Describe the growth of the pollen tube and its entry into the ovule followed by fertilisation (details of production of endosperm and development are not required)

SouravNovember 15, 2024

Answer

The growth of the pollen tube and its entry into the ovule is a critical process in the fertilization of flowering plants. Here’s a detailed description of this process:

Growth of the Pollen Tube

1. Pollen Germination:
   • When a pollen grain lands on a compatible stigma, it absorbs water and nutrients from the stigmatic surface, which triggers germination. The pollen grain swells, and its outer layer (exine) ruptures, allowing the inner layer (intine) to grow out as a pollen tube.
2. Pollen Tube Development:
   • The pollen tube emerges from a structure called the germ pore on the pollen grain. It grows by tip growth, where cell division occurs at the tip of the tube, allowing it to elongate rapidly.
   • As it grows, the pollen tube carries the contents of the pollen grain, including two sperm cells, toward the ovule. The growth is guided by chemical signals released by the ovule, a phenomenon known as chemotropism.
3. Pathway Through the Pistil:
   • The pollen tube travels through the style, which is part of the pistil, moving toward the ovary where the ovules are located. It may digest some of the surrounding tissue or grow through intercellular spaces to reach its destination.

Entry into the Ovule

1. Reaching the Ovule:
   • Upon arriving at the ovary, the pollen tube locates an ovule. The entry point for the pollen tube can vary depending on the plant species and is typically through one of three pathways:
     • Micropyle (Porogamy): This is the most common route where the pollen tube enters through a small opening called the micropyle, located at one end of the ovule.
     • Chalaza (Chalazogamy): In some species, such as Casuarina, the pollen tube enters through the chalaza, which is located at the base of the ovule.
     • Integuments (Mesogamy): Rarely, some plants allow pollen tubes to penetrate directly through the integuments surrounding the ovule.
2. Penetration and Discharge:
   • Once inside or at the micropyle of the ovule, the pollen tube penetrates into the female gametophyte (embryo sac). This is where fertilization occurs.
   • The contents of the pollen tube are discharged into the embryo sac, releasing two sperm cells.

Fertilization

• One sperm cell fertilizes the egg cell within the embryo sac, forming a diploid zygote. The other sperm cell typically fuses with two polar nuclei in a process known as double fertilization, resulting in triploid endosperm tissue that will nourish the developing embryo.