Tropisms
Tropisms is how plants grow toward or away from things like light, water, or gravity.
Think of it this way
A sunflower seedling on a windowsill shows how tropisms work. The stem bends and grows toward the bright window light. Roots grow down toward gravity and out toward water in the soil. Each growth direction is a response to a specific signal in the environment.
Explaining tropisms by grade level
Put a plant near a window. After a few days, it leans toward the light. Roots grow down into the soil. Plants move slowly toward what they need.