Sexual Reproduction in Fern

Life Cycle

The life cycle of fern plant completes in two phases:-

• Sporophyte (Asexual)
• Gametophyte (sexual)

Sporophytic generation:-

This stage involves theuse of spores. The plant structure is diploid (2n). Sori are the dark brown structures plant on the under surface of fern-leaves.

Sporangium:- (Bursting)

The sporangium is the sac-like structure which bears spore mother cells and spores in fern. When the spores get mature, they are released out by bursting of the sporangium.

Inside the sporangium spore mother cells are present. The spore mother cells (2n) meiotically divides and produces four spores (n).

What do you mean by sporophytic generation in fern plant?

Ans: It is the stage of asexual reproduction in the life cycle of fern plant which occurs with the help of spores.

Gametophyte :-

This stage involves male gamete called antherozoids and female gametes called eggs.

When the haploid spores are set free into the soil, they start germinating under the favourable condition i.e. presence of moisture, air and enough sunlight. The germinating spore produces prothallus.

Prothallus is the green, flat and heart-shaped structure in the life cycle of the fern plant. It serves as the breeding ground for gametes. Prothallus has the capacity of photosynthesis. The energy required to grow gametes is provided by it. Rhizoids are the root-like structures that grow at the base of the prothallus and help prothallus to anchor. Rhizoids also function as roots as they absorb water and minerals from the soil.

The male reproductive organ in fern is Antheridia. Antheridia contain male gametes "The Antherozoids". It grows just above rhizoids. Each antheridium is circular in shape and has nearly 32 antherozoid mother cells. These antherozoid mother cells give rise to spiral shaped antherozoids. Antherozoids move with the help of flagella (tail). The female reproductive organ in fern is Archegonia. Archegonia contain female gametes "The Eggs". Archegonia grows near the top of the prothallus. Each archegonium is divided into two parts lower "venter" and upper "neck". Venter allows the place to grow female egg and neck allows passage for antherozoids to reach egg cell.

When archegonia and antheridia mature, archegonia secretes the malic acid which directs antherozoids to reach the egg for fertilization. The fertilization takes place in prothallus when freely swimming antherozoid fuses with the egg(generally anchored to the prothallus). From there diploid structure, fern germinates.

Note: Up until Bryophyta, gametes (male) need water to move around and reach female gamete. But from Pteridophyta gametes don't need water for fertilization (fertilization base is provided by prothallus). This way plants are evolving to adapt to land. As we go up in the classification need of water for fertilization is decreasing. These plants can be called as true land plants.

Alternation of generation:-

The occurrence of sporophytic and gametophytic one after the other in fern plant is called alternation of generation.

OR

The life cycle of fern completes in (2 stage), the sporophyte (2n) and the gametophyte (n). The two stages occur one after the other to complete the life cycle. This process is known as alternation of generation.

• Alternation of generation in a diagram.

Draw the labelled diagram of archegonium.