I found it very interesting that some of my stem plants are capable of closing up at night. When the lights come on they open within minutes. I posted this question at tptf and was told it was Nyctinasty. I had never heard of this so I looked it up. Very interesting to read the why, and so forth. Even Charles Darwin's name is mentioned.
Nyctinasty
Botany online: Growth Movements, Turgor Movements , and Circadian Rhythmics