Hi JLR,
Sensitive plant aka Mimosa pudica is probably the one you are thinking of.* I saw this growing wild in the Amazon jungle.* It can become a weed in frost free areas if planted outside.
http://en.wikipedia.org/wiki/Sensitive_plant
http://www.plantoftheweek.org/week147.shtml
There is also the prayer plant aka Maranta leuconeura that folds its leaves up at night.* There is a very pretty red veined version and one that has blotches on it's leaves.
http://www.plantoftheweek.org/week205.shtml
http://www.plantoftheweek.org/week123.shtml
http://aggie-horticulture.tamu.edu/c...ayerPlant.html
Newt
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.