The outer cell layer of a leaf is called the epidermis. While it's true that the epidermis is transparent, allowing sunlight to pass through, it's not the only reason for its transparency.

Here's a more detailed explanation:

* Transparency: The epidermis is primarily composed of parenchyma cells which are thin-walled and lack chloroplasts (the structures responsible for photosynthesis). This allows light to pass through relatively unhindered.

* Waxy Cuticle: The epidermis is also covered by a waxy layer called the cuticle. This layer helps to prevent water loss and also contributes to the transparency of the epidermis by reflecting light.

* Protection: The epidermis serves as a protective layer for the leaf, shielding the inner tissues from physical damage and pathogens.

So, while transparency is important for sunlight penetration, it's just one of the many functions of the leaf epidermis.