Life Cycle of Pteridophytes

Pteridophytes are also known as the vascular amphibians/ reptiles of the plant kingdom.

Habitat

• Limited to narrow geographical regions.
• Prefer cool, shady and damp places.
• Require water for fertilization.

Life cycle

Diplo-haplontic life cycle.

1. The sporophyte (2n) bears sporangia on its sporophylls. The spore mother cells inside them divide by meiosis to form haploid spores (n).
2. The spores germinate to form a small, multicellular, free-living, mostly photosynthetic, thalloid gametophyte called the prothallus (n). It needs cool, damp and shady places to grow.
3. The prothallus bears the male sex organs (antheridia) and the female sex organs (archegonia). The antheridia release the antherozoids (n).
4. Water is required for the antherozoids to swim to the mouth of the archegonium.
5. An antherozoid fuses with the egg (n) in the archegonium, forming a diploid zygote (2n).
6. The zygote develops into an embryo, which grows into a new multicellular, well-differentiated sporophyte (2n) – the dominant phase. The young sporophyte is attached to the prothallus at first; the prothallus later dies, and the sporophyte becomes independent.

Body structure

Sporophyte

• Main, photosynthetic, independent plant body.
• Well differentiated into true roots, stem and leaves.
• Vascular tissues, i.e. xylem and phloem, are present (first tracheophytes).
• Stem – often modified into underground rhizomes.
• Roots – adventitious roots arise from the rhizome.
• Leaves – leaves of ferns show circinate vernation.
  • Small-sized – microphyllous. E.g. Selaginella, Equisetum.
  • Large-sized – macrophyllous. E.g. Salvinia, ferns.

Gametophyte

• Multicellular, reduced, inconspicuous and heart-shaped, called the prothallus.
• Photosynthetic, and thus an independent plant body.
• Dorsiventrally flat.
• Homosporous species – produce monoecious gametophytes.
• Heterosporous species – produce dioecious gametophytes.

Reproduction

Gamete formation

• The gametophyte bears the antheridium and archegonium (male and female sex organs respectively) and is responsible for the formation of gametes.
• Male gametes – antherozoids:
  • Biflagellate/ multiflagellate.
  • Spirally coiled.
• Female gametes – egg/ oosphere:
  • Large, non-motile.

Spore formation

• Leaves modified to produce haploid spores by meiosis are called sporophylls.
• Process of formation of spores – sporogenesis.
• Site of spore formation – sporangium.
• Cells responsible for spore formation – Spore Mother Cells.
• Most pteridophytes are homosporous, i.e. they produce only one type of spore.
• A few pteridophytes are heterosporous, i.e. they produce 2 different types of spores (microspores and megaspores).
  • Homosporous species – Pteris, Dryopteris, Lycopodium, Equisetum.
  • Heterosporous species – Selaginella, Salvinia, Marsilea, Azolla, Isoetes.

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