The thick outer layer of the pollen grains is known as ______ .

Explanation

To answer the question "The thick outer layer of the pollen grains is known as ______," we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Structure of Pollen Grains**:
- Pollen grains are the male gametophytes in plants and are crucial for the process of pollination. They consist of two main layers.

2. **Identify the Layers**:
...