Life Cycles of Selected Plants

Life cycle and alternation of generations in plants

Plants undergo two major phases during their life cycle:

1. Vegetative phase: The plant grows and develops without reproducing sexually.
2. Reproductive phase: The plant sets flowers for sexual reproduction, which involves the production of gametes.

This cycle is common across all flowering plants, regardless of their form (e.g., herbs, grasses, trees, or shrubs) or life span (annual, biennial, or perennial). Even lower plants like bryophytes and ferns follow a similar cycle, although they do not produce flowers.

Alternation of generations in plants

In the life cycle of plants, there is an alternation between two distinct stages: the sporophyte generation and the gametophyte generation. The two generations alternate during the plant's life cycle, and the transition between them involves different stages of cell division, meiosis, and fertilization. The process ensures genetic variation and the continuity of the plant species.

Sporophyte generation (diploid phase)

• The sporophyte is the diploid (2n) phase of the plant's life cycle, meaning it contains two sets of chromosomes. It is the dominant phase in most plant species, including vascular plants like ferns, gymnosperms, and angiosperms.
• The sporophyte produces spores through meiosis, a type of cell division that reduces the chromosome number by half, producing haploid (n) cells.
• In seed plants, such as gymnosperms and angiosperms, the sporophyte is the mature plant (trees, shrubs, flowers, etc.), which consists of leaves, stems, roots, and reproductive organs.

Gametophyte generation (haploid phase)

• The gametophyte is the haploid (n) phase of the plant's life cycle. It is formed from the germination of a spore produced by the sporophyte.
• The gametophyte produces gametes (sperm and egg cells) through mitosis, which are genetically identical to the parent plant.
• In non-vascular plants like mosses and liverworts, the gametophyte generation is the dominant phase. It is often more visible and larger than the sporophyte.
• In vascular plants like ferns, the gametophyte is small and inconspicuous. It is often a single cell or a few cells that develop from the spore.

Bryophytes (mosses, liverworts, hornworts)

• Bryophytes are non-vascular plants, meaning they lack specialized tissue for water and nutrient transport (like xylem and phloem).
• The gametophyte is the dominant and visible phase. It consists of leafy structures that perform photosynthesis.
• The sporophyte is dependent on the gametophyte for nutrients. It consists of a stalk and a capsule that produces spores.
• Fertilization: Water is essential for fertilization in bryophytes because the sperm are flagellated and must swim through water to reach the egg in the archegonium (female structure).

Pteridophytes (ferns, horsetails)

• Pteridophytes are vascular plants that include ferns, horsetails, and clubmosses.
• The sporophyte is the dominant phase and consists of a large plant with fronds (leaves) and rhizomes (underground stems).
• The gametophyte is reduced to a small, heart-shaped structure called the prothallus, which produces both male and female gametes.
• Spore Production: Sporangia, the spore-producing structures, are typically found on the underside of the fronds.
• Fertilization: Like bryophytes, ferns require water for fertilization because the sperm need to swim to the egg.

Angiosperms (flowering plants)

• In angiosperms, the sporophyte is the mature plant (trees, shrubs, or flowers).
• The reproductive organs of angiosperms are found in the flowers, which consist of male and female parts.
  • The male part is the stamen, consisting of the anther and filament, which produces pollen (male gametes).
  • The female part is the pistil or carpel, consisting of the stigma, style, and ovary, which contains the ovules (female gametes).
• Pollination: Pollination is the transfer of pollen from the anther to the stigma. This can occur through wind, insects, birds, or water.
• Fertilization: After pollination, the sperm cells travel through the pollen tube to reach the egg cell in the ovule. One sperm fertilizes the egg, and the other fertilizes the polar nuclei to form the endosperm (which nourishes the developing embryo).
• After fertilization, the fertilized egg (zygote) develops into an embryo inside the seed, and the ovary becomes the fruit.