Plant Tropism Notes

Tropism

  • Tropism is defined as the directional growth of a plant, or a part of a plant, in response to an external stimulus.
  • This growth is controlled by a hormone called auxin.
  • Auxins are produced by meristems and are found at the tips of stems, buds, and roots.
  • Auxins stimulate cell/stem elongation.

Types of Tropism

  • Phototropism: Response to sunlight/light.
  • Geotropism: Response to gravity.
  • Hydrotropism: Response to water.
  • Thigmotropism: Response to touch.

Phototropism

  • Phototropism is the growth response of a plant towards light.
  • Stimulus: Light.
  • Response:
    • Shoots grow towards light (positive phototropism).
    • Roots grow away from light (negative phototropism).

The Presence of Sunlight to the Right Side of a Stem

  • Auxin accumulates on the left/shaded side.
  • Auxin causes cells on the left side to elongate, while growth is inhibited on the right side.
  • The shaded side/left side grows, bending towards the light source.

The Presence of Sunlight to the Left Side of a Stem

  • Auxin accumulates on the right/shaded side.
  • Auxin causes cells on the right side to elongate, while growth is inhibited on the left side.
  • The shaded side/right side grows, bending towards the light source.

Sunlight Directly Over a Stem

  • Even distribution of auxin in the growing tip and the immediate left and right sides.
  • Cells in these regions elongate, resulting in upward growth.

Geotropism

  • Geotropism is the growth response of a plant towards gravity.
  • Stimulus: Gravity.
  • Response:
    • Roots grow towards gravity (positive geotropism).
    • Shoots grow away from gravity (negative geotropism).
  • Auxin accumulates at the lower surface of roots and inhibits growth.
  • Cells on the upper surface of roots, with lower auxin concentration, elongate and cause the roots to grow downwards.

Hydrotropism

  • Hydrotropism is the growth response of plant roots towards a water source.
  • Stimulus: Water.
  • Response:
    • Roots grow toward a water source (positive hydrotropism).
    • Shoots grow away from the water source (negative hydrotropism).
  • Roots have a stronger response to water than to gravity.
  • Roots grow towards water instead of continually growing straight downwards.
  • Roots also grow in all directions when water is dispersed throughout the soil, due to the presence of auxin in the tips of roots.

Thigmotropism

  • Thigmotropism is the growth response of a plant to touch.
  • Stimulus: Surfaces/matter that the plant touches.
  • Response:
    • The roots and shoot system of vine plants grow onto surfaces they touch.
    • These surfaces or structures act as a skeletal support system for vines and roots.