ch 45

Plant Reproduction and Development

45.1 How Do Plants Reproduce?

Plants reproduce through both sexual and asexual mechanisms, with their life cycles alternating between diploid and haploid stages. This alternation of generations is fundamental to plant biology and underpins the diversity of plant reproductive strategies.

• Asexual Reproduction: New plants are produced by mitosis, resulting in offspring genetically identical to the parent.

• Sexual Life Cycle: Plants alternate between diploid sporophyte and haploid gametophyte stages.

  • The sporophyte produces spores via meiosis; these spores develop into gametophytes.

  • Mosses and ferns: Gametophyte is independent and dominant in mosses; ferns also have independent gametophytes.

  • Gymnosperms and angiosperms: Sporophyte is dominant; gametophyte is not independent.

  • In angiosperms, the male gametophyte is the pollen grain, and the female gametophyte is the egg.

• Alternation of Generations: The plant life cycle alternates between two distinct forms:

  • Diploid sporophyte (2n)

  • Haploid gametophyte (n)

• Example: In mosses, the green, leafy gametophyte is the dominant form, while in flowering plants, the sporophyte is the visible plant.

45.2 Functions and Structures of Flowers

Flowers are specialized reproductive structures in angiosperms, adapted to attract pollinators and facilitate sexual reproduction, anatomy is key to understanding plant reproduction.

• Pollination: Most flowers attract animals for pollination; wind pollination is less efficient.

• Flower Structure: Complete flowers have four sets of modified leaves:

  • Sepals: Protect the flower bud; differ in monocots and dicots.

  • Petals: Attract pollinators.

  • Stamens: Male reproductive structures; consist of filament (stalk) and anther (produces pollen).

  • Carpels: Female reproductive structures; include stigma (sticky, receives pollen), style (tube), and ovary (contains ovules).

• Incomplete Flowers: Lack one or more floral parts; e.g., grasses lack petals and sepals, zucchini may lack stamens or carpels.

• Pollen Grain: The male gametophyte; formed in anthers from microspore mother cells via meiosis and mitosis.

• Female Gametophyte: Forms within the ovule from a megaspore mother cell; only one megaspore survives and undergoes mitosis to produce eight nuclei.

• Example: The ovary develops into fruit, and the mature ovule becomes a seed.

45.2 Pollen and Fertilization

• Pollen Formation: Each anther contains microspore mother cells; meiosis produces four haploid microspores, which undergo mitosis to form tube and generative cells.

• Pollination: Occurs when pollen lands on the stigma; triggers formation of pollen tube.

• Double Fertilization:

  • One sperm fertilizes the egg, forming a diploid zygote.

  • One sperm fuses with two nuclei to form the triploid endosperm.

• Equation: $\text{Egg} + \text{Sperm} \rightarrow \text{Zygote}$

• Equation: $\text{Central Cell} + \text{Sperm} \rightarrow \text{Endosperm}$

45.3 Fruit and Seed Development

After fertilization, the ovary develops into a fruit, and the ovule becomes a seed. Fruits and seeds are adapted for dispersal and survival.

• Fruit: Develops from the ovary; may be fleshy, hard, winged, or spiked to aid dispersal.

• Seed: Develops from the ovule; consists of seed coat, endosperm, and embryo.

• Seed Coat: Formed from thickened integuments.

• Endosperm: Triploid tissue formed by double fertilization; stores nutrients.

• Embryo: Develops from zygote; differentiates into shoot and root.

• Cotyledons: Seed leaves; monocots have one, dicots have two.

• Example: Beans (dicots) have two cotyledons; corn (monocot) has one.

45.3 Table: Seed Structure

45.4 Seed Germination and Growth

Germination is the process by which a seed develops into a seedling. Dormancy ensures seeds germinate under favorable conditions.

• Dormancy: Period during which seeds do not germinate; ensures timing is optimal.

