Fertilisation: The Fusion of Gametes

Introduction: What is Fertilisation?

Fertilisation in flowering plants is the culmination of the reproductive process that begins with pollination. It is the fusion of the male gamete (from the pollen grain) with the female gamete (the egg cell, inside the ovule) to form a zygote. This zygote will then develop into an embryo, which is contained within the seed. In angiosperms (flowering plants), this process is unique and involves two fusion events, collectively known as double fertilization.

Events from Pollination to Fertilisation

The period between the landing of a pollen grain on the stigma and the actual fusion of gametes is a dynamic and crucial phase. Here are the sequential events that lead to the formation of the zygote:

1. Germination of the Pollen Grain:

   • After a compatible pollen grain lands on the stigma, it absorbs moisture and nutrients (like sugars) from the stigma's surface. This triggers the pollen grain to germinate.
   • The inner wall (intine) of the pollen grain grows out through a weak spot in the outer wall (exine) called a germ pore. This outgrowth forms a thin tube known as the pollen tube.

2. Growth of the Pollen Tube through the Style:

   • The pollen tube, carrying two male gametes and a tube nucleus, grows down through the tissues of the stigma and the style, heading towards the ovary.
   • The growth of the pollen tube is a chemotropic movement, meaning it is directed by chemical signals secreted by the ovule. The tube nucleus guides the path of the pollen tube.

3. Entry of the Pollen Tube into the Ovule and Embryo Sac:

   • Once it reaches the ovary, the pollen tube enters an ovule, usually through a small opening called the micropyle.
   • After entering the ovule, the pollen tube penetrates the embryo sac (the structure within the ovule that contains the egg cell and other associated cells).

4. Release of Male Gametes and Formation of the Zygote:

   • The tip of the pollen tube ruptures, releasing the two male gametes into the cytoplasm of the embryo sac.
   • One of the male gametes moves towards the egg cell and fuses with its nucleus. This fusion of the male and female gametes is called syngamy, and it results in the formation of a diploid zygote (2n). The zygote is the first cell of the new embryo.

Double Fertilization and Triple Fusion: A Unique Angiosperm Feature

Flowering plants exhibit a unique process where two fusion events occur within the embryo sac. This is called double fertilization.

• Triple Fusion: The second male gamete (the one that did not fuse with the egg) migrates towards the two polar nuclei located in the center of the embryo sac. It fuses with them to form a triploid (3n) nucleus called the Primary Endosperm Nucleus (PEN). This fusion, involving three nuclei (one male gamete and two polar nuclei), is known as triple fusion.

• Double Fertilization: This is the term for the two fertilization events that occur simultaneously in the embryo sac:

  1. Syngamy: Fusion of one male gamete with the egg cell to form the zygote.
  2. Triple Fusion: Fusion of the second male gamete with the two polar nuclei to form the primary endosperm nucleus.

Significance of Double Fertilization:

• It ensures that the nutritive tissue (endosperm) is formed only when fertilization is successful, thus saving the plant's energy.
• The zygote develops into the embryo, while the primary endosperm nucleus develops into the endosperm, which provides nourishment to the developing embryo.

Fruit and Seed: The Products of Fertilisation

After fertilization, the flower undergoes a series of changes. The petals, sepals, stamens, style, and stigma usually wither and fall off, while the ovary and ovules develop into the fruit and seeds, respectively.

Fruit

• Definition: A fruit is a mature or ripened ovary, which develops after fertilization. The wall of the ovary develops into the pericarp, or the wall of the fruit.
• Significance:
  1. Protection of Seeds: The fruit protects the enclosed seeds from mechanical injury, harsh climatic conditions, and pests.
  2. Seed Dispersal: Fruits, especially fleshy and colored ones, attract animals that eat them and disperse the seeds to distant places. Some fruits have specialized structures (wings, hooks) that aid in dispersal by wind or animals.
  3. Source of Food: Many fruits are a source of food, vitamins, and minerals for humans and animals.

Seed

• Definition: A seed is a mature ovule, containing an embryo and a store of food (endosperm or cotyledons), all enclosed within a protective seed coat.
• Significance:
  1. Contains the Embryo: The seed contains the young, dormant plant (embryo) which will develop into a new plant under favorable conditions.
  2. Nourishment for Embryo: The seed stores food that nourishes the embryo during germination until it can produce its own food through photosynthesis.
  3. Dispersal: The seed is the unit of dispersal, allowing the plant species to colonize new areas.
  4. Dormancy and Perennation: The protective seed coat allows the embryo to survive unfavorable conditions (like drought or cold) in a dormant state, ensuring the continuity of the species.