Thorns and Physical Armor

African acacias adjust their thorn length in response to browsing pressure — branches within a giraffe's reach grow longer thorns than ones it cannot reach. Bullhorn acacias go further, housing entire armies of aggressive ants inside their hollow thorns.

Plant physical defenses are flexible and responsive. Resources are allocated between growth and protection depending on the threat.

From chemical warfare to ant armies to microscopic syringes, plants defend themselves with remarkable engineering.