PLANT LIFE CYCLE, DURATION OF LIFE OF PLANTS, AND GROWTH FORMS OF PLANTS

Learning Objectives

At the end of this lesson, students should be able to:

• Define the plant life cycle.
• Explain the stages of the plant life cycle.
• Differentiate between sporophyte and gametophyte generations.
• Classify plants based on their duration of life.
• Explain annual, biennial, and perennial plants with examples.
• Describe different growth forms (habits) of plants.
• Identify the ecological significance of different plant growth forms.

1. Plant Life Cycle

Definition

The plant life cycle refers to the series of developmental stages a plant passes through from the formation of a seed (or spore), through growth and reproduction, until it produces new seeds or spores and eventually dies.

One unique characteristic of plants is that they exhibit alternation of generations, where two distinct multicellular phases alternate throughout their life cycle.

The Two Generations

1. Sporophyte Generation (2n)

The sporophyte is the diploid generation, meaning it contains two complete sets of chromosomes.

Characteristics:

• Dominant stage in flowering plants
• Produces spores
• Develops from a fertilized egg (zygote)
• Undergoes meiosis to form spores
• Larger and longer-living than the gametophyte in higher plants

Examples:

• Mango tree
• Maize plant
• Hibiscus
• Fern plant

2. Gametophyte Generation (n)

The gametophyte is the haploid generation, possessing only one set of chromosomes.

Characteristics:

• Produces gametes (egg and sperm)
• Develops from spores
• Undergoes mitosis during growth
• Usually much smaller in flowering plants

Examples:

• Pollen grain (male gametophyte)
• Embryo sac (female gametophyte)

Alternation of Generations

Alternation of generations is the regular alternation between the diploid sporophyte generation and the haploid gametophyte generation during the life cycle of plants.

Sequence:

• Sporophyte (2n)
  • Meiosis
• Spores (n)
  • Mitosis
• Gametophyte (n)
  • Produces Gametes
• Fertilization
• Zygote (2n)
  • Mitosis
• New Sporophyte

Simple Life Cycle of a Flowering Plant

Step 1: Seed Formation

The mature plant produces flowers. The flower contains:

• Stamen (male reproductive organ)
• Carpel/Pistil (female reproductive organ)

Step 2: Pollination

Pollination is the transfer of pollen grains from the anther to the stigma.

Types:

• Self-pollination
• Cross-pollination

Step 3: Fertilization

The pollen tube grows into the ovule. The male nucleus fuses with the egg nucleus to form the zygote.

In flowering plants, double fertilization occurs:

• One sperm + egg → Zygote
• Second sperm + polar nuclei → Endosperm

Step 4: Seed and Fruit Formation

After fertilization:

• Ovule becomes seed
• Ovary becomes fruit
• Zygote develops into embryo

Step 5: Seed Dispersal

Seeds are dispersed through:

• Wind
• Water
• Animals
• Explosion
• Humans

Step 6: Germination

When conditions become favorable:

• Water enters the seed.
• Enzymes become active.
• Embryo resumes growth.
• Radicle emerges first.
• Plumule develops later.

Step 7: Seedling Development

The seedling develops:

• Roots
• Stem
• Leaves

Photosynthesis begins.

Step 8: Mature Plant

The mature plant:

• Flowers
• Produces fruits
• Produces seeds

The cycle begins again.

Factors Necessary for Germination

A seed requires:

• Water
• Oxygen
• Suitable temperature
• Viable embryo

Some seeds additionally require:

• Light
• Darkness
• Scarification
• Breaking of dormancy

Importance of the Plant Life Cycle

• Ensures continuity of plant species.
• Produces genetic variation.
• Enables adaptation.
• Maintains biodiversity.
• Supports ecosystem stability.

Key Terms to Memorize

• Sporophyte
• Gametophyte
• Spore
• Gamete
• Fertilization
• Pollination
• Germination
• Double fertilization
• Alternation of generations

2. Duration of Life of Plants

Definition

The duration of life is the length of time a plant completes its entire life cycle - from germination to flowering, fruiting, seed production, and death.

Plants are grouped into:

• Annuals
• Biennials
• Perennials

A. Annual Plants

Definition: Annual plants complete their entire life cycle within one growing season or one year.

They:

• Germinate
• Grow
• Flower
• Produce seeds
• Die

All within a single year.

Characteristics:

• Fast-growing
• Produce many seeds
• Usually herbaceous
• Short lifespan

Examples:

• Rice
• Maize
• Wheat
• Groundnut
• Beans
• Tomato
• Pepper
• Okra
• Sunflower

Advantages:

• Quick food production.
• Easy cultivation.
• Rapid reproduction.

Disadvantages:

• Must be replanted yearly.
• Short lifespan.

B. Biennial Plants

Definition: Biennial plants complete their life cycle in two growing seasons.

First Year:

• Vegetative growth
• Leaves
• Stem
• Root development
• Food is stored.

Second Year:

• Flowering
• Fruit production
• Seed production

The plant dies afterward.

