SMITHSONIAN MISCELLANEOUS COLLECTIONSVOLUME 92, NUMBER 11
PHOTOTROPIC SENSITIVITY IN RELA-TION TO WAVE LENGTH(With Two Plates)
BYEARL S. JOHNSTONDivision of Radiation and Organisms, Smithsonian Institution
(Publication 3285)
CITY OF WASHINGTONPUBLISHED BY THE SMITHSONIAN INSTITUTIONDECEMBER 6, 1934
BALTIMORE, UD., V. S. A.
PHOTOTROPIC SENSITIVITY IN RELATION TOWAVE LENGTHBy earl S. JOHNSTONDk'ision of Radiation and Or(/anisnis, Smithsonian Institution(With Two Plates)INTRODUCTIONAsymmetric growth resulting from unilateral stimulus has heen des-ignated tropism. Growth curvatures following unilateral illuminationare usually classified under the term phototropism. Different plantsrespond in different degrees to light, but perhaps those most fre-quently used in phototropic experiments are the sporangiophores ofPhycomyces and the coleoptiles of Az'ena. In such studies the in-tensity, the wave length, and the duration of exposure to light eachacts as a contributing factor toward the final result. Just as thereappears to be a threshold of intensity for a given duration of lightexposure, so there are wave lengths which seem to exert no influenceon these growth responses, but with exposures to other wave lengthsthe plants show distinct degrees of sensitivity. Not only do differentplants vary in their sensitivity, but separate portions of the same plantrespond differently. Recent work on growth substances indicates thepresence of factors other than light in this complex plant-response.In the present paper the subject is limited, in the main, to the influ-ence of radiation of different wave lengths on phototropism as shownby the response of the coleoptiles of Avena sativa. The variety usedis Culberson, C.I. no. 272,, for which the author wishes to thankMr. T. Ray Stanton, of the United States Department of Agriculture.All the light intensity measurements were made by Dr. E. D.McAlister, to whom credit for that part of this work is given.HISTORICAL SURVEYMany of the early experiments on phototropism have been reviewedby Parr (1918) and the data classified under four general theories:I. The " intensity " theory originating with De CandoUe in 1832 andadhered to in a more or less modified form by Wiesner, Darwin,Engleman, Oltmanns, Yerkes, Loeb, and Davenport. 2. The ray-
Smithsonian Miscellaneous Collections, Vol. 92, No. 11
2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92direction theory advanced by Sachs in 1876 and supported by theexperiments of Strasburger, Davenport, and Canon. 3. The wave-length theory first investigated by Payer in 1842. 4. The energytheory first mentioned by Miiller in 1872 in which the maximumresponse of cress seedlings shifted in the spectrum for differentenergy values of the wave lengths studied.The basis for much of the recent quantitative work on phototropismwas laid by Blaauw (1909, 1914, 191 5, 1919)- His studies wereperhaps the first serious attempt made to interpret this growth re-sponse in terms of modern physics. Plant responses were studied indifferent spectral regions of sunlight and of the carbon arc and com-pared with the energy values calculated from Langley's (1884) tables.Blaauw found the most effective region of the carbon spectrum forphototropic response of Az'ena seedlings to lie between 4660 and4780 A, while the red and yellow regions were ineffective. Accordingto Blaauw (1914), the curvature of a plant resulting from unilateralillumination is caused by the light-growth responses on the oppositesides which are illuminated differently. The minimum amount ofradiation required to produce phototropic response was found to be20 meter-candle-seconds. It also appears from his work that forequal effects the product of light intensity and time of exposure is aconstant.It is impossible to evaluate the effect of wave length in manj^ of theearly phototropic experiments because of the lack of accurate physicaldata. Some 10 years after the early quantitative studies of Blaauw.Parr (1918) made a study of the responses of Pilobohis to differentwave lengths and intensities of carefully measured artificial light.The results of these quantitative studies are best summarized in herown words
:
(i) Pilobohis responds to the light of all the regions of the visible spectrum.(2) The presentation time decreases gradually from red to violet. There is noindication of intermediate maxima or minima. (3) The presentation time doesnot vary in direct ratio vi^ith the measured value of the energy of the light inthe different regions of the spectrum. (4) The presentation time varies ininverse ratio to the square roots of the wave frequency. (5) The product of thesquare root of the frequency times the presentation time, decreases with thedecrease in the energy value of the spectral regions, and is an approximate con-stant for a given light-source. (6) The spectral energy in its relation to thepresentation time may be expressed approximately in the Weber-Fechner formula,if the wave-frequencies be made a function of the constant. (7) The relationof the spectral energy to the presentation time may also be approximatelyexpressed in the Trondle formula, the wave-frequencies being made a functionof the constant.
