Mimosa has a doubly compound leaf. The petiole, or leaf stalk, is attached to the stem at the node and bears four smaller stalks that carry two rows of leaflets. As you can see here, a mechanical stimulus, such as the touch of a finger, will cause the leaflets to fold rapidly, in less than one second. The same response can be elicited with other stimuli such as rain, wind, shaking, or severe heat. Interesting research has been conducted on the propagation of the signal from the site of stimulation to other parts of the leaf. When the terminal leaflets are given a light pinch, the leaflets close in sequence. In some cases, the signal from the stimulus reaches the node and causes the petiole to flop down. Eventually, the signal reaches the base of the group of leaflets and causes the leaflets to close sequentially from the base to the tip. Credit: Courtesy of Graham R. Kent and Rebecca L. Turner, Smith College.