Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
Why are some plants glossy?
Plants that grow in dry areas and plants that don’t lose their leaves in the fall have to protect themselves from drying out in hot summers and in winters when the water is frozen.
There are several tricks the plants can use to protect their leaves from drying out. One trick is to make the leaves into long, thin needles, so there is less surface to dry out. Pine trees use this trick.
Another trick is to curl the leaf up, like live oak leaves. This reduces water loss by reducing the wind and sun the leaf gets. Some leaves also have tiny hairs on them to reduce the water loss due to wind.
Yet another trick is to coat the leaves with a thick coat of protective wax, so the water stays trapped in the leaf. This protective coating of hard wax makes a glossy coat on the leaf, just like waxing a car.