Egg apparatus is present towards the micropylar end of an ovule. Egg apparatus has two lateral synergid cells and one centrally located egg cell. During entry of pollen tube within the ovule synergid cells become disintegrate and provide path for entry of pollen tube within the chamber of embryo sac.

As pollen tube enters embryosac it destroys one of the synergid while entering the embryosac....so its called degenerated synergid....as it gets destroyed by pollen tube's entry....

Pollen Tube Entry into the Embryo Sac: Degenerated Synergid

The pollen tube enters the embryo sac through the degenerated synergid. This process is crucial for successful fertilization in angiosperms. The degenerated synergid is one of the two synergid cells found at the micropylar end of the embryo sac. The other synergid cell is referred to as the persistent synergid.

Role of Synergid Cells
Synergid cells play a significant role in guiding the pollen tube towards the embryo sac. They secrete various molecules and establish a communication pathway between the pollen tube and the ovule. The synergid cells attract and guide the pollen tube towards the embryo sac through the micropyle, which is a small pore at the tip of the ovule.

Degeneration of Synergid Cells
As the pollen tube approaches the embryo sac, it releases enzymes that cause the degeneration of one of the synergid cells. This process is known as synergid degeneration. The degenerated synergid cell breaks down and creates a passage for the pollen tube to enter the embryo sac. The persistent synergid remains intact and continues to function.

Reasons for Pollen Tube Entry through Degenerated Synergid
There are several reasons why the pollen tube enters the embryo sac through the degenerated synergid:

1. Passage Creation: The degenerated synergid provides a route for the pollen tube to penetrate the embryo sac successfully. Without the degenerated synergid, the pollen tube would face barriers and might fail to reach the egg cell.

2. Nutrient Supply: The degenerated synergid cell undergoes programmed cell death, releasing nutrients that can support the growth of the pollen tube. These nutrients help sustain the pollen tube during its journey towards the egg cell.

3. Guidance: The degenerated synergid cell acts as a guide, directing the pollen tube towards the embryo sac. The breakdown of the degenerated synergid cell creates a path that the pollen tube can follow.

4. Selective Entry: The degenerated synergid cell ensures the selective entry of the pollen tube into the embryo sac. This selective entry is crucial for preventing multiple pollen tubes from fertilizing the egg cell simultaneously.

In conclusion, the pollen tube enters the embryo sac through the degenerated synergid cell. This process is essential for successful fertilization in angiosperms, as it allows the pollen tube to reach the egg cell and establish fertilization. The degenerated synergid cell provides a passage, releases nutrients, guides the pollen tube, and ensures selective entry into the embryo sac.