Lecture Notes on Ferns

Introduction to Ferns

• Ferns have long been appreciated for their beauty, shelter, and food.
• Young coiled fronds are depicted in art (e.g., Koru).
• Around 12,000 species exist globally, ranging from tiny to tree-sized.
• Ferns reproduce by producing spores.

Structure of Ferns

• Familiar fern is known as a sporophyte.
• Saurus: Small circular areas on the underside of fern fronds that contain spores.
  • Some sauri are unprotected; others have a protective covering called endosium.
  • Endosium can be cup-shaped.

Sporangium

• Contains numerous sac-like structures called sporangia.
• Outer wall: protective jacket cells.
• As sporangium matures:
  • Jacket cells enlarge to form a band called annulus.
  • Thin outer wall; delicate lip cells on the opposite side.
  • Below jacket: two layers of cells called tapetum, nurturing sporocyte cells.

Sporocyte Cells

• Diploid cells (two chromosome sets).
• Undergo meiosis to produce haploid daughter nuclei.
• Cytoplasm cleavage results in tetrad (four adhering cells).
• Tapetum breaks down, forming a tough protective coat called sporopolinone, resulting in spores.

Spore Release Process

• Sporangium opens via annulus contraction due to water evaporation.
• The release of spores occurs rapidly once the annulus springs forward.
• Thousands of spores released from each leaf.

Germination and Gametophyte Formation

• Spores germinate on moist surfaces, forming bisexual gametophytes (prothallus).
• Rhizoid: structure that anchors the gametophyte to the soil.
• Notch meristem: dividing cells forming at the notch of the prothallus; leads to heart-shaped gametophyte.
• Male and female gametangia form:
  • Anthuridia (male) at posterior end.
  • Archegonia (female) at anterior end.

Fertilization Process

• Antheridiogen hormone stimulates male gametophyte development.
• Water triggers sperm release from antheridia.
• Sperm swims towards archegonia due to attraction from neck canal contents.
• Fertilization creates a diploid zygote, attached to the gametophyte.

Development of the Sporophyte

• Zygote divides to form embryo's sporophyte plant.
• Embryo components:
  • Foot: absorbs nutrients.
  • Root: grows into soil.
  • Leaf: grows through notch to sunlight.
  • Rhizome: provides additional leaves and roots.

Conclusion

• The fern reproductive cycle involves two plants: the large sporophyte and the tiny gametophyte.
• Spores allow for long-distance travel; gametes provide genetic variability, aiding fern evolution.