NO. II PHOTOTROPIC SENSITIVITY JOHNSTON 3About the same time Hurd (1919) showed wave-length effect onyoung rhizoids by equalizing the intensity of the light coming througha series of Wratten filters. Only the blue (4700 to 5200 A) andviolet (4000 to 4700 A) lights produced phototropism, negative indirection. The other lights at the intensity of 1800 meter-candles hadno effect. However, with a greater intensity the green light (5200 to5600 A) exerted a negative phototropic effect as well as the blue andviolet.For the purpose of investigating the wave-length effects of radia-tion on phototropic bending of young plants, Johnston (1926) con-structed and described a simple plant photometer. The apparatusconsisted of a long box divided into three compartments. Each endcompartment contained an electric lamp which could be moved towardor away from the light-filter window in the partition separating itfrom the central or plant compartment. Plants which easily respondin their directional growth to differences in light intensities were em-ployed in place of the adjustable indicator or photometer screen inthe ordinary Bunsen photometer.Sonne (1928-1929) determined the necessary amount of energy ofdifferent wave lengths to produce a minimum phototropic response inoats. The young plants were so placed that about i cm of their tipswere exposed at different distances from the light of a mono-chromator for different exposure periods. The visible part of thespectrum of a Hefner lamp was used as a standard of comparison.Minimum response was obtained at 0.86 x io~^ g. cal. per cm^ in isecond. The energy was measured by a thermo-element. The resultsare summarized in table i.
Table i.—Sonne's Data showing Phototropic Sensitivity Determined from theAmount .of Energy Required to Produce a Minimum Responsein OatsWave length
4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92
It will be seen from this table that the amount of energy whichbarely causes phototropic curvature varies with the wave length. Theyellow (5700 A) is about 600 times as intense as is the white lightnecessary to bring about the same response, while the green (5460 A)is approximately 400 times as intense, and the blue (4360 A) only
.03 as strong as the energy of his standard white light. The blue isthus approximately 10,000 times as efifective phototropically as thegreen and 20,000 times that of the yellow. The violet (4050 A) isalso very efifective but only about half that of the blue.
350
Fig. I.—Graphs from Bachmann and Bergann showing the sensitivity of Avenasativa to wave lengths of light (continuous line) compared with their cor-rected values of Blaauw (crosses), of Sonne (circles), and of Koningsberger(horizontal lines).Bergann (1930) made a very careful study of the effects of mono-chromatic light on the growth and bending of Az'cna sativa as well asthe effects produced by a change of intensity and length of exposure.Employing the method of placing the young plant between two oppos-ing lights, he concludes that the regions other than the red and infra-red produce corresponding growth reactions for suitable intensities.In unilateral light equal bending is shown for corresponding intensi-ties, first positive, then negative, and finally positive. Light curvaturesand light-growth reactions are parallel processes. The stronger thelight-growth reaction in a given wave-length region, the greater willbe the phototropic response. The seedlings " choice " in the com-pensation experiments between two wave-length ranges is always thatwhich corresponds to the stronger growth reaction.
NO. II PHOTOTROPIC SENSITIVITY JOHNSTONBachmann and Bergann ( 1930) review the early work of Blaauwand correct the energy values of his data for light absorbed by CUSO4and water filter, surface reflections, and color filter in order to com-pare his results with those obtained by Bergann. The results of Sonneand Koningsberger are also corrected and compared. These data arerepresented gra])hically in figure i, in which the continuous line isthe sensitivity curve. The data from Blaauw's work are indicated as
inn
6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92
sporangiophores were placed between two light sources. The intensi-ties were adjusted until the phototropic effects of the differentspectral regions were equal. At this point the efficiency of each regionwas taken as proportional to its relative energy content. Wrattenfiiters were used in conjunction with a copper chloride filter. The mostsensitive region proved to be in the violet (4000-4300 A). In figure 2Castle compares his results with those obtained by Blaauw and Parr.It is pointed out that because of the presence of " accessory " pig-ments in these sporangiophores care must be taken in correlating theseresults with those obtained from the absorption spectrum of thephotosensitive substance.PRELIMINARY EXPERIMENTSThe general method of studying the wave-length effects on photo-tropism as described by Johnston (1926) was used by Johnston,Brackett, and Hoover (1931) with an improved plant photometer forevaluating four spectral regions in terms of plant response. The gen-eral procedure was to place an oat seedling between two different andoppositely placed lights, and after an interval observe the growthcurvature. If, for example, when the seedling