The movement of leaves of the sensitive plant (Mimosa pudica) is a nastic movement, specifically a thigmonastic movement, which does not depend on the direction of the stimulus. This movement is triggered by touch or mechanical stimulation and happens rapidly due to the sudden loss of water from cells in the pulvini (swelling at the base of the leaflets), resulting in collapse or folding of the leaves. It is a reversible, non-growth movement mainly for defense. In contrast, the movement of a shoot towards light is a tropic movement called phototropism, which depends on the direction of the light stimulus. This movement is slower and caused by unequal growth on the sides of the shoot due to the redistribution of the plant hormone auxin, leading to bending of the shoot towards the light. Phototropism is a growth-dependent, directional response aimed at optimizing light capture for photosynthesis.
Key Differences:
Aspect| Sensitive Plant Leaves (Thigmonastic)| Shoot Towards Light
(Phototropism)
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Stimulus| Touch (mechanical stimuli)| Light
Directionality| Non-directional (independent of stimulus direction)|
Directional (depends on light direction)
Mechanism| Rapid water movement causing turgor changes in pulvini|
Differential growth caused by auxin
Nature of Movement| Rapid, reversible, non-growth movement| Slow, growth-
dependent movement
Purpose| Defense mechanism| Optimizes light capture for photosynthesis
Thus, the sensitive plant's leaf movement is a rapid, non-growth response to touch, while the shoot's movement towards light is a slower, growth-based directional response to light stimulus.
