The correct option is (a) 1 only.
Explanation
The 'touch-me-not' plant (Mimosa pudica) exhibits a specific type of plant movement known as thigmonasty or seismonasty. This is a non-directional nastic movement that occurs in response to touch, shock, or vibration.Statement-wise Analysis:
- Statement 1 is Correct: Plants respond to various environmental stimuli such as light, gravity, and touch. The Mimosa plant is a prominent example of a plant responding to mechanical stimuli. Upon contact, an electro-chemical signal is transmitted through the plant tissues, triggering a physical response.
- Statement 2 is Incorrect: The leaves of the Mimosa do not remain rigid when touched. Instead, they rapidly fold inwards and droop. This movement is driven by a sudden change in turgor pressure within the pulvinus (the swollen base of the leaf stalk). Water rapidly exits specific cells in the pulvinus, causing the structure to lose rigidity and the leaf to collapse.
- Statement 3 is Incorrect: Mimosa pudica is not a desert plant (xerophyte). It is native to tropical and subtropical regions and thrives in areas with adequate moisture. While the folding mechanism may assist in reducing transpiration or protecting against herbivores, the plant lacks the primary morphological adaptations of desert flora, such as fleshy stems or modified spines for water conservation.
Key Takeaway: The rapid movement of Mimosa pudica leaves is a nastic response caused by the loss of turgor pressure in the pulvinus cells, not by growth or permanent structural rigidity.