was exposed to blue andto green lights, a distinct bending was noted toward the blue side,the lights were so adjusted as to increase the green or decreasethe blue intensity. Another seedling was then used and the processrepeated until a balance point was reached where the effect of onelight neutralized the effect of the other. When this balance point wasdetermined, a specially constructed thermocouple replaced the plantand the relative light intensities were measured. From these experi-ments it was found that no measurable phototropic response wasfound for wave lengths longer than 6000 A ( Wratten no. 24—redfilter), while a noticeable bending was found with the yellow filter(Coming's heat-resisting yellow—yellow shade), whose cut-off onthe short-wave-length side was 5200 A. The threshold for wave-length influence was found to lie somewhere between 5200 and6000 A. The effects of green and blue light (Wratten filters nos. 61and 47 respectively) were progressively greater, being in round num-bers 1,000 for the green and 30,000 for the blue times that of theyellow.These results justified a more elaborate and better controlled ex-periment wherein narrower spectral regions could be investigated.For this purpose Johnston (1931) used the specially constructedmonochromator illustrated in plate i. Care was taken to eliminatescattered light and to keep the conditions surrounding the coleoptile
NO. II PHOTOTROPIC SENSITIVITY—JOHNSTON 7
symmetrical, with the exception of the wave-length region beinginvestigated. A double-walled glass cylinder with water between thewalls slowly rotated about the axis of the coleoptile. Two strips ofpaper blackened on the inside and separated i cm from each otherwere wrapped about the cylinder in order to shield all but a restrictedregion at the tip of the coleoptile from the light. The cylinder wasencased in a light-proof box which contained two oppositely placedside windows. Through one window, light was passed from themonochromator, and through the other, light from the standard lamp.The standard used was a 200-watt, 50-volt projection Mazda lampwith the filaments in a plane. The standard lamp was enclosed in anair-cooled brass housing with one small glass window opening towardthe plant. The light from the standard was passed through a number6.0 Corning line filter, a heat-absorbing glass, and a water cell beforeentering the rotating cylinder surrounding the plant. The number6.0 Corning line filter transmitted wave lengths from about 4400 Ato 5800 A and from 7000 A to 12800 A of the light transmitted by thewater filter. The radiation intensity of the standard was 0.37 micro-watts/cm- at a distance of 25 cm. This value, of course, varied withdifferent lamps and also with the same lamp as it aged. A photo-graphic red lamp was used behind the small rear window of the plantbox for properly placing a coleoptile at the beginning of each expo-sure. Previous experiments showed the coleoptile to be insensitivefor all practical purposes to this particular light. The monochromatorlamp was located outside the phototropic room, which was a smallroom with no outside walls located in the west basement of the Smith-sonian building. Very little daily temperature fluctuation occurredin this room because of its ideal location.Coleoptiles of oats, Avena sativa Culberson, were used in all theseexperiments. The seeds were germinated at approximately 25° C.between glass plates covered with moist filter paper. The plates wereso placed in moisture chambers that the seedlings grew vertically. Acareful selection of the seedlings was made for straightness whenthey had attained a length of 2 to 4 cm. One was then transferred toa small Erlenmeyer flask fitted with a cork stopper. It was supportedby means of a little cotton in a small hole of the stopper. The flaskwas filled with distilled water so that the roots were entirely im-mersed. With the cross hairs in a small telescope as a guide, theseedling was adjusted to a vertical position within the glass cylinderlocated between the two lights.The general experimental procedure was to illuminate the coleoptileon its two opposite sides, preferably the narrow edges, and after a
8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92time interval to note the resulting growth curvature. If the lightadjustment was very much out of balance as indicated by the plant,a bending similar to that shown in plate 2 occurred in 20 to 30 min-utes. An adjustment was then made in the proper direction and theused seedling discarded for a new one. As the balance point wasapproached the exposure time necessarily increased. Finally on mov-ing the standard light back and forth through a distance of i cm, theplants could be made to curve repeatedly toward one light then towardthe other. The balance point was taken to be the midpoint betweenthese two positions. Care was always used not to expose the freshseedlings to any light but red in the preliminary handling. Priestley(1926) has shown that light afifects normal and etiolated shoots verydiiferently. The amount of light required to induce phototropic curva-ture in normal light-grown shoots is greater, and must be continuedlonger, than that required to bring a similar curvature in etiolatedshoots.After a balance point had been determined and tested by usingseveral