Waterproof layer present on the outer wall of epidermal cells.

Explanation

**Step-by-Step Solution:**

1. **Identify the Structure**: The question asks about a waterproof layer present on the outer wall of epidermal cells. In plants, this structure is known as the cuticle.

2. **Understand the Location**: The cuticle is located on the outer side of the epidermal layer. The epidermis is the outermost layer of cells in a plant.

3. **Function of the Cuticle**: The primary function of the cuticle is to provide a waterproof barrier. It is composed of a waxy substance that prevents water loss from the plant, thereby reducing transpiration.

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