Lecture 4 - Plant Reproduction and Dispersal

Lecture Overview

  • Lecture Topic: Plant Reproduction and Dispersal (BS1007/AG1007)

  • Key Areas Covered:

    • What is alternation of generations?

    • Pollination: Definitions and Types

    • Double fertilization and fruit development

    • Dispersal: Definitions and Syndromes

Green Plants Classification

  • Taxonomical Hierarchy:

    • Streptophyta

    • Land plants

    • Bryophytes

    • Tracheophytes

      • Euphyllophytes

      • Seed plants

        • Mosses

        • Hornworts

        • Lycophytes

        • Ferns and Allies

        • Gymnosperms

        • Angiosperms

  • Green Algae:

    • Red Algae

    • Chlorophytes

    • Charophytes

  • Ancestral Alga and Evolution:

    • Evolution of terrestrial plants is closely linked to the advent of pollination.

    • Plants that do not use pollen typically rely on spores for reproduction.

Alternation of Generations

  • Definition:

    • The life cycle of land plants includes an alternation between a haploid (n) generation and a diploid (2n) generation.

  • Chromosome Sets:

    • Haploid (n) => 1 set of chromosomes

    • Diploid (2n) => 2 sets of chromosomes

  • Dominant Generations:

    • Bryophytes: Dominant gametophyte generation

    • Other groups (Lycophytes, Pterophytes, Seed plants): Dominant sporophyte generation

    • Function: 2n sporophyte produces n spores by meiosis

  • Bryophytes Characteristics:

    • Mature sporophyte small (dependent)

    • Produces n spores via meiosis.

  • Further Classification:

    • Lycophytes and Pterophytes: Sporophyte is the dominant generation.

    • Seed plants: Separation of male (pollen) and female (ovules) reproductive parts.

  • Angiosperms:

    • Sporophyte is the dominant generation; gametophyte is small (8 cells), enclosed within ovary inside ovule.

    • Female gametophyte contains endosperm which nourishes the embryo sac.

    • Male gametophyte forms the pollen tube and divides to form 2 sperm cells.

Pollination

  • Definition:

    • The process by which pollen is placed on the stigma of a flower.

  • Types of Pollination:

    • Self-Pollination:

    • Pollen from a flower’s anther pollinates its own stigma.

    • This may happen between flowers on the same plant.

    • Cross-Pollination (Outcrossing):

    • Pollen from one plant pollinates a flower on another plant.

    • Often relies on pollinator attraction (e.g., bees, birds, moths).

  • Historical Context:

    • Early seed plants primarily relied on wind for pollination.

  • Advantages of Outcrossing:

    • Benefits for eukaryotic organisms include genetic diversity.

    • Frequency of self-pollination in scarce pollinator environments leads to better adaptation.

Strategies Promoting Outcrossing

  • Methods to Enhance Genetic Diversity:

    1. Separation of Stamens and Pistils:

    2. Dioecious plants:

    • Only produce ovules or only pollen.

    1. Self-incompatibility:

    • Mechanism preventing self-fertilization.

    1. Dichogamy:

    • Maturity of stamens and pistils at different times.

Types of Pollination

  • Modes of Pollination:

    • Hydrophily (Water Pollination):

    • Rare (<1% of all plants)

    • Anemophily (Wind Pollination):

    • Gymnosperms and 12% of angiosperms.

    • Zoophily (Animal Pollination):

    • Responsible for >87% of angiosperms (up to 98% in rainforests).

    • Includes insect (entomophily), bird, and bat pollination.

Co-evolution of Plants and Pollinators

  • Adaptations Agreeing with Pollination Syndromes:

    • Plants evolve traits to attract specific pollinators, ensuring successful pollen transfer.

    • Pollinators are primarily herbivores seeking food, not intentionally pollinating.

    • Rewards Offered by Plants:

    • Nectar (high in carbohydrates)

    • Pollen (rich in vitamins & proteins)

    • These traits aid in attracting pollinators crucial for plant reproduction.

Specific Pollinator Characteristics

  • Beetle-pollinated Flowers:

    • Typically early pollinators, often dull in color (white or green).

    • Moderate nectar producers.

  • Moth-pollinated Flowers:

    • Tend to be white or dull colored with strong scents and bloom at night.

  • Butterfly-pollinated Flowers:

    • Found in clusters, bright colors (red, yellow), and ample nectar.

  • Fly-pollinated Flowers:

    • Generally dull colors with a putrid odor, resembling carrion.

  • Bee-pollinated Flowers:

    • Ample nectar, blue/yellow colors, aromatic, and generally open during the daytime.

    • Crucial for pollinating 30% of the world’s crops and 90% of wild plants.

  • Bat-pollinated Flowers:

    • Typically large, pale, fragrant, open at night, and provide dilute nectar.

  • Bird-pollinated Flowers:

    • Tubular in shape, brightly colored, odorless, and nectar-rich.

Unique Plant-Pollinator Interactions

  • Bee Orchids (Ophrys):

    • Mimic female bees for sexual deception to ensure pollen transfer.

  • Hammer Orchids (Drakaea):

    • Display attributes that attract male Thynnid wasps following the same sexual deception strategy.

Wind Pollination Characteristics

  • Wind-pollinated Flowers:

    • Lacking bright colors or nectar.

    • Typically small, with no petals and exposed reproductive organs.

    • Large quantities of smooth, easily airborne pollen.

Dispersal

  • Definition:

    • Seed dispersal refers to the mechanisms plants use to spread their seeds across the landscape.

  • Adaptations for Dispersal:

    • Plants must adapt to utilize biotic (living organisms) or abiotic (non-living) vectors.

  • Types of Dispersal Syndromes:

    • Anemochory: Wind dispersal.

    • Barochory: Gravity dispersal.

    • Hydrochory: Water dispersal.

    • Zoochory: Animal dispersal, which includes:

    • Endozoochory: Ingestion and defecation.

    • Synzoochory: Mouthpart interactions.

    • Epizoochory: Accidental attachment to animals.

Dispersal Strategies and Examples

  • Mymecochory (Ant Dispersal):

    • Critical in Australian ecosystems with approximately 1,500 plant species relying on this method.

  • Dipterocarps in Borneo:

    • Seeds are dispersed locally by wind and later by animals such as bearded pigs.

  • Mistletoe Birds:

    • Feed on sticky seeds, requiring them to land on a host tree to germinate successfully.

Summary Points

  • Alternation of Generations:

    • Switching between haploid (gametophyte) and diploid (sporophyte).

  • Dominance of Generations:

    • Bryophytes have a dominant gametophyte; most others have a dominant sporophyte.

  • Reproductive Strategies:

    • Bryophytes, Lycophytes, and Pterophytes utilize spores.

    • Gymnosperms and Angiosperms utilize seeds and pollen.

  • Key Aspects of Pollination:

    • Process of transferring pollen; includes strategies for self-pollination and cross-pollination.

    • Various types of pollination syndromes lead to co-evolution of plants and pollinators.

  • Double Fertilization:

    • Involves two fertilization events creating zygote (2n) and endosperm nucleus (3n).

  • Fruits:

    • Defined as mature ovaries, with fruits that can develop without seed formation.

  • Dispersal:

    • The movement of seeds influenced by type of dispersal and dispersal syndromes, critical for plant survival and spread.