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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...

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. In a study of the effects of photoperiod and nitrogen nutrition on the expression of certain morphological characteristics of soybean, plants of Morse, Virginia, Lincoln, and T-48 varieties were grown either in controlled-environment rooms or in the greenhouse and in either soil or sand cultures. 2. In experiments in which nitrogen nutrition was not a factor, all varieties flowered on all durations of photoperiod, but first-flower primordia were formed at a higher node on the plant axis with the longer photoperiods. 3. Position of first-flower primordia did not vary with nitrogen treatment when the plants were grown with short photoperiods. When grown with long photoperiods, however, plants of certain varieties initiated first-flower primordia at higher nodes as the amount of nitrogen in the nutrient solution was increased. Variety T-48 was outstanding in this respect, Lincoln was intermediate, while Morse and Virginia were least responsive. 4. Plants of Lincoln and Morse grown on long photoperiods sho...

Action spectra between 4100 and 8500 A for seed germination and radicle elongation were measured for Nemophila insignis and Lactuca sativa var. Great Lakes and between 6500 and 8500 A for seed germination of Lamium amplexicaule. The responses observed are features of the continued excitation of the photomorphogenic pigment in those regions of the spectra where both pigment forms have appreciable absorptivities.

1. The purpose of this study was to examine effects of photoperiod on various stages of development of soybean plants of several varieties and to attempt to evaluate the relative effects of small photoperiod differences such as occur at different latitudes or as a consequence of different planting dates. 2. Each of four experiments included six varieties, some of which were present in every experiment. Four or more photoperiod treatments were used in each. In three experiments photoperiods of several fixed durations were applied either throughout the entire period of plant growth or during part of it. In one experiment the photoperiods simulated the seasonal changes in daylength of three different latitudes. 3. Plantings were made on several dates in one fixed-photoperiod experiment and in the variable-photoperiod one. The dates on which certain stages of reproductive development were started or completed, as well as plant heights, number of nodes, and position of first flowers, were observed. 4. Signific...

1. Internodes of young seedlings of Pinto bean (Phaseolus vulgaris L.), grown under fluorescent lights, elongated to about three or more times their normal length in response to low radiant energies of unfiltered incandescent-filament light applied at the close of daily high-energy fluorescent-light periods. The lengthening response was due to the far-red radiant energy of the supplemental light and was completely and repeatedly reversible with red. Maximum lengthening occurred when the plants received cycles of about 4 hours of light and 20 hours of dark. Plants that received no daily high-energy period of light did not elongate in response to far-red treatments unless given sugar solution through their severed hypocotyls. 2. The lengthening response was found to result from the action of a photoreversible pigment which also controls flowering, seed germination, and certain other photoregulated phenomena. Experiments with bean internode elongation show that this pigment is present and functioning in the ...

1. A study was made of the several effects of photoperiod and nitrogen nutrition upon the bulb and flower-stalk development of onion. Plants were grown from seed, sets, and mature bulbs, and with soil and in sand culture. 2. Plants grown on photoperiods substantially longer than that critical for bulb formation showed no difference in bulb development when supplied different amounts of nitrogen. 3. Plants grown on a photoperiod at or near that critical for bulb formation exhibited the greatest bulb development with the lowest nitrogen concentration and the least with the highest concentration. 4. Under certain environmental conditions nitrogen and photoperiod can interact to influence bulb development of certain onion varieties. 5. Onion plants of sister lines gave similar bulbing response on photoperiods of identical duration. However, the two lines differed in their production and development of visible floral stalks, especially on the shorter of the photoperiods. Development of floral stalks was favore...

The germination of Kentucky bluegrass, (Poa pratensis L.), seeds is profoundly influenced by two light reactions. One, the phytochrome reaction (P), is promotive, and the other, the so-called “high-energy reaction (HER)”, is inhibitory to germination. The level of germination displayed in 14 days as the resultant of these two opposing reactions is appreciably influenced by temperature. In darkness, the seeds germinate well at certain temperature alternations but not at constant temperatures. At 15-25°C, the promotive effects of temperature alternation are accomplished in the first 5 to 6 cycles. The promotive effects of alternations are displayed in darkness when the daily period at 25° is between 4 and 14 hours. Brief daily high-intensity fluorescent illuminances (approximately 4,000 ft-c) during otherwise continuous darkness at constant 20°C induce high germination in most lots. Continuous medium-to-high intensity illumination (approximately 1,200 ft-c) very weakly promotes germination and in potentially promoted seeds inhibits germination to about the level of the dark controls. Inhibitory effects of continuous light on potentially promoted seeds are best displayed at 20°C constant although, in ‘Newport’ they are observable at 15-25°. The inhibition of ‘Newport’ at 15-25° is to a level below that of the dark controls but above that caused by prolonged illumination at 20°. Thus conditions most promotive to germination are 15-25°C alternations and brief daily illuminances of high intensity. Simultaneous application of these 2 conditions causes higher germination than when either is applied alone and in one lot of ‘Newport’ the effects are strikingly additive.

