Plants bend toward the light

A tulsi plant by a window bends toward the window. Its shoot is the part above the soil.

Drag Days. Watch which side grows more.

A plant has no nerve cells and no muscles. It responds with chemicals.

Cells at the shoot tip make a hormone called auxin. Light from one side makes auxin move to the shaded side. There auxin makes cells grow longer. The shaded side becomes longer than the lit side. So the tip bends toward the light. This is phototropism.

Roots respond differently from shoots. Roots grow down, with gravity. This is geotropism.

A pea tendril is a thin climbing thread. The side away from a stick it touches grows faster. So the tendril curls round the stick. A response to touch is thigmotropism. A root that bends toward water shows hydrotropism.

Not every plant movement is growth. A touch-me-not plant folds its leaves at a touch.

The rule

Plants coordinate by chemistry, not nerves. Light makes a growth chemical gather on the shaded side. The shaded side grows more, so the shoot bends toward light.

For example, turn a plant's pot halfway round. New growth bends toward the light again. The old part keeps its bend.

Try the lab to finish this lesson.