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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.

Seeds of loblolly pine, Pinus taeda L., and of white pine, P. strobtus L., which require a long moist treatment at a low temperature (stratification) for complete germination, were chosen for this study. Seeds of the former species require 30 to 90 days at 2 to 3 C and those of the latter 30 days at 10 C or 60 days at 4 C (27). The purpose of the work was to investigate responses of these seeds to red and far-red radiation, which controls germination (4) through the phytochrome pigment (13). Phytochrome exists in two interconvertible forms (4). The P660 form absorbs red light and is converted to the P730 form believed to be active biologically. The P730 form absorbs far red and is converted to the inactive P660 form. The P730 form also reverts in darkness to the P660 form (13). Most seeds in which the action of phvtochrome has been demonstrated germinate promptly and completely after one brief exposure to light (23). A few seeds in a sample of seed of Virginia pine, P. virginiana Mill., also germinate in response to a single brief irradiation, but most of them require a short period at a low temperature before they germinate in response to light (26). This change in light requirement of Virginia pine seeds after a short period at a low temperature made it important to measure similar changes in loblolly and white pine seeds, which reportedly require appreciable stratification.

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.

The responses to light of lettuce and Lepidium seeds depend on the reversible red, far-red photoreaction (3,5). These light-sensitive seeds are rather small in comparison with seeds of several species of southern pines reported by Nelson (4) to be stimulated to germinate by light. The current experiments were conducted to determine whether seeds of Pinus virginiana respond to the reversible red, far-red photoreaction in the same way as lettuce and Lepidiunm seeds, and how the responses are modified by temperature and condition of imbibition.

Action spectra were measured for suppression of germination of Poa pratensis L. and Amaranthus arenicola I. M. Johnston seed under prolonged or continuous irradiation. The action maxima for both types of seeds are near 720 nm. The maxima are unchanged in position or magnitude in the presence of radiation in the region of 600 to 670 nm adequate to maintain phytochrome predominantly in the far-red-absorbing form. A reversible potentiation of germination to change in form of phytochrome was observed for both seeds. The bearing of these findings on a high-energy regulatory light response is discussed.

