Plant Life Cycles

Plant Life Cycles: Alternation of Generations

• Plants have a life cycle with two multicellular stages: haploid and diploid.
• They alternate between these generations, which is called the alternation of generations.
• Gametophyte: Haploid stage (one set of chromosomes).
• Sporophyte: Diploid stage (two sets of chromosomes).
  • The gametophyte produces haploid gametes via mitosis.
  • Male gametophytes produce male gametes, and female gametophytes produce female gametes.
• Fertilization: Gametes fuse to form a diploid zygote.
• The zygote undergoes mitotic divisions to become a diploid sporophyte.
• The sporophyte produces spores via meiosis (haploid).
• Spores give rise to new gametophytes via mitosis, continuing the cycle.

Evolution and Dominance of Generations

• Simpler plants exhibit a dominant gametophyte stage, which is more prominent and long-living.
• As plants evolved and became more complex, the dominance of the gametophyte reduced.

Bryophytes (Mosses):

• Simplest plants with a dominant and independent gametophyte.
• The main plant with leaf-like, root-like, and stem-like structures is the gametophyte (haploid).
• Gametophyte contains sex organs producing male and female gametes (haploid).
• Fertilization results in a diploid zygote.
• The zygote develops into a short-lived, barely noticeable sporophyte, which depends on the gametophyte for nutrients, water, and shelter.
• The sporophyte (2n) is attached to the gametophyte.
• The mature sporophyte produces haploid spores via meiosis. These spores are released and develop into new gametophytes via mitosis under ideal conditions.

Pteridophytes (Ferns)

• The sporophyte generation is dominant and independent.
• The main plant with leaves, roots, and stems is the diploid generation sporophyte.
• Sporophyte produces haploid spores via meiosis.
• Each spore divides mitotically to form a tiny (1 cm) independent haploid gametophyte.
• Gametophytes use thread-like structures called rhizoids to gather water and minerals from the soil.
• The gametophyte bears sex organs, producing male and female gametes.
• Gametes fuse (fertilization) to form a diploid zygote.
• The zygote develops into a diploid embryo, which further differentiates into roots, stems, and leaves of the sporophyte.

Gymnosperms and Angiosperms (Seed Plants):

• The sporophyte remains dominant and independent.
• Seed plants produce seeds that protect and nourish the embryo.
• The gametophyte generation is extremely reduced and found inside the sporophyte.
• Germination: A seed develops into a new sporophyte under the right conditions.

Gymnosperms (Conifers)

• Gametophyte generation occurs inside cones (on the sporophyte).
• Haploid spores form inside cones and develop into haploid gametophytes.
• Male cones contain male spores that develop into male gametophytes with male gametes.
• Female cones contain female spores that develop into female gametophytes with female gametes.
• Male gametes are transferred to the female cone for fertilization.
• Fusion of haploid gametes results in a diploid zygote.
• Zygote develops into an embryo inside a seed on the scales of the cone.

Angiosperms (Flowering Plants):

• The gametophyte generation occurs inside diploid flowers on the diploid sporophyte.
• Flowers have sex organs producing haploid spores via meiosis denoted as M.
• Haploid spores develop into haploid gametophytes via mitosis denoted as M.
• Male sex organs produce haploid male spores that develop into male gametophytes with male gametes.
• Female sex organs produce haploid female spores that develop into female gametophytes with female gametes.
• Gametes fuse (fertilization) to form a diploid zygote.
• The zygote develops into an embryo within a seed via mitotic divisions.
• The seed germinates into a new sporophyte under ideal conditions, and the cycle continues.

Life Cycle Classifications

• Haploidyplontic Life Cycle: Most plants exhibit this.
  • Meiosis does not directly produce gametes.
  • The Diploid sporophyte (2n) produces haploid spores via meiosis
  • Spores undergo mitotic divisions to produce a multicellular haploid gametophyte.
  • The gametophyte produces male and female gametes.
  • Sporophyte is multicellular and diploid, while the gametophyte is multicellular and haploid.
• Diplontic Life Cycle:
  • Characterized by unicellular haploid gametes formed directly via meiosis.
  • Only the diploid stage is multicellular.
  • Example: Animals, including humans.
  • Human sex organs produce unicellular haploid gametes directly through meiosis, and from the zygote onwards, humans are multicellular and diploid.
• Haplontic Life Cycle: Primarily in algae.
  • Only the haploid stage is multicellular.
  • The main organism is haploid and produces haploid gametes via mitosis.
  • Gametes fuse to form a diploid zygote, which immediately undergoes meiosis to produce haploid spores.
  • Haploid spores undergo mitotic divisions to develop into the haploid organism.
  • The diploid zygote is unicellular, and the haploid organism is multicellular.
  • The haplontic life cycle is, in a way, the opposite of the diplontic life cycle.

Nonvascular Plants

• Plants turn carbon dioxide into oxygen.
• Plants evolved from a single species of algae that moved onshore about 1,200,000,000 years ago.
• Around 475 million years ago, the first simple plants evolved.
• Descendants of these early plants still exist today as nonvascular plants: liverworts, hornworts, and mosses.

Characteristics

• Nonvascular plants are less complex than vascular plants like orchids or oak trees.
• They have unique features and habits, especially in their reproductive systems.

Reproduction

• Nonvascular plants perfected the reproductive cycle inherited from algae, which is now used by all plants.
• Traces of this cycle are even found in animal reproductive systems.