Coordination and response in plants

Tropisms

A tropism is a growth response of a plant to a directional stimulus. Phototropism: shoots grow towards light (positive), helping leaves absorb light. Gravitropism: roots grow down with gravity (positive), shoots grow up (negative). These are controlled by auxin, a hormone made in the shoot tip. In a shoot, auxin moves to the shaded side, where it makes cells elongate more, so the shoot bends towards the light.