Plant Hormones

• Auxin

  a plant hormone that controls growth near the tips of the roots and shoots
• Auxin controls the growth of a plant in response to light (phototropism) and gravity (gravitropism or geotropism)
• Auxin is produced in the tips and moves backwards to stimulate the cell elongation (enlargement) process which occurs in the cells just behind the tips
• If the tip of a shoot is removed, no auxin is available and the shoot may stop growing
• Extra auxin promotes extra growth in the shoot, but inhibits growth in the root, producing the desired result
• When a shoot tip is exposed to light, more auxin accumulates on the side that's in the shade than the side that's in the light - this makes the cells grow (elongate) faster on the shaded side, so the shoot bends towards the light
• When a shoot is growing sideways, gravity produces an unequal distribution of auxin in the tip, with more auxin on the lower side - this causes the lower side to grow faster, bending the shoot upwards
• A root growing sideways will also have more auxin on its lower side
• In a root, extra auxin inhibits growth - this means the cells on top elongate faster when growing sideways, due to gravity, and the root bends downwards