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1. Four lots of barley seedlings were grown on 16-hour photoperiod to ages of 5, 10, 15, and 20 days, respectively, at 65⚬ F. Each lot was then divided into three groups and all were grown for another week under the same conditions of light and photoperiod. One group of each lot, however, was continued during the week at 65⚬ F., one was transferred to 45⚬ F., and the third to 35⚬ F. The plants were then removed from the control rooms and transplanted out of doors for final differential photoperiodic treatment. 2. At the beginning and end of the temperature treatments representative plants from each lot were photographed and dissected. The total number of nodes in the main axis of each was determined, and enlarged photographs of the terminals of the main axes were made. 3. Ten plants from each age and temperature group were subjected to 12,- 16-, 20-, and 24-hour photoperiods after they were moved outside and remained on these photoperiods at natural temperatures for the duration of the experiment. 4. Numb...

1. When whole plants of Biloxi soy beans are subjected to 8 hour photoperiods, initiation occurs if the intensity of light during the photoperiod is above 100 foot candles. Below 100 foot candles no flower primordia are initiated. 2. When plants are given an 8 hour photoperiod of natural light supplemented by 8 hours of Mazda light, initiation occurs if the intensity of the supplemental light is below 0.5 foot candle, but does not occur if the intensity is above 0.5 foot candle. 3. The stimulus that causes initiation of flower primordia at the growing points arises in the leaves and moves to the growing plants. 4. Flower primordia may be initiated at growing points that are kept, either in complete darkness or on photoperiods above the critical, provided the leaves are kept on short photoperiods. 5. Photoperiods shorter than the critical, applied directly to growing points, have no effect on flower bud initiation. Control of initiation is exercised only through application of photoperiods of proper length...

1. The purposes of this investigation were to obtain quantitative data on the photoreactions that prevent flowering of short-day plants, from which an action spectrum relating wave length to photoperiodic effectiveness of light could be derived, and to draw such inferences concerning the nature of the photoreactions as the action spectrum would permit. 2. Experiments designed to give the action spectrum made use of a specially designed prism spectrograph having a dispersion of 15 A. per cm. at 5000 A. At this wave length and with an effective slit width of 100 A. the energy was about 3000 ergs per sq. cm. per second with the slit illuminated by a carbon arc operated at 12 kw. input. 3. Plants investigated were soybean, Soja max (L.) Piper var. Biloxi, and cocklebur, Xanthium saccharatum Wallr. To facilitate irradiation, the foliar surface of the plants of each species at the beginning of an experiment was reduced to a single leaflet or leaf, respectively. 4. The experimental treatment was based on the fac...

1. Tomato seedlings were decapitated above the second lobed leaf and a mixture of 20 mg. of indoleacetic acid in 1 gm. of lanolin was applied to the cut surface. Similarly, decapitated plants were kept as controls, either untreated or with pure lanolin applied to the cut surface. 2. Gross observations and histological and microchemical studies were made of treated and control material collected at frequent regular intervals for a period of 216 hours after treatment. 3. The internode terminated by the cut surface fails to enlarge after decapitation unless the cut surface is treated with indoleacetic acid-lanolin mixture. The internodes so treated enlarge at about the same rate as do the internodes below. 4. The first change observed after treatment was enlargement of cells of the epidermis and cortex. 5. Many of the tissues of the stem became meristematic in response to the treatment, although most of this activity was confined to a zone 0.5 to 2 mm. from the treated surface. Most parenchymatous cells in t...

1. Biloxi plants or parts of Biloxi plants, subjected continuously to daily photoperiods of 17 or more hours, were used as one component of all grafts reported in this work. Plants of this variety do not develop sufficient flower-forming stimulus on long photoperiods to cause floral initiation. They therefore served to determine whether or not a flower-inducing stimulus was transmitted across the graft unions. They are referred to as receptors and the plant or plant parts grafted to them as the donors. 2. The donor components were from Agate, Batorawka, or Biloxi varieties of Soja max, or from Red Kidney, Plentiful, Black Valentine, or Dwarf Horticulture varieties of Phaseolus vulgaris. 3. Methods of grafting employed were approach grafting of stems, splice grafting of petioles, splice grafting of stems, and bud grafting. 4. A total of 490 Agate-Biloxi approach grafts were made and all formed strong unions. Approximately 50 per cent of the Biloxi receptors formed flower primordia. This percentage was some...