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.

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. The effects of photoperiod on the growth of several tree species-American elm (Ulmus americana), flowering dogwood (Cornus florida), horse chestnut (Aesculus hippocastanum), red maple (Acer rubrum), sweet gum (Liquidambar styraciflua), tulip poplar (Liriodendron tulipifera), paulownia (Paulownia tomentosa), Asian white birch (Betula mandshurica), catalpa (Catalpa bignonioides and C. speciosa), and pine (Pinus taeda, P. virginiana, and P. sylvestris)-were investigated. 2. In general, short days induced dormancy, and long days prolonged growth. The various species differed in their response to short (8-hour) days, ranging from tulip poplar, which stopped further growth after about 10 8-hour days, to elm, which required 20 weeks of 8-hour days before the plants stopped elongating new structures. However, most of the species tested seemed to required about 4 weeks of 8-hour days before they stopped growing. At higher temperatures it took a greater number of 8-hour days to induce dormancy, and at temperatur...

Dark-germinating seeds of Ace and Porte tomato varieties can be inhibited by far-red radiation. Germination of inhibited seed is promoted by red radiation and in turn is reversed by far-red radiati...

1. This paper is concerned with the development of the female gametophyte, the pollen tubes, and the embryo of Daucus carota. 2. A single archesporial cell functions directly as a macrospore mother cell which produces a linear tetrad of macrospores. 3. The macrogametophyte, which arises from the chalazal macrospore, is of the 7-celled type characteristic of most angiosperms. 4. The presence of a cellulose filiform apparatus was demonstrated by microchemical tests. 5. The structure and path of the pollen tubes were investigated. Tubes were found to grow intercellularly down through the conducting tissue of the style to its base, and then superficially along a groove leading to a canal communicating with each locule. Tubes growing down one style may enter the locule immediately below or grow through the transverse canal and into the other locule. 6. A filamentous 8-celled embryo is usually formed before longitudinal divisions occur. The three cells farthest from the micropyle give rise to all of the embryo ...

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.

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...

The rate of dark reversion of the far-red absorbing form of phytochrome (Pfr) to the red absorbing (Pr) was measured by means of the flowering response of Chenopodium rubrum L. It is approximately 0.8%/min of the total phytochrome over the range of 2–90% Pfr. Degrees of interconversion of Pr ⇄ Pfr under irradiation in the region of 380–500 mμ were measured at several intensity levels. Equilibrium was attained in about 8 min with the highest intensity (1.7 mw) at a Pfr level of about 35%. The average absorbancies of Pfr and Pr in the region of 380–500 mμ (average 440 mμ) are about 3 × 106 and 6 × 105 cm2 moles-1, respectively.

1. Seeds of lettuce variety Grand Rapids vary with duration of imbibition in their sensitivity to promotion of germination by red radiation and inhibition of germination by infrared radiation, the changes being opposite for the two effects after about 12 and 20 hours of imbibition at 20⚬ C. 2. Respiration changes in nongerminating seed of lettuce varieties Big Boston and Slobolt that are correlated with these changes in sensitivity to radiation also occur. 3. Respiration of the nongerminating seed is constant during the periods between the changes in sensitivity to radiation. 4. The relation of these phenomena to the photoreaction controlling seed germination is discussed in terms of limiting factors.

1. How light acts in controlling flowering of several varieties of chrysanthemums was studied with greenhouse-grown plants from rooted cuttings and transferred to plant-growth rooms for the duration of the experiments. The radiation treatments, which were of 8 days' duration, were preceded and followed by non-inductive photoperiodic conditions. In most experiments the treatments consisted of brief irradiations with red or far red or both near the middle of the dark period, and the plants were dissected 2 weeks later to observe the flowering conditions. The stage of floral initiation was designated by numbers from 0 to 10; 0 represents a vegetative condition of the terminal meristem, and 10 the most advanced condition, in which perianth primordia were present on all florets. Dissection results were found to agree very closely with macroscopic observations made on other branches of the same plants after the plants had developed for several weeks after the light treatments. 2. Floral initiation was markedly ...

