WEATHER INFLUENCED. “Silene saxifraga flowers close during the day and open during the night for about 5 d. The petals roll up during the day. The inrolling (closing) movement was due to net water loss. Outrolling (opening) was apparently due to refilling of the cells with water. The flowers remain open longer on dry days if the soil is wet, and do not close at all on wet days.”
http://jxb.oxfordjournals.org/content/54/389/1801.full