Life Cycle of Plants: Sexual & Asexual Reproduction
Life Cycle of Plants
Sexual Reproduction
Plants are formed from the union of two specialized cells called gametes during fertilization to create a zygote.
Advantages:
- Genetic diversity through recombination.
- Different genotypes in each generation.
Asexual Reproduction
New plants develop from one or more cells of a progenitor plant.
Disadvantage:
- Produces clones with identical genotypes to the parent.
Advantage:
- Faster reproduction requiring only one individual.
- Effective colonization.
Haploid and Diploid Phases
Haploid Phase (Gametophyte): Begins and ends with haploid spores, leading to gamete formation. Gametes include male sperm and female egg cells.
Diploid Phase (Sporophyte): Starts with the diploid zygote and ends with meiosis producing haploid spores in sporangia.
Life Cycle of Spermatophytes
Spermatophytes are vascular plants with seeds inhabiting terrestrial habitats.
Gymnosperms (e.g., Pine)
Gymnosperms are wind-pollinated plants with naked seeds.
Male Cones: Small cones grouped together, each with microsporophylls carrying pollen sacs. Meiosis in the pollen sacs produces pollen grains, initiating the male gametophyte.
Female Cones: Larger cones with spirally arranged seed scales, each containing ovules with a nucellus, integument, and micropyle.
Pollination: Wind carries pollen grains to the ovules. The pollen grain germinates and sends a pollen tube through the micropyle.
Fertilization: One megaspore undergoes meiosis and forms the female gametophyte with archegonia containing egg cells. The pollen tube reaches the female gametophyte, releasing sperm cells to fertilize the egg, forming a zygote that develops into an embryo within the seed.
Angiosperms
Angiosperms are plants with flowers and seeds enclosed in fruits.
The Flower
Formed at the shoot apical meristem, the flower is essential for sexual reproduction. It comprises:
- Calyx: Formed by sepals.
- Corolla: Formed by petals.
- Perianth: Calyx and corolla combined.
- Androecium: Formed by stamens.
- Gynoecium: Formed by one or more carpels.
Flower Types
- Hermaphrodite: Contains both stamens and carpels.
- Unisexual: Has either stamens (male) or carpels (female).
- Monoecious: Both male and female flowers on the same plant.
- Dioecious: Separate male and female plants.
- Polygamous: Unisexual and hermaphrodite flowers on the same plant.
- Apetalous: Lacking petals.
- Asepalous: Lacking sepals.
- Naked: Lacking both petals and sepals.
Angiosperm Life Cycle
Gamete Formation: Meiosis in the ovule produces megaspores, one of which develops into the female gametophyte. In the anthers, meiosis produces microspores that develop into pollen grains (male gametophytes).
Pollination: Transfer of pollen from anther to stigma. Self-pollination occurs within the same flower or plant, while cross-pollination occurs between different plants.
Double Fertilization: One sperm fertilizes the egg cell to form the zygote, while the other fuses with polar nuclei to form the endosperm.
Seed Development: The zygote develops into the embryo, the endosperm provides nutrients, and the integuments form the seed coat.
Fruit Formation: The ovary develops into the fruit, which encloses the seeds.
Seed Germination: The radicle emerges first, followed by the shoot. Epigeal germination has the cotyledons above ground, while hypogeal germination has them below ground.
Asexual Reproduction Structures
- Bulbs: Short stems with fleshy leaves storing reserves.
- Corms: Solid underground stems.
- Stolons: Horizontal stems producing new plants.
- Rhizomes: Underground stems storing reserves.
- Tubers: Thickened rhizomes storing reserves.
Asexual Reproduction in Agriculture
- Cuttings: Propagating plants from stem or leaf cuttings.
- Grafting: Joining two plant parts together.
- Layering: Rooting a stem while still attached to the parent plant.
New Techniques
In vitro culture includes cell culture, tissue culture, and apical meristem culture.