Plant Reproduction » Fertilisation and Seed Formation

What you'll learn this session

Study time: 30 minutes

The process of pollination and how it leads to fertilisation in plants
The structure of flowers and their role in reproduction
How fertilisation occurs in flowering plants
The formation and structure of seeds
Different methods of seed dispersal
The process of germination and factors affecting it

Introduction to Plant Reproduction

Plants, like animals, need to reproduce to ensure the survival of their species. Flowering plants have evolved a fascinating reproductive system that involves flowers, pollination, fertilisation and seed formation. This process ensures genetic diversity and allows plants to spread to new areas.

Key Definitions:

Pollination: The transfer of pollen from the anther to the stigma of a flower.
Fertilisation: The fusion of male and female gametes to form a zygote.
Seed: A structure containing an embryo plant and food store, surrounded by a protective coat.

Flower Structure and Function

Flowers are the reproductive organs of flowering plants (angiosperms). Understanding their structure is essential to comprehend how fertilisation and seed formation occur.

🌼 Male Reproductive Parts

Stamen: The male reproductive organ consisting of:

Anther: Produces pollen grains containing male gametes
Filament: Supports the anther

Pollen grains contain the male gametes (sperm cells) needed for fertilisation.

🌸 Female Reproductive Parts

Carpel/Pistil: The female reproductive organ consisting of:

Stigma: Receives pollen during pollination
Style: Connects stigma to ovary
Ovary: Contains ovules with female gametes (egg cells)

Each ovule contains an egg cell that can develop into a seed after fertilisation.

Pollination: The First Step

Before fertilisation can occur, pollen must be transferred from the anther to the stigma in a process called pollination. This can happen in two main ways:

🐝 Insect Pollination

Many flowers are pollinated by insects like bees, butterflies and beetles. These flowers typically have:

Brightly coloured petals to attract insects
Nectar as a food reward
Pleasant scent to attract pollinators
Sticky or spiky pollen that attaches to insects

🌬 Wind Pollination

Some plants rely on wind to carry their pollen. Wind-pollinated flowers usually have:

Small, inconspicuous petals or no petals at all
Large, feathery stigmas to catch pollen
Lightweight pollen produced in large quantities
Stamens that hang outside the flower

From Pollination to Fertilisation

After a pollen grain lands on the stigma, a series of events leads to fertilisation:

Pollen tube growth: The pollen grain germinates and forms a pollen tube that grows down through the style towards the ovary.
Sperm cell transport: The pollen tube carries two sperm cells down to the ovule.
Double fertilisation: A unique process in flowering plants:
- One sperm cell fertilises the egg cell to form a zygote (which develops into the embryo)
- The other sperm cell fuses with two polar nuclei to form a triploid endosperm nucleus (which develops into the food store)

Did You Know? 💡

Double fertilisation is unique to flowering plants. It ensures that the food store (endosperm) only develops if the egg is fertilised, preventing wasted resources. This evolutionary adaptation has contributed to the success of flowering plants, which are the most diverse and widespread plant group on Earth!

Seed Formation and Structure

After fertilisation, several changes occur in the flower as the seed and fruit develop:

The fertilised egg (zygote) develops into an embryo plant
The triploid endosperm nucleus divides to form the endosperm (food store)
The ovule develops into a seed
The ovary develops into a fruit
Other parts of the flower (petals, stamens) usually wither and fall off

Seed Structure

A seed consists of three main parts:

🌱 Embryo

The young plant that will grow when the seed germinates. It includes:

Plumule: Embryonic shoot
Radicle: Embryonic root

🍞 Food Store

Provides nutrients for the embryo during early growth. It can be:

Endosperm: In some seeds (like wheat)
Cotyledons: In other seeds (like beans)

🛡 Seed Coat

Protective outer covering that:

Protects the embryo from physical damage
Prevents the embryo from drying out
May prevent premature germination

Seed Dispersal

For plants to colonise new areas, seeds need to be dispersed away from the parent plant. This reduces competition and increases the chances of survival. Plants have evolved various mechanisms for seed dispersal:

🌬 Wind Dispersal

Seeds adapted for wind dispersal often have:

Wings (like maple seeds)
Parachute-like structures (like dandelion seeds)
Very small, light seeds (like orchids)

🐮 Animal Dispersal

Seeds can be dispersed by animals in two main ways:

Inside animals: Fleshy, tasty fruits are eaten and seeds pass through the digestive system
On animals: Seeds with hooks or sticky substances attach to fur or feathers

💦 Water Dispersal

Seeds adapted for water dispersal often have:

Waterproof outer coatings
Fibrous, buoyant tissues
Air spaces to help them float

💥 Explosive Dispersal

Some plants have pods that explode when dry, shooting seeds away from the parent plant. Examples include:

Gorse
Peas and beans
Himalayan balsam

Germination: The Beginning of a New Plant

Germination is the process by which a seed develops into a seedling. For germination to occur, several conditions must be met:

Water: To activate enzymes and soften the seed coat
Oxygen: For aerobic respiration to release energy
Suitable temperature: Usually between 5-30°C for most plants

The germination process follows these steps:

The seed absorbs water (imbibition)
The embryo produces enzymes that digest the food store
The radicle (embryonic root) emerges first, growing downwards
The plumule (embryonic shoot) grows upwards towards light
The seedling begins photosynthesis once leaves develop

Case Study Focus: The Coconut

The coconut provides an excellent example of seed adaptation for dispersal and survival. Its seed has a waterproof outer layer that allows it to float in seawater for months without damage. Inside, it has a large food store (the coconut "meat" and milk) that sustains the embryo during its journey and early growth. This adaptation has allowed coconut palms to colonise tropical coastlines around the world, sometimes travelling thousands of kilometres across oceans. When the coconut washes up on a suitable beach, it can germinate and establish a new palm tree, demonstrating the remarkable effectiveness of its seed design.

Summary

Plant reproduction through flowers, fertilisation and seed formation is a complex but efficient process that has contributed to the success of flowering plants worldwide. The process involves:

Pollination - the transfer of pollen from anther to stigma
Fertilisation - the fusion of male and female gametes
Seed formation - the development of the fertilised ovule into a seed
Seed dispersal - the movement of seeds away from the parent plant
Germination - the growth of a new plant from a seed

Understanding these processes helps us appreciate how plants reproduce and spread and has practical applications in agriculture, horticulture and conservation.

🧠 Test Your Knowledge!