A leaf is made of many layers that are sandwiched between two layers of tough
skin cells (called the epidermis). The epidermis also secretes a waxy substance
called the cuticle. These layers protect the leaf from insects, bacteria, and other
pests. Among the epidermal cells are pairs of sausage-shaped guard cells. Each
pair of guard cells forms a pore (called stoma; the plural is stomata). Gases enter
and exit the leaf through the stomata.
Diagram of leaf structure. Note