The movement of leaves in a sensitive plant (Mimosa pudica) is a rapid, involuntary response to touch, known as thigmonasty, causing the leaves to fold inward quickly. In contrast, the movement of a shoot towards light, known as phototropism, is a slow, directional growth response where the plant grows towards a light source for optimal light absorption, a process driven by growth hormones.
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Rapid Response of Sensitive Plant Leaves
The sensitive plant, Mimosa pudica, exhibits a unique movement known as thigmonasty or seismonasty. This movement is a rapid, involuntary response to physical stimuli such as touch, shaking, or thermal stress. When touched, the leaves of the sensitive plant quickly fold inward and droop. This response is thought to deter herbivores or protect the plant from damage.
Mechanism behind Thigmonastic Movements
The mechanism of thigmonasty in Mimosa pudica involves a rapid change in cell turgor pressure. When the plant is stimulated, cells at the base of the leaflets lose water rapidly, leading to a loss of turgor pressure. This causes the leaves to fold and the petioles to droop. The movement is quick, often occurring within seconds of stimulation.
Phototropism in Shoots
On the other hand, the movement of a shoot towards light, known as phototropism, is a growth-oriented response. This slow, directional movement allows the plant to optimize its exposure to sunlight, which is crucial for photosynthesis. Phototropism is most commonly observed in young shoots, which bend towards the light source.
Role of Plant Hormones in Phototropism
Phototropism is primarily driven by the uneven distribution of plant hormones called auxins. When a shoot is exposed to light, auxins accumulate on the shaded side of the plant. This higher concentration of auxins on the darker side stimulates cells to elongate more than those on the light-exposed side, causing the shoot to bend towards the light.
Distinct Nature of Movements
In summary, the movement of leaves in the sensitive plant is a rapid, protective response to physical stimuli, involving changes in cell turgor pressure, while the movement of a shoot towards light is a slow, growth-driven process regulated by hormonal gradients. These distinct movements reflect the plant’s adaptation to environmental interactions – one being a defensive mechanism and the other a growth optimization strategy.