Lecture Notes on Fern Reproduction and Biology

Introduction to Ferns

• Ferns have been valued throughout history for their beauty, shelter, and food.
• Young ferns have coiled fronds that are often featured in art.
• Commonly found in parks, gardens, and indoor spaces.
• Approximately 12,000 species of ferns exist worldwide.
• Size variation: from small (fingernail size) to large (tree height).

Reproduction in Ferns

• Ferns reproduce and spread through spores.
• Sporangium: The familiar fern is known as a sporophyte.
  • On the underside of fronds, small circular areas called sori can be seen.
  • Indusium: Protective covering of the sorus, sometimes cup-shaped.

Structure of Sporangium

• Contains numerous sack-like structures called sporangia.
• Annulus: A band formed by enlarged jacket cells that surrounds the sporangium.
• Outer wall: Thin and delicate, with lip cells below the jacket.
• Tapetum: Two layers of nourishing cells within the sporangium.
• Sporocyte cells: Diploid, containing chromosomes from both parents.

Meiosis and Spore Formation

• Sporocyte undergoes meiosis, resulting in haploid daughter nuclei.
• Formation of tetrad: Cluster of four adhering cells.
• Breakdown of the tapetum deposits a tough coat around each cell, forming spores.

Spore Release Mechanism

• The annulus dries out, increasing tension as water evaporates.
• This contraction causes the annulus to tear lip cells apart, releasing spores.
• Thousands of spores can be released from one leaf at a time.

Germination and Gametophyte Formation

• Spores germinate on moist surfaces, forming bisexual gametophytes (prothalli).
• Rhizoids: Initially attach spores to soil; followed by young prothallus development.
• Notch meristem: Structure that allows for further growth.
• Formation of heart-shaped gametophyte.

Gametangia Development

• Male gametangia (antheridia) located at the posterior end of the gametophyte.
• Female gametangia (archegonia) at the anterior end, containing an egg cell.
• Timing of gametangia development influences fertilization type (self vs. cross-fertilization).
• Hormone antheridiogen can stimulate adjacent plants to form male gametangia in response to flooding.

Fertilization Process

• Water plays a critical role in fertilization by facilitating sperm movement.
• Flagellated sperm swims towards the egg cell using water as a medium.
• Sperm attracted to archegonia by diffusible signals from the egg cell.
• Fertilization occurs when sperm and egg nuclei fuse, forming a zygote.

Embryo Development and Growth

• Zygote divides to form the embryo within the archegonia.
• The foot of the embryo absorbs nutrients from surrounding tissue.
• Development leads to the formation of roots, leaves, and stem (rhizome).
• Emergence of the rhizome allows the plant to grow and mature into another sporophyte.

Conclusion

• The fern life cycle consists of two distinct plants: the familiar sporophyte and the smaller gametophyte.
• Use of spores and genetic variability has allowed ferns to spread and evolve globally.