1. The influence of low temperature in suppressing flower-bud initiation in Biloxi soybean has been considered from three points of view: its influence upon the differentiation of flower buds from the meristems; its influence on translocation of a flower-forming stimulus from the leaves to the growing points; and its influence upon those processes in the leaf that bring about flower-bud formation. Data are presented on the first two of these points; the last is being subjected to further investigation. 2. Fewer flower buds were formed in response to a 4-day induction treatment on plants bearing a single leaf if the terminal buds or the petioles were cooled to 3⚬ C. than if they were not cooled. When the temperature was raised to 10⚬ C. the inhibiting effect on flowering decreased in both types of experiments, but even at this temperature only slight growth of the terminals occurred during the induction treatment. 3. Petiole-cooling experiments involving plants with two leaves indicate that inhibition of f...

1. Initiation of flower primordia in Biloxi soybean was limited by controlling photosynthesis during induction. This was done in one case by controlling the CO2 supplied to the plants and in another by controlling the duration of high-intensity light. 2. When no CO2 was supplied during 8-hour photoperiods, no initiation of primordia occurred. Plants that received the CO2 contained in natural air during 2, 4, 6, and 8 hours of each photoperiod produced flower primordia in proportion to the duration of time the natural air was supplied. 3. Plants that received only 1 hour of high-intensity light during an 8-hour photoperiod formed no flower primordia. Those that received 2 hours or more of high-intensity light during the photoperiod produced increasingly larger numbers of flower primordia as the duration of high-intensity light increased. 4. Increased concentration of CO2 in natural air resulted in increased floral initiation.

1. Flower primordia were present on plants of Taraxacum kok-saghyz collected from several localities in the central or northern parts of the United States during the late fall or winter months. Photoperiods of 8, 10, 12, 14, 16, and 18 hours did not inhibit the development of these primordia into mature flowers when the plants were transplanted in the greenhouses. 2. Wide variability of leaf pattern was observed in the seedlings, but morphological characteristics of the flowers indicated that almost all were kok-saghyz plants. Seedling plants subjected to six different lengths of photoperiods varying from 8 to 18 hours flowered under all conditions, but more of them flowered with photoperiods of 12 hours or longer than with shorter photoperiods. 3. Plants that received low temperatures during their early development flowered more abundantly than those that received no low temperature. Ninety-eight per cent of one lot of seedlings that had received several weeks of cold weather during the spring flowered, ...

1. Wintex barley grown with an 11.5-hour photoperiod and a 12.5-hour dark period remained vegetative. If the dark period was interrupted with a brief period of irradiation of sufficient intensity, spikelet formation was stimulated. 2. By use of this technique of interrupting the dark period, quantitative data on the photoreaction that promoted flowering were obtained, and action spectra relating wave-length to photoperiodic effectiveness of light were derived. 3. The most effective time to apply the dark-period interruptions was the 2-hour period beginning 6.5 hours after the start of the dark period. Within this time and with the intensities used, the reciprocity law held. Energy required to promote flowering, if applied continuously throughout the 12.5-hour dark period, was about tenfold greater than if applied within the 2 hours near the middle of the dark period. 4. The action spectrum for the production of spikes in barley was very similar to the action spectra for the prevention of floral initiation...

1. Biloxi soy bean plants with flower primordia initiated upon them were transferred to photoperiods of 8, 10, 12, 13, 14, 15, 16, and 18 hours. The development of these primordia and the flowering and fruiting responses of the plants were determined. 2. The plants transferred to photoperiods of 8 to 13 hours bloomed nearly simultaneously and all produced fruits. The yield of fruits on the 8 hour lot was somewhat less than on the 10, 12, and 13 hour lots. 3. Flowering on the 14 and 15 hour plants was later than on those of shorter photoperiod, the flowers were less numerous, and no fruits were formed. 4. No flowers opened on the 16 and 18 hour plants during the experiment. 5. When plants were transferred to a range of photoperiods after flower primordia were initiated it was found that the longest photoperiod on which fruit formation occurred was 13 hours and the shortest one on which no flowering took place was 16 hours. 6. Plants were grown for biochemical studies on 8, 13, and 16 hour photoperiods afte...