• Germination Requirements: Adequate temperature, moisture, and sometimes drying, cold, or seed coat disruption.

• Germination Process:

  • Embryo absorbs water, swells, and breaks seed coat.

  • Root emerges first, followed by shoot.

  • Dicots: Hypocotyl hook and epicotyl hook guide shoot emergence.

  • Monocots: Coleoptile protects shoot tip.

• Example: In beans, the cotyledons nourish the seedling; in corn, the coleoptile protects the shoot.

45.5 Plant-Pollinator Interactions

Plants and their pollinators have coevolved, resulting in diverse adaptations for pollination. Flowers may provide food, act as mating decoys, or serve as nurseries for pollinators.

• Coevolution: Mutual adaptations between plants and pollinators.

• Food for Pollinators: Nectar and pollen; flowers may have UV markings visible to bees.

• Specialized Flowers: Tube-shaped flowers for butterflies/moths; foul-smelling flowers for scavenging insects; red/orange tubular flowers for hummingbirds.

• Mating Decoys: Orchids mimic female wasps to attract males for pollination.

• Nurseries: Yucca moths pollinate and lay eggs in yucca flowers; offspring feed on seeds but do not destroy all.

• Example: Bees pollinate flowers with UV patterns; hummingbirds pollinate tubular flowers.

45.6 Fruit and Seed Dispersal

Plants have evolved various mechanisms to disperse seeds, increasing their chances of survival and colonization.

• Animal Dispersal: Clingy fruits stick to fur; edible fruits are eaten and seeds are dispersed.

• Explosive Dispersal: Some fruits, like mistletoe, shoot seeds away from the parent plant.

• Wind Dispersal: Lightweight fruits (dandelions, milkweeds, elms) are carried by wind; maples have propeller-like fruits.

• Water Dispersal: Floating fruits (coconuts) can travel long distances.

• Example: Dandelion seeds are dispersed by wind; coconuts by water.

Key Terms

• Alternation of generations: Life cycle alternating between diploid and haploid stages.

• Anther: Pollen-producing part of stamen.

• Carpel: Female reproductive organ of flower.

• Coleoptile: Protective sheath in monocot seedlings.

• Complete flower: Flower with all four floral parts.

• Cotyledon: Seed leaf in embryo.

• Dormancy: Period of inactivity in seeds.

• Double fertilization: Two sperm fertilize egg and central cell.

• Endosperm: Nutrient-rich tissue in seeds.

• Epicotyl hook: Part of dicot shoot.

• Filament: Stalk of stamen.

• Flower: Reproductive structure of angiosperms.

• Fruit: Mature ovary containing seeds.

• Gametophyte: Haploid stage producing gametes.

• Generative cell: Forms sperm cells in pollen.

• Germination: Growth of seed into seedling.

• Hypocotyl hook: Part of dicot seedling.

• Imperfect flower: Lacks stamens or carpels.

• Incomplete flower: Lacks one or more floral parts.

• Integument: Protective layer around ovule.

• Megaspore: Haploid cell from megaspore mother cell.

• Megaspore mother cell: Diploid cell in ovule.

• Microspore: Haploid cell from microspore mother cell.

• Microspore mother cell: Diploid cell in anther.

• Ovary: Base of carpel; contains ovules.

• Ovule: Structure containing female gametophyte.

• Petal: Modified leaf attracting pollinators.

• Pollen grain: Male gametophyte.

• Pollination: Transfer of pollen to stigma.

• Seed: Mature ovule containing embryo.

• Seed coat: Protective layer of seed.

• Sepal: Modified leaf protecting bud.

• Spore: Haploid cell from sporophyte.

• Sporophyte: Diploid stage producing spores.

• Stamen: Male reproductive organ.

• Stigma: Sticky tip of carpel.

• Style: Tube connecting stigma to ovary.

• Tube cell: Forms pollen tube.

• Zygote: Fertilized egg cell.