Biology Topic 3 Part 4/4

Control of plant growth: auxin and phototropism

• Plants respond to light by growing toward it

  • This maximizes the amount of light available for the plant’s leaves

• Darwin’s experiment of phototropism had observations that showed that the shoot tip must be exposed to the light for phototropism

Scientists have since discovered that there is a hormone called auxin

• It is produced in the tips of plants and promotes cell elongation

• When the shoot tip is exposed to light, auxin, moves to the cells on the shaded side of the stem, stimulating elongation in the cells

  • This causes the shaded side to be longer than the sunny side, which causes the stem to grow towards the light (positive phototropism)

Negative phototropism occurs in roots, causing them to grow away from the light

Gravitropism is a coordinated process of differential growth by a plant in response to gravity pulling on it

• Roots have positive gravitropism

• Stems have negative gravitropism

• Auxin causes elongation on the upper side of the root and on the lower side of the stem