Types of Responses

Plants cannot move from place to place, yet they still respond to changes in their surroundings. A response that involves a change in the direction of growth of part of the plant is called a tropism. Tropisms are slow, permanent growth responses controlled by the uneven distribution of growth substances.

Directional growth

A tropism is described by the stimulus that causes it and by its direction. Growth towards a stimulus is a positive tropism; growth away from a stimulus is a negative tropism.

• Phototropism — response to the direction of light. Shoots grow towards light (positive), so they capture more light for photosynthesis. Roots are usually negatively phototropic.
• Geotropism (gravitropism) — response to gravity. Roots grow downwards, in the direction of gravity (positive), anchoring the plant and reaching water; shoots grow upwards, against gravity (negative).
• Hydrotropism — response to water. Roots grow towards a source of moisture (positive), improving water absorption.
• Thigmotropism — response to touch or contact, seen when tendrils of climbing plants coil around a support.

Survival value

Tropisms are adaptations that improve survival. Positive phototropism of shoots maximises light absorption; positive geotropism and hydrotropism of roots secure anchorage, water and mineral salts. Because these responses depend on growth, they are far slower than the responses of animals, which use nerves and muscles.

Nastic movements

Not every plant response is a tropism. A nastic movement is a non-directional response — its direction does not depend on the direction of the stimulus. The rapid folding of the leaves of Mimosa pudica when touched is a nastic response, not thigmotropism, because the leaflets always fold the same way regardless of where they are touched.