Leaflet movement of Mimosa pudica L. Indicative of phytochrome action

1. Closing movements of Mimosa pudica pinnae, upon change from light to darkness, depend upon the presence of phytochrome in the far-red-absorbing form. 2. The potentiated control of closing movements by phytochrome can be repeatedly established and reversed by repeated alternations of red and far-red radiation, respectively. 3. Action spectra were measured for the potentiation of closure and for its reversal. 4. The response to phytochrome action is evident in 5 minutes and is fully expressed in 30 minutes. 5. This rapid response and the more rapid potentiation with half times of less than 1 minute for several other responses to phytochrome action indicate that the primary action of phytochrome ist not gene activation, but rather metabolic control at the substrate level.