Plants cannot move away from pathogens, so they rely on defences that are always in place. Physical defences try to stop a pathogen getting in at all. The surface of a leaf is covered by a waxy layer called the cuticle. The cuticle is waterproof, so water does not collect easily on the leaf, and this makes it harder for bacteria and fungi to settle and grow. It is also a barrier that a pathogen has to get through.
Every plant cell is surrounded by a cell wall made of cellulose. The cell wall is tough, so it forms a second barrier that stops many pathogens entering the cells. When a plant is attacked, it can also thicken the cell walls near the damage to make the barrier stronger.
