Plant Tropism

• A tropism is a directional growth response to a stimuli.
• If part of the plant grows towards the stimulus it is called a positive tropism; away from a stimulus- a negative tropism.
• Tropisms are designed to help the roots and shoots keep in the most favourable environment.
• Indole Acetic Acid (IAA) belongs to a group of chemicals called auxins. IAA is the plane hormone that is responsible for tropic responses.
• IAA is made by a group of actively dividing cells that cause growth a the tip of the root of shoot by affecting the length of cells.
• Tropism in shoots:

  In shoots IAA causes cell elongation. Positive Phototropism.

  Omnidirectional light produces an even distribution of IAA and straight growth.

  Unidirectional light produces an uneven distribution of IAA- more IAA accumulates in the shaded side, so the shoot grows towards the source of light.
• Tropism in shoots:

  Negative gravitropism.

  IAA accumulates on the lower side of the stem. Shoot cells elongate more on the lower surface and bends upwards against gravity.
• Tropism in roots:

  In roots higher concentration of IAA causes less cell elongation. As IAA accumulates on the lower side as a result of gravity, it inhibits cell growth on the lower side so the root grows down.