpous ovary meet in the centre to form the placenta to
which the ovules are attached e
...
Tomato, Hibiscus, Citrus and Solanum
Basal placentation (monocarpous) or Superficial (syncarpous): The ovules are attached to the
base of a syncarpous ovary e
...
sunflower, Sonchus, Water lily, Helianthus and Compositae
...
e
...
However, in a few types of
plants, the flowers are either male or female (unisexual)
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 When male and female flowers are found on the same plant, the plant is said to be
monoecious, e
...
g
...

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Accessory floral structures:
These include the nectary and the bracts
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It secretes a
sugary liquid called nectar
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Bracts: they are special leaves from the axil of which a single or cluster of flowers arises
...
In some flowers, they become;
 Large, coloured and showy petaloid, e
...
g aroids, banana and maize cob and
 Small and dry to form glumes, found only in grasses and sedges
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A whorl of bracteoles
outside the calyx is known as the epicalyx e
...
hibiscus
...
It is the first step
which leads to the eventual coming together of the male and female gametes for the sake of
fertilization



Types of pollination: There are two types of pollination, they are;
Self-pollination
Cross-pollination
Self-pollination
This is when mature pollen grains from the anther of a flower fall on the stigma of the same
flower, or that of another flower on the same plant
...

Self-pollination brings the male gametes and egg cells of the same plant together
...

Cross-pollination
This is when mature pollen grains of a flower are transferred to the stigma of a flower of
another plant of the same or closely related species
...

Cross-pollination brings the male gametes and egg cells of two different parent plants
together
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Conditions or Features Which Aid Self-Pollination
Self-pollination can only occur in bisexual flowers and unisexual flowers of monoecious plants
...

6






Homogamy:
This refers to the ripening of the anthers and stigmas of a bisexual flower at the same time
...

Pollinating agents readily carry pollen grains from the mature anthers to the receptive stigmas
...
The anthers then
burst and discharge the pollen grains directly on the stigmatic surfaces
...
g
...
In some species, the stigmas bend over to touch the mature anthers when cross-pollination
fails, a common practice in members of the sunflower and hibiscus families
...
g
...
As
the carpels mature, the stigmas push their way out through the tube, brushing against the mature anthers
as they do so
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Here the bisexual flowers never open, they are closed
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Advantages of Self-Pollination
1
...

2
...

3
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Disadvantages of Self-Pollination
1
...

2
...

3
...

Conditions or Features Which Aid Cross-Pollination
A
Dichogamy:
This is the maturation of stamens and carpels of bisexual flowers at different times
...
There are two types of dichogamous
flowers:
i
Protandrous (Protandry):
This is when the stamens of a flower mature earlier than the carpels of that flower or other
flowers of the same plant so that the mature pollen grains are only useful to flowers of other plants
which have mature stigmas to receive them
...
g
...

Usually, the carpels only mature and stigmas receptive when the stamens have finished shedding all
their pollen
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Protogyny (Protogynous):
This is when the carpels of a flower mature earlier than its stamens or those of other flowers of
the same plant so that it can only receive pollen grains from flowers of other plants
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B
...
Unisexual flowers occur either in Dioecious or Monoecious plants
...
g
...

ii
In monoecious plants or plants with male and female flowers borne on the same plant e
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pumpkin, maize and oil palm trees, self-pollination is possible but the likelihood is reduced by having
Female flowers above and male flowers below or
Male flowers and female flowers ripen at different times
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Self-sterility (self-incompatibility)
This is when bisexual flowers of some species may bear mature stamens and carpels at the same
time
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This is called self-sterility or incompatibility
...
E
...
Tea,
Tobacco, Passion flowers, Crotalaria and Leguminous flowers
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Position of stigmas and Anthers
In many erect flowers, the stigmas are kept higher than the stamens so that a visiting insect
reaches the stigma before the anthers
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Advantages Of Cross-Pollination
1
...

The seeds develop into stronger plants which will produce more fertile offspring
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Offspring or individual produces are more adapted to the environmental conditions
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It leads to greater variations among species and a faster rate of evolution
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It is less certain to occur since it involves two separate plants which may be at a great distance
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Pollen grains are likely to be wasted during the process as the pollinating agents may be
transferring them to the wrong plants
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3
...

Differences between Self Pollination and Cross Pollination
SELF POLLINATION
Takes place only in bisexual plants
Only one parent is involved
Pollination may occur without an external
agent
...
g
...

It results in the formation of new varieties
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Pollination and Pollinating Agents
Pollination in most flowering plants is dependent on external pollinating agents or pollinators
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Of these, the two major agents of pollination are wind and insects
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AGENTS OF POLLINATION

A
...

C
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Water
This is not a very common method of pollination
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Ceratophyllum and bladderwort are examples of flowers pollinated by Water
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Wind: They are anemophilous flowers and they have the following characteristics

Characteristics of wind-pollinated flowers
A
...

C
...

E
...

G
...

I
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A
...

C
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They are often inconspicuous and not brightly coloured
They are scentless
They have no nectary
The calyx and corolla are reduced to perianth segments or sometimes absent
Anthers are comparatively large and loosely attached to long filaments which are exposed to the air
The male inflorescence is usually in an uppermost position where there is no obstacle to wind action
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3
...
Where the flowers are small,
they are massed together in a conspicuous inflorescence eg clerodendron, lantana and sunflower
Flowers are often scented ( the scent and colour of petals attract the insects and other animals)
Flowers have nectaries at the base which secrete nectar or flower juice
...
There is little or no wastage since pollination is
controlled
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E
...


Pollen grains are rough, spiny or sticky so that they can stick to the body of the insect or other animal
pollinators (pollen grains are food for insects, bats and birds
...

The stigmas are broad or rounded and sticky for effective retention of pollen grains after pollination
...
It is monoecious with
terminal male inflorescences and axillary female inflorescences (cobs)
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Male Inflorescence
This is a branched structure with flower groups called spikelets arranged along the branches
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Each spikelet is enclosed within glumes and contains two male flowers
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The male flowers are borne on an inflorescence called the Tassel, above the leaves
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The spikelets are arranged in rows on a fleshy
inflorescence stalk
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The fertile female flower only has a monocarpous pistil made up of
an ovary with a single ovule (basal placentation) and
a long flexible style with a feathery upper portion that is stigmatic
...
The flowers are
usually unisexual and dioecious i
...
the male and female flowers are borne on separate trees
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(See modern biology for more on Paw-paw)

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