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. The observations of Flint and McAlister that imbibed seed of some lettuce varieties can be promoted in germination by irradiation in the red portion of the spectrum and inhibited in the infrared were verified. 2. The action spectra for promotion and inhibition were measured in detail for wave lengths greater than 4000 A. Maximum sensitivity for promotion was found in the region 6400-6700 A (red) and for inhibition in the region 7200-7500 A (infrared). 3. Absorption of radiation in the red or in the infrared region changes the effective pigment into the infrared- or the red-absorbing form, respectively. The alternation of form can be repeated many times. 4. The action spectrum for lettuce-seed germination is the same as that effective for photoperiodic control of floral initiation. The two phenomena involve the same initial photoreaction. 5. The photoreaction involves monomolecular isomerization of the effective pigment. 6. A reaction by which the pigment changes from the infrared- to the red-absorbing ...

1. Anthocyanin synthesis induced by radiation in apple skin and in red cabbage and turnip seedlings depends upon continued excitation of the photomorphogenic pigment in spectral regions where absorptivities of both pigment forms are appreciable. 2. The rates of anthocyanin synthesis calculated for absorptivities obtained from measurements of bean stem elongation and germination of lettuce and other seeds are in close agreement with observed values. 3. Photomorphogenic responses arise both from the enzymatic action of the far-red-absorbing form of the effective pigment and from continued photoexcitation. 4. Photomorphogenic responses to radiation in the blue part of the spectrum also arise from absorptions of the photomorphogenic pigment.

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.

Effects of active phytochrome (Pfr) on flowering in cocklebur, Xanthium pensfivanicum Wallr., subjected to unusually short photoperiods were examined. For interpretation of the results, Pfr is considered to be an enzyme that catalyzes a reaction that controls many phenomena including flowering. In cocklebur plants phytochrome is in the Pfr form at the end of the photoperiod. After photoperiods of 11/2 or 2 hr the Pfr, although present, does not act during the first 6 or 8 hr of darkness. This failure is attributed to delay in arrival of substrate for Pfr action. Between the eighth and tenth hours Pfr acts to promote flowering. After the tenth hour it is no longer present, and, if it is then reintroduced by a red irradiation, it inhibits flowering. Dark reversion of Pfr is completed simultaneously with completion of its flower-promoting action and is regarded as a consequence of its action. The amount of radiant energy received by cockleburs during a 3-hr photoperiod determines whether Pfr action is necess...

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. Flowering of chrysanthemum, Chrysanthemum morifolium Ramat., is prevented by dark-period interruptions of 4 hours, which convert phytochrome to the flower-inhibiting far-red-absorbing form (Pfr) and maintain it in this form throughout the light period. 2. Pfr is also maintained at an effective level by intermittent lighting provided the dark periods between successive periods of light are so short that dark reversion of Pfr to an ineffective level does not occur. If the light is from incandescent-filament lamps, the dark periods between successive light breaks must not greatly exceed 30 minutes. Cycles of light and dark from 30 minutes to 1 minute are equally effective. 3. Effective cycle length depends on the ratio of red to far-red energies in the light. As the relative amount of red increases, the amount of Pfr formed and the time required for its reversion in darkness to a level ineffective for flower inhibition also increase. For these reasons light from ruby-red, incandescent-filament, and fluore...

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.

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...

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.

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...

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.

Flowering of chrysanthemum (Chrysanthelmum morifolium Ramat ) is inhibited by a few hours of continuous or intermittent fluorescent or incandescent- filament illumination of low intensity. It is also inhibited by a 1-min fluorescent illuminance of a few hundred ft-c but not by one of several thousand from an incandescent-filament source. This difference in effectiveness of 1-min illuminances of fluorescent and incandescent light results from differences in their red-far-red energy ratios which are made even greater by chlorophyll screening in the leaf. Screening of red by chlorophyll tends to increase the amount of far red relative to red reaching phytochrome. Fluorescent light contains so little far red that screening does not cause a biologically significant change in the red-far-red ratio. In sunshine and incandescent-filament light, however, screening changes the ratio to such an extent that it can have striking biological consequences. The success of a 1-min fluorescent illuminance in preventing flow...