Why does the touch-me-not plant close in the evening till the whole night?

well,this an exceptional case as the real reason for its closing is the loss of tugidity, when we touch it ,the water loss occurs from the base of its leaves so it is more likely to close in the daytime because it will lose more water at that time but i find no reason for its closing in the night

Why does the touch-me-not plant close in the evening till the whole night?
The same reason that you have mentioned. its leaves are too small. The leaves are used to generate its food along with sunlight and when in night it does not get the sunlight so it closes its leaves to avoid the loss of water vapour and many of the trees and plants do this. only during day time that also when sun's rays are sen they open the leaves .This plant also have small thorns for its own protection.But in day time the sun's rays and its chlorophyll joint together produce its food and hence day time it is open

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