The best answer - dermal tissue.
Dermal tissue in plants forms the first line of defense against physical damage and infection from the outside world.
Dermal tissue covers the outside of the plant, except in woody shrubs and trees, which have bark. The most common cell type in dermal tissue is epidermal cell. Generally, a thin waxy layer called a cuticle covers the epidermal cells and protects them.
Other cells in the dermal tissue are guard cells that surround the stomata, which are openings in the leaves. Gases enter and leave the dermal tissue through the stomata.
No they cant be mechanical because then something would have to generate their rays through the inventing of a machine.
