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1. Mimosa pudica L. pinnae close in darkness when phytochrome is predominantly in the far-red-absorbing form (Pfr) and remain open when Pfr is low [6]. The leaflets remain open, however, in normal light periods irrespective of the form of phytochrome. Pinnae, after closing in darkness, regularly reopen in light. 2. An action spectrum for the opening response shows maxima for effectiveness near 710 and 480 nm. This action spectrum is similar to that for a high-energy response affecting morphogenesis in many plants. 3. Dropping of the petiole of M. pudica can be photostimulated by irradiation of the primary pulvinus after holding the plants in darkness [4]. 4. The photostimulation of the primary pulvinus is effective only at wavelengths less than 520 nm. Wave bands in the region of 400 to 470 nm are about equally effective. 5. These photoresponses of M. pudica are related to current discussion about the nature of the high-energy and phytochrome photomorphogenic reactions.

Hypocotyl elongation in two varieties of Petunia and in Grand Rapids lettuce is shown to be affected by a high-energy reaction and by phytochrome action. These two photoreactions interact in such a way that, on the one hand, shortening of the hypocotyls due to the high-energy reaction can be entirely masked by brief terminal far-red light treatment, while on the other hand, there is no evidence of phytochrome action unless brief exposures to red light are preceded by relatively long exposure of high-intensity.