Which one of the following events takes place after double fertilisation ?

Options

A. The pollen grain germinates on the stigma
B. The pollen tubes enter the embryo sac
C. Two male gametes are dischared into the embryo sac.
D. The PEN (Primary Endosperm Nucleus) develops into endosperm.

Explanation

To answer the question "Which one of the following events takes place after double fertilization?", we will analyze each option provided and determine which one correctly describes an event that occurs after the process of double fertilization in flowering plants.

### Step-by-Step Solution:

1. **Understanding Double Fertilization**:
- Double fertilization is a unique process in flowering plants where one male gamete fuses with the egg cell to form the zygote (embryo), while the other male gamete fuses with two polar nuclei in the central cell to form the primary endosperm nucleus (PEN), which will develop into the endosperm.

2. **Analyzing the Options**:
- **Option A**: "The pollen grain germinates on the stigma."
- This event occurs before double fertilization. The pollen grain must germinate on the stigma to form a pollen tube that will deliver the male gametes to the ovule.

- **Option B**: "The pollen tube enters the embryo sac."
- This event also occurs prior to double fertilization. The pollen tube must enter the embryo sac to facilitate the fertilization process.

- **Option C**: "Two male gametes are discharged into the embryo sac."
- This event occurs during double fertilization, not after. The male gametes are released into the embryo sac just before the actual fertilization takes place.

- **Option D**: "The primary endosperm nucleus develops into endosperm."
- This is the correct answer. After double fertilization, the primary endosperm nucleus (PEN) develops into the endosperm, which provides nourishment to the developing embryo.

3. **Conclusion**:
- The correct answer is **Option D**: "The primary endosperm nucleus develops into endosperm."