SEXUAL REPRODUCTION

Sexual reproduction involves the fusion of the nuclei of male and female gametes to form a
zygote. The sexual reproductive organs of higher plants are known as flowers.

FLOWER
The flower is that part of the shoot modified for sexual reproduction They are developed
from flower buds and held on to the stem by flower stalk called pedicle.

- The outermost structure of the flower is composed of leaf-like sepals together called calyx.
The calyx protects the inner whorls during the bud stage. They are usually green and at
times brightly coloured to attract insect pollinators.

-The petals possess nectar which together with the brightly coloured petals, attract insect
pollinators.
In monocotyledonous flowers, the calyx and corolla are similar and are collectively called
perianth.
A collection of flowers borne on the same stalk is called inflorescence.

-The androecium is the male reproductive organ of the flower. It consists of a number of
stamens each of which comprise of a filament and an anther. Each anther has two pollen
sacs, which contain pollen grains.

-The gynoecium consists of one or more carpels forming the female reproductive organs of
the flower. Each carpel consists of the ovary, style and stigma.

A superior ovary is an ovary located above the other flower parts on the receptacle. A
flower that bears a superior ovary is referred to as hypogynous
An inferior ovary is an ovary located below. the other flower parts on the receptacle. A
flower that bears a superior ovary is referred to as epigynous

Terms Associated with whole plants and flowers.

Hermaphrodite. Male and female sex organs borne on the same plant, e.g., hibiscus,
flamboyant, crotalaria, etc.

Dioecious: Male and female sex organs borne on separate plants, i.e., the plant is either
male or female. E.g., Pawpaw.

Monoecious plant: Separate male and female flowers borne on the same plant, such as oak
maize, coconut, oil palm and sycamore. Such plants are hermaphrodite.

Actinomorphic flower: An actinomorphic flower is one which is radially symmetrical

Zygomorphic: This is an irregular flower which is bilaterally symmetrical, that is can be

divided into two equal halves when cut vertically.

Unisexual flower: This only has male or female part

Bisexual Flower: This is a flower with both gynoecium and androecium

Protandrous Flower: This is a bisexual flower in which the male and female parts mature at
different times. The anthers mature first before the stigmas

Protogynous Flower: This is a bisexual flower in which the male and female parts mature at
different times. The stigmas mature first before the anthers. Protandrous flowers are much
more common than protogynous.

A monocarpous pistil is one which consists of only one carpel. E.g., Crotalaria.

An apocarpous pistil is one with more than one carpel. The carpels remain entirely separate
from one another, e.g., Bryophyllum and rose. A syncarpous pistil is one with more than one
carpel. All its carpels or at least the ovaries are fused. For example, in Hibiscus the ovaries
and styles of the five carpels are fused but the stigmas are separate.

FLORAL FORMULAR
A floral formula can define as the numeric and symbolic expression, which reveals
the flower morphological characteristics by employing different symbols, letters
and figures.

It is the conventional method accustomed to formulate the structure of the flower. It

elucidates the information about the number of whorls and a relative relationship
between each other.

The floral formula uses discrete letters, signs and figures to represent the specific
feature of the flower.

Letters used in Floral Formula

1. K: This letter denotes the sepals that form an outermost whorl called
the calyx.
2. C: This letter represents the group of petals that constitute the second whorl
called the corolla.
3. P: It is used to denote the tepals, which indicates the undifferentiated
condition of the perianth members (sepals and petals).
4. A: It specifies the male reproduction part or stamens, which includes the
filament and anther that all together makes up the third floral whorl known
as androecium.
5. G: It denotes the female reproductive part, i.e. carpel, which includes the
stigma, style and ovary that colloquially forms the innermost whorl
called gynoecium.
6. Br: It represents the bracteate condition of the flower.
7. Ebr: It indicates the ebracteate condition, in which a flower lacks bract.
8. Brl: It indicates the presence of bracteoles or represents the bracteolate
condition.
9. Epik: It represents the presence of a secondary whorl surrounding the calyx
called epicalyx.
10. Ebrl: It is used to indicate the absence of bracteoles or to represent the
ebracteolate condition.
Symbols used in Floral Formula

1. 0: It indicates the absence of a particular member in flower.

2. ∞: It is generally used when the number of specific flower parts is more

3. ⊕: It indicates an actinomorphic condition.

than 10.

4. %: It denotes a zygomorphic condition.

5. ⚥: It represents the bisexuality of flowers.
6. ⚦: It represents the unisexual, staminate flower.
 7. ♀️: This represents the unisexual, pistillate flower.

