Plant Reproduction Notes

Alternation of Generations

• Both asexual and sexual reproduction increase population size.

• Sexual reproduction introduces genetic variations (genotype and phenotype).

• Asexual reproduction is beneficial in stable environments for well-suited organisms.

• Sexual reproduction is more important in unstable environments.

• Every plant life cycle includes:

  • Alternation of Generations: A cycle alternates between a haploid (n) and a diploid (2n) phase. This involves cells with a specific number of chromosomes (haploid) alternating with cells having twice that number (diploid).

Sporophyte and Gametophyte

• Sporophyte phase ends with spore production via meiosis in sporangia (singular: sporangium).

• Gametophyte phase ends with gamete production via mitosis in gametangia (singular: gametangium).

• Types of Gametangia:

  • Archegonia (singular: archegonium): Egg-producing gametangia in mosses and ferns.

  • Antheridia (singular: antheridium): Sperm-producing gametangia in mosses and ferns.

Plant Life Cycles

• Mosses: Some believe they evolved from algae, others from ferns.

  • Unique among land plants due to dominant gametophyte generation.

  • Sporophyte depends on gametophyte for nutrients and support.

  • Sporophyte's sole function is spore production, which germinate into gametophytes.

• Ferns:

  • Dominant sporophyte generation.

  • Gametophyte is a small, vulnerable, and transitory phase (heart-shaped prothallus).

Fern Life Cycle

• Spores are released from sporangium.

• Spores develop into a prothallus (gametophyte).

• The gametophyte produces antheridia (sperm) and archegonia (eggs).

• Sperm fertilizes the egg, forming a zygote.

• Zygote develops into an embryo (young sporophyte).

• The embryo grows into a mature sporophyte (fern plant), with fronds, rhizomes and sori (clusters of sporangia).

Angiosperms and Gymnosperms

• Dominant sporophyte generation (the visible plant).

• Gametophytes are greatly reduced and often not visible.

• Mosses and ferns disperse via mature spores, developing into independent gametophytes, susceptible to damage.

• Seed plants retain gametophytes within the flower.

• Female gametophyte (embryo sac) remains within the parent sporophyte.

• Male gametophyte (pollen grain) is released during pollination.

  • Immature pollen grain: Two nuclei (generative and tube).

  • Mature pollen grain: develops after pollination.

• Pollination: Pollen grains are transferred to the stigma.

Pollination and Fertilization

• After pollination, the tube nucleus forms a pollen tube, growing down the style to the ovary and ovule.

• The generative nucleus divides (mitosis) into two sperm nuclei.

• The pollen tube releases the sperm nuclei into the embryo sac.

Double Fertilization

• Occurs in the embryo sac (female gametophyte) within the ovule inside the ovary.

• Embryo sac components: egg and two polar nuclei.

• One sperm nucleus + egg = zygote.

• Other sperm nucleus + two polar nuclei = triploid (3n) endosperm.

• The triploid endosperm nourishes the zygote and developing embryo.

• Zygote develops into an embryo.

• Ovule becomes the seed; ovary becomes the fruit.

Seed Structure

• Mature seed contains an embryo with cotyledons and a seed coat.

• Cotyledons absorb endosperm.

• Embryo parts:

  • Radicle: Develops into the root system.

  • Hypocotyl: Transition zone between root and stem.

  • Epicotyl: Develops into stem, leaves, and flowers.

• Monocots: Seeds with one cotyledon (e.g., corn, grasses).

• Dicots: Seeds with two cotyledons (e.g., bean).

Fruit Development and Seed Dispersal

• Fruits develop from the ovary after pollination.

• Ovary wall expands to accommodate developing seeds.

• Major function of fruit: seed dispersal.

• Dispersal Methods:

  • Fleshy fruits: Dispersed by animals via feces.

  • Dry fruits: Dispersed by wind (e.g., dandelion with parachute-like top).

  • Fruits with barbs: dispersed via attachment to animal fur.

Asexual Reproduction

• Reproduction without seeds or gametes (sperm and egg).

• No fertilization, less genetic variation.

• Offspring are genetically identical to the parent plant.

• Asexual Reproduction Methods:

  • Tubers: potatoes.

  • Runners/Stolons: horizontal stems (e.g., strawberries).

  • Rhizomes: underground stems (e.g., ferns, bamboo).

  • Bulbs: short stems with storage leaves (e.g., tulip, onion).