What is alternation of generation? Briefly describe the phenomenon of life cycle of fern

The fern plant is a sporophyte. It is differentiated into roots, rhizome and leaves. The fern plant exhibits true alternation of generation.

What is alternation generation?

In some plants there are two morphologically distinct phases in life cycle; the sporophytic phase and the gametophytic phase, which alternate each other. The sporophyte is diploid in nature, long-lived and produces spores.

The gametophyte, on the other hand, is relatively short-lived and thalloid. It bears the sex organs. The young sporophyte is dependent on gametophyte. The sex organs or gametes are haploid (i.e. in numbers). These haploid gametes fuse to for the diploid zygote, which is the mother cells of the sporophyte. This type of life cycle is known, as alternation of generation i.e. there is alternation of sporophyte with gametophyte.

Life cycle of fern:

External morphology- The sporophyte is differentiated into roots, rhizomes and leaves. The roots are adventitious. The rhizome is subterranean creeping and dichotomously branched. Leaves develop at the apex of the rhizome in acropetal succession. Young leaves show circinate type of vernation.

Reproduction:

The reproduction takes place by means of vegetative methods and by spores.

(1) Vegetative reproduction:

It takes place by means of adventitious buds, which develop on the rhizome. These buds give rise to new plants. Fragmentation of rhizome also helps in vegetative propagation.

(2) Reproduction by spores:

There are special spore-producing organs known as sporangia. The leaves bearing sporangia are known as sporophylls. Many sporangia arise irregularly from placental tissue developed at the tip of an ultimate vein. The sporangia are aggregated in clusters called sori. These are arranged in two rows one on each side of the main vein. Sorus covered by a thin membranous shield-like kidney shaped outgrowth of the leaf known as inducium.

The development of sporangium is leptosporangiate type. Mature sporangium consists of a stalk and capsule. The stalk is long, slender and multicellular. The capsule is lens-shaped with distinct stomium and annulus. Capsule encloses 8 or 16 spore mother cells. Each of them divides meiotically to form 4 hapliod spores is similar in shape structure, so it is homosporous. The spore is the first cell of the gametophyte generation.

Gametophyte:

The spore germinates to form a green filamentous structure. The germ tube attaches itself to the soil by rhizoids. The apical cell divides and redivides to form a heart-shaped structure known as prothallus. The apical part of the prothallus has an apical notch where the growing point is situated. It bears rhizoids on the ventral surface. It is green and, therefore, autotrophic.

The sex organs:

The male and female sex organs are known as antheridia and archegonia respectively. The antheridia are developed at the basal region among the rhizoidal cushion. The archegonia are produced near the apical notch, which is deeply sunken in the prothallus tissue.

Antheridia:

Each antheridium is spherical. It contains antherozoid mother cells. There are usually 32 spirally coiled multiflagllete antherozoids present.

Archegonia:

Each archegonium is flask-shaped structure with its venter embedded in the prothallus. The venter is swollen and contains an egg cell and a venter canal cell.

Fertilization:

The anthrozoids swim towards the tip of the archegonium and enter through neck canal. One of the antherozoids fuses with egg and forms zygote. The diploid zygote is the first cell of the sporophyte.

Young sporophyte:

The zygote divides vertically and then successive division occurs forming an octant. These octant cells are arranged in two tiers. The upper epibasal tier forms the primary root and primary leaf. The lower hypobasal tier forms foot and the embroynal stem. Subsequent divisions are irregular. The young sporophyte gets nourishment from the gametophyte till it grows independently into a fern plant.