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

1. Biloxi soybeans were grown in the greenhouse for four to five weeks and transferred to a series of control rooms where various combinations of temperature during the photoperiod and the dark period were applied for five days. Photoperiods of both 8 and 16 hours were used. 2. The effect of these various temperatures on the initiation of flower primordia has been determined. 3. Initiation of flower primordia was influenced to a much greater extent by variation in temperature during the dark period than by variation during the photoperiod. 4. When the temperature during the photoperiod was constant, a 55⚬ temperature during the dark period limited the amount of initiation that occurred. At 65⚬ initiation was much more extensive. This rise of 10⚬ in temperature during the dark period produced a greater difference in initiation than any other temperature variation of the same amount. 5. Variation in temperature resulted in the formation of different numbers of nodes. These differences, however, were not gre...

1. Vegetative plants of Hyoscyamus niger can be induced to flower by application of relatively small amounts of radiant energy near the middle of dark periods that would prevent flowering if uninterrupted. 2. Interruptions of 12-hour dark periods with energies corresponding to about 100 foot-candle-minutes of light were effective in causing initiation of flower primordia. Minimum energy to cause a given effect was relatively constant if the interruption was made during the 2-hour period following the middle of the dark period. 3. The action curve for floral initiation of H. niger was readily established in the region from 7300 A to 5600 A. Limitation of energy from 5600 A to 4000 A made it impossible to establish a continuous response curve for this region, but sufficient points were obtained to establish the limits of effectiveness. This curve is essentially the same as those for control of flowering in soybean, cocklebur, and barley, and for regulation of leaf size in etiolated peas. 4. Similarity of th...

1. Individual leaves on a Biloxi soybean when subjected to photoperiodic stimulation differ in their capacities to cause floral initiation. 2. The most effective leaf on the plant is the one that has most recently attained its full size. Young leaves increase in their capacity to effect floral initiation until they attain full size, after which they gradually decline in effectiveness. 3. The most active leaf, operating alone, is able to cause initiation of as many flower buds per plant as are formed when all leaves function simultaneously. 4. The capacities of different leaves to supply a flower-forming stimulus is correlated with their relative states of maturity and not with their distances from the growing points where flowers are formed. When the flower-forming stimulus from the third compound leaf passes downward through two internodes to the bud in the axil of the first compound leaf, it induces the formation of more flower primordia there than does the stimulus from the first compound leaf which is...

1. Apparatus was devised that made possible the application of controlled temperature to individual leaves of Biloxi soybean plants during the dark periods of a photoperiodic treatment, while the remainder of the plant received greenhouse temperatures and long photoperiods. 2. When a leaf was held at 50⚬F. or lower during a 5-day induction period, floral initiation was greatly inhibited. At 70⚬-90⚬, such initiation was in general equal to that of the controls held at greenhouse temperatures, but at 90⚬ or higher the extent of the initiation again was less. 3. On the basis of the data presented in this and two preceding papers, the inhibiting effect of low temperature on floral initiation in Biloxi soybean plants appears to be the result of its effect on the photoperiodic reactions occurring in the leaf blade during the dark period, rather than through its effect on translocation of a flower-inducing stimulus from the leaf to the terminal meristems or its effect at the terminal meristems upon the different...

INTERRUPTION of the dark period of a 24-hr, cycle with visible radiation has been found to control reproduction in representatives of the Spermatophyta, Chordata, Arthropoda and Mollusca. Elongation, possibly cell division, in certain seedling structures is controlled by the same photoreaction as that for the photoperiodic control of reproduction in plants. It seems quite probable that the plumage cycles of birds, production of wings by aphids, prevention of diapause in some insects and the regulation of hair cycles of certain animals are controlled by the same photoreaction that controls sexual reproduction. A considerable volume of descriptive literature has grown up on this day-length phenomenon, which is a major expression of species adaptation to changing season.