Examples:

• Carrot
• Beetroot
• Cabbage
• Onion
• Parsley
• Celery

Characteristics:

• Require two seasons.
• Store food during the first year.
• Reproduce during the second year.

C. Perennial Plants

Definition: Perennial plants live for more than two years. Many survive for decades or even centuries.

Characteristics:

• Long-lived
• Flower repeatedly
• Woody or herbaceous
• Extensive root systems
• Produce seeds many times

Examples:

• Mango
• Orange
• Coconut
• Oil palm
• Cocoa
• Rubber
• Guava
• Cashew
• Neem
• Mahogany
• Iroko

Advantages:

• Continuous production.
• Soil conservation.
• Carbon storage.
• Habitat for wildlife.

Comparison of Life Duration

Feature	Annual	Biennial	Perennial
Life span	One year	Two years	More than two years
Flowering	First year	Second year	Many years
Seed production	Once	Once	Repeatedly
Death	After seed production	After second year	After many years

3. Growth Forms (Plant Habits)

Definition

Growth form refers to the general external appearance or structural habit of a plant, determined by its stem, branching pattern, height, and adaptation to the environment.

Plants are classified into:

• Herbs
• Shrubs
• Trees
• Climbers
• Creepers

A. Herbs

Definition: Herbs are small plants with soft, green, non-woody stems.

Characteristics:

• Short-lived or perennial
• Soft stem
• Flexible
• Small size
• Little branching

Examples:

• Spinach
• Lettuce
• Basil
• Mint
• Coriander
• Rice
• Maize
• Wheat

Ecological Importance:

• Food source
• Medicinal value
• Ground cover
• Soil protection

B. Shrubs

Definition: Shrubs are medium-sized woody plants with several stems arising near the base.

Characteristics:

• Woody
• Multiple stems
• Medium height (typically 1-5 m)
• Long-lived

Examples:

• Hibiscus
• Rose
• Bougainvillea
• Cotton
• Coffee

Importance:

• Ornamental purposes
• Soil conservation
• Habitat for birds and insects

C. Trees

Definition: Trees are large woody plants with a single main trunk and a broad crown.

Characteristics:

• Tall
• Single trunk
• Deep roots
• Long lifespan
• Extensive branching above ground

Examples:

• Mango
• Neem
• Mahogany
• Iroko
• Cashew
• Oil palm
• Coconut

Importance:

• Timber production
• Oxygen production
• Carbon sequestration
• Shade
• Habitat for wildlife
• Climate regulation

D. Climbers

Definition: Climbers are plants with weak stems that require external support to grow upward.

Methods of Climbing:

• Tendrils
• Twining stems
• Hooks
• Adventitious roots

Examples:

• Yam
• Grapevine
• Passion fruit
• Pea
• Bean

Importance:

• Efficient use of sunlight
• Food production
• Ornamental value

E. Creepers

Definition: Creepers are plants with very weak stems that spread horizontally along the ground because they cannot support their own weight.

Characteristics:

• Weak stems
• Grow along the soil surface
• Produce runners or trailing stems

Examples:

• Pumpkin
• Watermelon
• Sweet potato
• Melon
• Cucumber

Importance:

• Reduce soil erosion
• Cover bare ground
• Conserve soil moisture

Summary Table of Growth Forms

Growth Form	Stem Type	Height	Examples
Herbs	Soft, green	Short	Maize, Rice, Spinach
Shrubs	Woody, many stems	Medium	Hibiscus, Rose
Trees	Woody, one trunk	Tall	Mango, Neem
Climbers	Weak stem with support	Variable	Yam, Grapevine
Creepers	Weak stem on ground	Low	Pumpkin, Watermelon

Ecological Significance of Different Growth Forms

Different growth forms enable plants to:

• Occupy different ecological niches.
• Reduce competition for light, water, and nutrients.
• Adapt to diverse environmental conditions.
• Increase biodiversity within ecosystems.
• Provide food and shelter for animals.
• Protect soil from erosion.
• Contribute to nutrient cycling and ecosystem stability.

Examination Tips (Important Points to Memorize)

Definitions

• Plant life cycle
• Alternation of generations
• Sporophyte
• Gametophyte
• Annual plant
• Biennial plant
• Perennial plant
• Growth form (plant habit)

Classification by Duration of Life

• Annual: Completes life cycle in one year.
• Biennial: Completes life cycle in two years.
• Perennial: Lives for more than two years and reproduces repeatedly.

Major Growth Forms

• Herbs
• Shrubs
• Trees
• Climbers
• Creepers

Stages of the Plant Life Cycle

1. Seed formation
2. Pollination
3. Fertilization
4. Fruit and seed development
5. Seed dispersal
6. Germination
7. Seedling growth
8. Mature plant and reproduction

Key Biological Processes

• Meiosis in the sporophyte produces spores.
• Mitosis in the gametophyte produces gametes.
• Fertilization restores the diploid condition and forms the zygote.
• Flowering plants exhibit double fertilization, a distinctive feature of angiosperms.