Explain the process of 'double fertilization' in angiosperms with the help of a diagram.

Double fertilization is a unique and characteristic feature of angiosperms (flowering plants) where two fertilization events occur within the ovule. This process involves the fusion of two male gametes with two different female gametes, leading to the formation of both the zygote and the endosperm.

Process of Double Fertilization

1. Pollen Germination and Pollen Tube Formation

   • A pollen grain lands on the stigma of the pistil and germinates, forming a pollen tube that grows down through the style to the ovary.
   • The pollen tube carries two sperm cells (male gametes) produced by the pollen grain.

2. Entry into the Ovule

   • The pollen tube enters the ovule through the micropyle (a small opening in the ovule).
   • The tube releases the two sperm cells into the embryo sac (female gametophyte).

3. Fertilization Events

   • First Fertilization (Syngamy):
     • One sperm cell fuses with the egg cell (female gamete) to form a zygote (2n), which later develops into the embryo.
   • Second Fertilization (Triple Fusion):
     • The second sperm cell fuses with the central cell (which contains two polar nuclei, each with n chromosomes) to form the primary endosperm nucleus (3n), which develops into the endosperm (a nutritive tissue for the embryo).

4. Formation of Seed and Fruit

   • The fertilized ovule develops into a seed, while the ovary matures into a fruit.

Significance of Double Fertilization

• Ensures the formation of both the embryo (from the zygote) and the endosperm (nutritive tissue).
• Provides genetic diversity and ensures proper seed development in angiosperms.

The diagram above illustrates the key steps of double fertilization, showing the fusion of sperm cells with the egg cell and the central cell, leading to the formation of the zygote and endosperm.