Introduction When someone touches you, you turn and look. When you hear a loud noise, you may jump. These are known as STIMULUS and RESPONSE. Something happens and causes another thing to respond.

TROPISMS In plants, a response to a stimulus is called a TROPISM. Forces cause the plant to respond. There are several different tropisms that you will need to know:

Geotropism (Gravitropism) – a plant’s response to the force of gravity Roots grow down – stems grow up (positive) (negative)

Gravitropism (geotropism)

Phototropism – A plant’s response to light – grows toward the light

Phototropism

Thigmotropism – a plant’s response to touch Turgor pressure – force of water pushing against cell walls in plants

Thigmotropism

Turgor Pressure Water pressure in a plant’s cells.