The exine of pollen grain is made up of a complex substance called__________ .

Explanation

**Step-by-Step Solution:**

1. **Understanding Pollen Grains**: Pollen grains are the male gametes in plants. They are typically spherical structures that play a crucial role in plant reproduction.

2. **Identifying the Layers of Pollen Grains**: Pollen grains have two main layers: the outer layer known as the exine and the inner layer known as the entine.

3. **Defining Exine**: The exine is the hard outer layer of the pollen grain. It provides protection and is significant for the pollen grain's durability.

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