Week 6 POLLINATION IN FLOWERING PLANTS

Pollination is defined as the process of transfer of pollen grains from the anthers of a flower to the
stigma of a flower. It is the first step which eventually leads to fertilization in plants. Flowers may be
either self-pollinated or cross-pollinated.
 In self-pollination, pollen is transferred from the anthers to the stigma of the same flower, or
of another flower on the same plant, e.g. pea, cotton, tomato, e.t.c.
 In cross-pollination, pollen is transferred from the anthers of a flower on one plant to the
stigma of a flower on another plant, e.g. paw-paw, hibiscus, pride of Barbados, e.t.c.
Cross-pollination is much more common than self-pollination as this ensures variation which helps the
plant survive. Common arrangements that encourage cross pollination is protandry, protogyny and
dioecious or unisexuality. Some bisexual plants have conditions that favour self-pollination. These are
homogamy and cleistogamy.

Agents of pollination
Pollinators include organisms and other means by which pollination occurs. These include air, water,
insects, wind and other animals like snails, birds, bats and man.

Characteristics of insect pollinated flowers

Insect pollinated flowers are also called entomophilous flowers. They have the following characteristics:
1. They have large conspicuous petals/sepals.
2. Flowers are usually bright coloured.
3. They possess scent.
4. Nectar is also present.
5. Pollen grains are rough, sticky and relatively few.
6. The stigma is flat with sticky surface to enable visiting insects become dusted with pollen grains
Examples of insect-pollinated flowers include Hibiscus, Delonix, Cowpea, Crotaloaria, Pride of
Barbados, e.t.c.
Characteristics of wind pollinated flowers
Wind pollinated flowers are also called anemophilous flowers. They have the following characteristics:
1. They have small, inconspicuous petals/ sepals.
2. Flowers are usually dull coloured.
3. There is absence of scent or nectar.
4. Large quantities of pollen grains are produced.
5. Pollen grains are small, smooth, light and not sticky.
6. Stigma is long and sticky with a large surface area.
7. Anthers are attached to flowers in such a way that they readily swing in the air and release the
pollen grains.
Examples of wind pollinated flowers are maize, guinea grass, rice millet,e.t.c.

Fertilization and zygote formation

Fertilization will only proceed if the pollen and stigma are mature and of the same species.
After pollination, the pollen grain absorbs a sugary liquid on the stigma, swells and germinates. The
outer coat (exine) of the pollen grain splits. The pollen tube grows out and down into the style.
Initially, the pollen grain has one nucleus. Later, this nucleus divides into two, a larger tube nucleus and
a smaller generative nucleus. The generative nucleus divides into two male nuclei. The male nucleus is
the male gamete.
The pollen tube grows into the ovule through the micropyle, and the end of the pollen tube bursts. The
two male nuclei released inside the embryo sac.
One male nucleus fuses with the egg cell(ovum) to form zygote. This zygote develops into the
embryo. This is the first fertilization.
The second male nucleus fuses with the two polar nuclei to form a triploid cell (endosperm nucleus)
that produces the endosperm. This is known as the second fertilization which takes place in plants.
The zygote grows by repeated cell-division(mitosis), cell enlargement and differentiation to form an
embryo. The embryo becomes differentiated into the plumule or embryonic shoot, radicle or
embryonic root, one or two cotyledons and sometimes, the endosperm. In a monocotyledonous plant,
the embryo has one cotyledon, while in dicotyledonous plants, it has two cotyledons.
The ovary develops into a fruit, the ovary wall develops into the fruit wall. The ovule develops into
seeds.
Changes which take part in a flower after fertilization;
 The ovule develops into a seed
 The zygote develops into an embryo of the seed.
 The ovary becomes the fruit.
 The rest of the flower usually withers.

Fig 1: Structure of Pollen grain and Fertilization in the Embryo sac

DISPERSAL OF FRUITS AND SEEDS

Dispersal is defined as the movement or spread of seeds and fruits from the parent plant to faraway
places by means of several dispersing agents. Fruits and seeds are dispersed by; animals, winds, water
and fruits (themselves).
Importance for dispersal for fruits and seeds are as follows;
i. It prevents overcrowding between members of same species
ii. It reduces the competition for necessary condition of light, water and nutrients for their
germination.
iii. It helps in colonization of new area.
 iv. It prevents diseases from spreading among plant species.
Means of Dispersal (Agencies)
Based on the agencies involved, the fruits and seeds may be dispersed by the following ways;
Explosive Mechanism (Autochory)
The dispersion of fruits and seeds by themselves is called autochory. It is also called mechanical
dispersion of seeds. Fruits such as; Okro, Oil bean, Castor oil and Pride of Barbados. The pericarp (outer
covering) dries and suddenly twists open and throwing out the seeds(burst). The mechanism of dispersal
is called explosive mechanism.
Dispersal by Wind (Anemochory)
Seeds and fruits dispersed by winds have the following adaptive features;
i. Seeds are light weight, minute and dusty so they can easily be blown to long distances by air,
e.g Sodom apple
ii. They have well developed thin, dry and membranous wing-like structures to carry them to
long distances. Examples of winged seeds include Moringa, African rose wood,
iii. Some fruits develop parachute-like structures called pappus. Examples Tridax, Eupatorium
Dispersal by Water (Hydrochory)
Seeds and fruits dispersed by water have some kinds of floating devices and a protective covering. The
coconut fruits possess fibrous mesocarp which encloses large air spaces to help them float in water. The
outer epicarp is smooth and waxy. Due to these adaptations, the fruits are carried away to long distances
along the seashores.
Dispersals by Animals (Zoochory)
The seed of such seeds that are dispersed by such means are provided with hooks, spines, stiff hairs and
sticky glands. These fruits and seeds get attached to the skin, fur or wool of passing animals, and carried
away to some distances. Some plants produce fleshy edible fruits and are eaten raw by animals including
human beings. Such fruits have attractive colours to and sweet smell when ripe. These seeds are thrown
away here and under suitable conditions of germination, they grow into new plants.

Fig 2: Means of Dispersal (Agencies)

Fig 1: Structure of Pollen grain and Fertilization in the Embryo sac