LIGHT TAKES part in a number of distinct physiological reactions in plants. Among these are chlorophyll formation, photosynthesis, photoperiodism, phototropism, and cell elongation. Pigments are necessarily involved in the initial absorption of light in each process. Measurement of the effectiveness of different wave lengths of light gives information about the pigment and possible basic similarity of the initial steps in different physiological processes. This method is here applied to establish a relationship between growth of etiolated seedlings and floral initiation in photoperiodically responsive plants. Spectral sensitivities or action curves have been obtained for floral initiation in both longand short-day plants by Parker et al. (1946, 1948). The response to light shows that the same pigment or same class of pigments is effective in both tvpes of plants and that the most effective absorption is in the red portion of the spectrum. Effects of light on stem and leaf growth of etiolated pea seedlings have been measured by Went (1941). Here too the red portion of the visible spectrum is most effective, the response being an increase in leaf area and suppression of internode elongation, while the blue is relatively ineffective. The spectrographic arrangement of Parker et al. (1946), which affords high dispersion, large irradiation area, and high intensity of a given wave length region but with low intensities of other wave lengths as impurity, meets the necessary conditions for use in a detailed study of the action curve for etiolated peas. MATERIALS AND METHODS.-Spectrograph apparatus.-The description of the spectrograph which has previously been given in detail (Parker et al., 1946) is repeated here for immediate reference and to indicate required modifications. The energy source is the crater of a high-intensity arc with rotating cathode (fig. 1). A diaphragm essentially serving as the slit of the spectrograph is placed in the convergent beam from the condensing lens. The essential focusing element is a front-aluminized concave spherical mirror of 2-m. focal length, conjugate foci of which are the slit and focal plane. Two large glass prisms, set for minimum deviation at 5780 A, are placed in the convergent beam from the mirror and the emergent light is reflected from a flat front-silvered mirror that can be rotated by a tangent screw. In experiments with etiolated pea seedlings the instrument was used essentially as a monochromator to throw a narrow wave length region on a slit in the focal plane. This narrow band of the spectrum 1 Received for publication September 8, 1948. had to be spread out to permit radiatinig the seedlings in the 35 cm. boxes and to give variation of intensity. This was accomplished by use of an achromatic lens as shown in the diagram. The F7 lens had previously been used for collimation on a large Steinheil Spectrograph and had a focal length of 195 mm. A 5.5-m. I beam was used as an optical bench in the diverging beam from the achromatic lens. Boxes of peas could be placed along this beam at predetermined distances from the lens corresponding to given magnifications. The bench was housed in black cloth to eliminate all light except that entering through the lens. A side opening in the housing led to the dark room in which the peas were held before treatment and to which they were returned. Intensity could be reduced as much as 1000-fold by the divergence and transmission of the lens (68 per cent). It was further reduced when desired by introducing neutral screens (blackened wire gauze) of known transmissions between the slit and the concave mirror, a more than 100-fold factor being obtainable in this way. Wave length calibration was carried out by use of a low-pressure mercury arc to illuminate the slit. Energy values for definite wave bands were measured with a ten-junction thermopile that was standardized with an N. B. S. certified lamp. Spectrographic procedure.-An action curve can be considered as expressing the energy required to give a definite effect as a function of wave length. This can be obtained in several ways for response of etiolated pea seedlings. The method adopted required two different types of treatment. It avoided investigation of variation of response with duration of illuminations which is intrinsically of interest but not immediately pertinent to the objective. In the first instance the variation of response was measured at different energies given in constant time. A 4-min. daily interval for illumination on 4 successive days was adopted on the basis of previous work by Went (1941). Energies were varied by approximately 20,000-fold for a given wave length band. Sufficient points were taken to give the response, change in leaf length, as a function of energy. These measurements were made for five regions varying from the blue to the red portion of the spectrum. Experiments, were performed as follows. Three stations were established on the optical bench in the diverging radiation from the lens in the focal plane to give relative energies of 18.5, 4.30, and 1.0. Two boxes of peas were irradiated at each of these stations. Neutral screens were then placed in the light beams to reduce the intensity to 1/80 of

Light from sunlight, incandescent, and fluorescent sources was filtered through fresh tobacco (Nicotiana tabaccum L.), corn (Zea mays L.), and soybean (Glycine max (L.) Merrill) leaves. The spectral quality of the leaf-filtered light showed that much more of the incident red energy was absorbed than the far-red. The effect of the leaf-filtered light on the phytochrome-controlled germination of six weed species was generally to inhibit germination of seeds given a stimulatory pre-irradiation of red light from a standard source. Germination of seeds with no pre-irradiation was either not promoted or promoted to various degrees. Unfiltered light, at intensities equivalent to those under the leaf filters, caused no comparable effects. These results indicate an effect of altered spectral quality of the leaf-filtered light on the ratio of inactive/active phytochrome (Pr/Pfr) in the underlying seeds. We suggest that such phenomena could influence the germination of weed seeds in the field.

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.

Flowering of chrysanthemum plants under short photoperiods, as is well known, is prevented when the plants are illuminated near the middle of the long night. Such illumination inhibits flowering whether it is given continuously or intermittently, and whether it comes from incandescent or from fluorescent lamps. We discovered, however, that fluorescent light applied intermittently (cyclically) throughout the entire 16-hour long night was far less inhibitory than when applied during only part of this dark period. By contrast, incandescent filament illumination is strongly inhibitory under these conditions. The cycles of fluorescent light usually lasted 15 minutes, 1.5 minutes of light followed by 13.5 minutes of dark. When such cycles were applied for only 12 hours, leaving 4 hours of uninterrupted darkness in each long night, inhibition of flowering was complete again.