Sensitive Plant

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The left photo shows fully turgid leaves (pinnae) with all the leaflets (pinnules) fully extended for maximum light absorption. In the right photo the leaflets have folded up and the leaves are barely discernable. Can you spot the five main compound leaf divisions (pinnae) that have closed up in the right photo?<FONT SIZE="3" COLOR="#000000" FACE="Arial" > </FONT></div> <bR> <bR> <div> The sensitive plant is a pantropical weedy herb in the legume family (Fabaceae). The pinnately compound leaves are composed of numerous tiny leaflets. When touched, the leaflets begin to fold up very rapidly and the leaf stalk (petiole) suddenly bends downward. [Sleep movements also occur in the sensitive plant and in many other species of leguminous trees and shrubs in which the leaflets slowly fold up at night.] These plant movements in response to a stimulus (called nastic movements) are associated with loss of tugor pressure in the leaves. The sensitive plant is especially interesting because of the rapidity of the wilting process, an entire leaf suddenly drooping after it has been touched. As one leaflet folds up, the stimulus moves to other parts of the leaf until all the leaflets and adjacent leaves have folded up. Two distinct mechanisms, one electrical and the other chemical, appear to be involved in the rapid spread of the stimulus in sensitive plants. At the bases of the leaflets are jointlike thickenings called pulvini, with a large pulvinus at the base of each petiole. When a leaf is stimulated by touch, heat or wind, there is a chain reaction in which potassium ions migrate from one side of each pulvinus to the other side. This is followed by a rapid shuttling of water molecules from parenchyma cells in one half of the pulvinus to cells in the other half. This action results in loss of turgor pressure that causes folding of the leaflets and eventually the entire leaf. The entire process may take only a few seconds. When the leaflets fold up and instantaneously become wilted, it is often difficult to see where the leaf was in its original turgid state. It has been suggested that this rapid wilting process may be an adaptation to grazing mammals or ravenous insects.<FONT SIZE="3" COLOR="#000000" FACE="Arial" > </FONT></div> <bR> <bR> <bR> <bR> <bR> <bR> <bR> <bR> </td> </tr></table></body>