PLACENTATION.

This is the term used to describe the arrangement of ovules within the ovary. The different
forms of placentation that exist include marginal, axile, parietal, and free – central.

Marginal: In this case the ovules are arranged along one edge of a monocarpous ovary.
Example Flamboyant, crotalaria, beans, etc.

Axile: Here the ovules are on a central column, e.g., tomato

Parietal: The ovules are arranged on several lines on the ovary wall, e.g., pawpaw.

Free-Central: The ovules are attached to a knob which projects from the bases of the ovary.
E.g., water leaf.

Basal-ovules are found at the base of the ovary eg, sunflower

Pollination.

This is the transfer of pollen grains from the anther to the stigma of a flower. When the
pollen grains are transferred from the anther to the stigma of the same flower, or a flower
on the same plant then it is referred to as self-pollination. When the pollen is transferred
from the anther of one plant to the stigma of another plant of the same species then it is
referred to as cross pollination.
Generally, cross pollination is more advantageous than self-pollination because the seeds
produced after cross pollination yield healthier plants than those produced after self-
pollination.

Adaptations for self-pollination.

Flowers are homogamous, i.e., the anthers and stigmas mature at the same time.

Flowers are cleistogamous, i.e., the flower remains closed until self-pollination has taken
place.

Flowers are bisexual.

Types of fruits.
There are three main types of fruits namely, simple fruits, aggregate fruits and multiple or
compound fruits.

Simple Fruits.
Simple fruits are formed from one flower with one carpel, or several carpels fused together.
This group includes majority of fruits and it can be divided into three main sub-groups
namely: dry indehiscent, dry dehiscent and succulent fruits

Dry Indehiscent fruits: These are fruits that do not split when they get dry
Examples include the following:
 Achene eg, sunflower
 Nut eg, groundnut, almond nut
 Caryopsis eg, maize, rice
 Cypsela eg, tridax
 Samara eg, combretum

Dry dehiscent fruits are those whose pericarp splits open to set the seeds free.
Examples include the following:
 Follicle eg, Sodom apple
 Legume eg, Caesalpinia, beans, flamboyant, crotalaria
 Capsule eg, cotton, castor oil, balsam
 Schizocarp eg, desmodium

Succulent fruits when ripe are usually soft and fleshy, and may be eaten by animals. They
exist in three main forms namely:
 Drupe eg, mango, coconut, oil palm
 Berry eg, tomato, orange, guava, grapes, banana
 Pome eg, apple, pear
Aggregate fruit: these are formed from one flower from several separate carpels
(apocarpous). The individual parts of the fruit are known as fruitlets. Eg cola, rose fruit,
soursop

Multiple fruits are formed from flowers known as inflorescence. Each multiple fruit is made
up of many separate fruits. The ovaries and other parts of the flowers become fused after
fertilization to form the fruit. Eg, pineapple, jackfruit, fig.
Fruit and seed dispersal
Dispersal is the process by which plants scatter their seeds and fruits away from themselves.
Dispersal prevents overcrowding and competition and so makes species less vulnerable to
epidermic attack by viruses, fungi and insects. Dispersal is carried out by various agents
namely, wind, water, animals, explosive mechanism and censer mechanism.

Dispersal by animals.
Fruits dispersed by animals are usually brightly colored with edible, sweet-tasing succulent
parts. Others have hooks or sticky hairs. Succulent fruits may be eaten by animals or birds
and their seeds discarded or passed out in their feces undamaged because of the enzyme-
resistant Testa. Examples are guava, pawpaw and tomatoes. Hooked or sticky fruits example
cleavers., desmodium, mimosa may cling to the fur of animals or in the clothing of people as
they walk along. They are carried away and later dropped.

Explosive Mechanism.
This is mainly brough about by unequal drying of the pericarp, which creates tension in it.
The fruits finally split open throwing their seed away from the parent plant. The pressure set
up is so great that each segment of the pericarp curls inwards. Fruits dispersed by this
method include legumes of many beans. Flamboyant, Crotalaria, okra, balsam etc.

Dispersal by wind

The nature of fruits and seeds dispersed by wind are as follows.

 They mostly have small size
 They are usually dry and light in weight
 Some have extended pericarp forming wings, hairs, etc.
These features enable them to be carried away from the parent plant-by air current.
Examples are cotton, Tridax, Tecoma, Narra, Combretum and Kapok.

Wind dispersal is a wasteful process because a great number of seeds may be carried to
unsuitable areas such the streets, roofing of buildings and ocean. Wind dispersed seeds are
produced in large quantities to cater to for several wasted ones,