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Flowering of Chenopodium rubrum L., selection 374, was examined with respect to an endogenous circadian rhythm, the state of phytochrome, and the result of changing the form of phytochrome during a single dark period of 2 to 96 hours interrupting continuous light. Darkness was imposed either 4 or 5 days after seeds were placed on moist filter paper in Petri dishes.The following working hypothesis, which is partly retrospective, is projected to explain the main features of the experimental results. Flowering is controlled by a product of the enzymatic action of the far-red absorbing form of phytochrome (Pfr) on a single but unknown substrate. In acting, Pfrfinally reverts to the inactive red-absorbing form of phytochrome (Pr) or is changed from the Pfrform in some other way. The available substrate, if not utilized by Pfraction, is soon depleted by other reactions. The substrate for Pfraction is low during the skotophile but high during the photophile phases. The significant time for phasing is the beginning of darkness. The initial substrate supply appears to be derived from the preceding light period but some time in the region of the 9th to 12th hour of darkness a significant rhythmic change of substrate starts up. The dependence of flowering on the time that darkness is interrupted by light is directly related to a rhythmic change in the optimum Pfrlevel required for the processes leading to flowering.The role of the endogenous rhythm in flowering under natural conditions is questioned. Similarities that are shown in the control of flowering, whether the display is governed by an endogenous rhythm or by a daily photoperiodic cycle, indicate that phytochrome acts as a "pacemaker". It is suggested that the distinct ecotypic populations of C. rubrum that differ in flowering response have dissimilar levels and rates of supply of substrate for phytochrome action. In C. rubrum-374, complete reversion or loss of Pfrdoes not occur during a long dark period of 72 hours at 20 °C, but Pfrdoes decrease to low levels.A hydrodynamic system is discussed as an analogy to rhythmic flowering response.

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.

Flowering of plants depends upon the length of the night. This control of flowering is one of the methods of adaptation of species by which an unfavorable season is anticipated. It implies a time-measuring system that distinguishes between light and darkness through mediation of a pigment. Ways of finding the pigment and explanations of some of the features of seasonal response are described. The pigment, now called phytochrome, is a blue or a bluish-green protein that exists in two forms interconvertible by light. Phytochrome is present to the extent of about 1 part in 10 million in many plant tissues. 18 references, 4 figures.

The first inflorescence of the potato was differentiated at approximately the same node under a range of conditions including size of seed piece, temperature and photoperiod. The small size of seed piece (5 grams), both the high temperature (8o° during the day, and 55° night), and the short photoperiod (9 hours) had a tendency to increase the node number to the first inflorescence. Flower primordia were differentiated in total darkness.