Here's why:
* Animals have immune systems: Animals, including humans, have complex immune systems with white blood cells (leukocytes) that circulate in their blood and lymphatic systems. These cells are crucial for fighting off infections and diseases.
* Plants have a different defense system: Plants don't have the same type of circulatory system as animals, and they don't have specialized cells that travel through their bodies to fight off invaders. Instead, plants have evolved a different, but equally effective, defense system.
* Plant defenses: Plants have various mechanisms to defend themselves, including:
* Physical barriers: Thick cell walls, waxy cuticles, thorns, and spines act as physical barriers against pathogens.
* Chemical defenses: Plants produce a wide range of toxic compounds, enzymes, and signaling molecules that can kill or inhibit pathogens.
* Hypersensitive response: This is a localized, programmed cell death that prevents pathogens from spreading.
* Systemic acquired resistance: A plant-wide immune response triggered by an initial infection, making the plant more resistant to future infections.
In summary: While animals have white blood cells that circulate in their blood, plants rely on a different set of defense mechanisms to protect themselves from disease.