Abstract
Lanthanum and ethylenediaminetetraacetate (EDTA) profoundly affect the rapid leaf movements of Mimosa pudica L. Lanthanum, which mimics calcium but does not penetrate the plasmalemma, inhibits the closing response but does not affect reopening. A low concentration of EDTA retards the reopening process while a higher EDTA concentration prevents the closing movement. There is evidence that the EDTA effects result from chelation of calcium ions rather than chelation of other cations. These results are discussed with regard to the role of calcium in leaf movements.