seedlings, a specially constructed thermocouple was insertedinto the glass cylinder occupied by the seedlings and the light intensi-ties measured at the balance position. The junction of the thermo-couple was made of a short length of fine bismuth wire and one ofbismuth-tin alloy, each about 25 microns in diameter. The alloy wasmade up of 95 percent bismuth and 5 percent tin. Utmost care wasneeded in measuring the light intensities since the plants were foundto be much more sensitive to the light than the best physical instru-ments available. It should be remembered, however, that the seedlingintegrates the effect of radiation over a relatively long period, whilethe thermocouple responds in a few seconds.The results of this experiment are presented in table 2. The ratioof the intensity of the monochromator light to that of the standardlight is given in the third column for corresponding wave-lengthranges shown in the first column. Where filters were used in combina-tion with the monochromator they are indicated in the second column.No phototropic responses were obtained in any of the first six wave-length ranges. The first quantitative measurements that could bemade were for the range 5040 to 5160 A. In the last column of thetable the relative phototropic effectiveness of the different wave-length ranges is given. The ratio 29.10 was arbitrarily taken asunity.With unilateral illumination through the monochromator and anumber 'j'j Wratten filter in the region 5430 to 5670 A, bending oc-
NO. II PHOTOTROPIC SENSITIVITY JOHNSTON 9
curred in four hours. This indicated the approximate threshold regionof phototropism. In order to determine this point more accurately amercury arc in pyrex glass was substituted for the Mazda lamp of themonochromator. and by passing this light through a number "jyWratten filter, a seedling was unilaterally illuminated by the 5461mercury line. In five such tests, each lasting from two to severalhours, two gave positive bending and three no bending. With reason-Table 2.
—
Data from the Preliminary Experiment Shozving PhototropicEffectiveness of Restricted Regions of the Spectrum. That forWave-length Region 3040-3160 A is Taken as UnityFilter " RelativeWave-length used with Light intensity ratio phototropicrange (A) monochromator (Monochromator/standard) effectiveness7250-7700 W 886900-7300 W 886550-6950 CLF 26250-6600 CLF 25940-6270 CLE 35670-5950 TR5430-5670 W 22; CLF 5.15200-5430 W 775040-5160 CLF 6.1 29.10 i.o4940-5070 CLF 6.1 2.49 1 1.
7
4810-4930 CLF 6.1 0.68 42.84670-4800 CLF 6.1 0.54 53.94550-4670 CLF 7 0.29 100.34450-4550 0.27'^ 107.84410-4500 0.34" 85.64360-4450 0.36" 80.84280-4360 0.41" 71.04210-4280 0.47 67.04130-4220 0.84 34.64070-4135 1.49 jg^
^W, Wratten; CLF, Corning line filter; TR, thermometer red.With the standard lamp at a fixed position from the plant, the intensity of monochromatorlight was varied by changing the resistance in its lamp circuit until a balance point wasobtained.
able certainty it can be concluded that under these particular experi-mental conditions the threshold wave-length efi'ect is at or very near5461 A.When the phototropic efifectiveness is plotted against wave length,a curve is obtained as shown in figure 3, with its maximum at aboutwave length 4550 A. The horizontal lines represent the wave-lengthranges for which balance points were determined. Points where filterswere used in addition to the monochromator are represented as circles.
lO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92There is a slight suggestion of two other maxima, one on each sideof the peak. It could not be determined from these data whether ornot these secondary maxima were real. Furthermore, certain condi-tions existed during this preliminary experiment which make it impos-sible to consider this sensitivity curve more than approximately cor-rect. Although an attempt was made to burn the lamps at a constantvoltage, there was some fluctuation during the exposure of the seed-
L \. -U
^
L_—
1
1 1 1 1— ^1 I I <=>—4tO0 4200 4^00 4500 4600 4700 4900 5000 SIOO 5200 5300Fig. 3.—Phototropic sensitivity curve of preliminary experiment (continuousline). The ordinates are relative sensitivity values, the abscissae, wave lengthsin angstroms, and the horizontal bars indicate the wave-length ranges of thebalance points. Circles indicate points obtained with filters combined with themonochromator. Points more accurately determined are indicated by crossesand connected by dash lines.
lings and during the intensity measurements. Also, in some of thework the standard lam]> as well as its filter cell was cooled by tapwater. This resulted in an accumulation of iron on the glass surfacesduring the time required for determining the balance points. Theseuncontrolled factors undoubtedly modified to some extent the char-acter of light transmitted.Because of the suggested secondary maximum on the longer-wave-length side, three points on this side were again determined. This timethe lamps were connected to a battery of storage cells and the currentheld more nearly constant. These three wave-length regions with the
NO. II PHOTOTROPIC SENSITIVITY JOHNSTON II
corresponding phototropic effectiveness are given in table 3 and themidpoint of each band plotted in figure 3. Here a distinct break inascent of the curve is shown.
Table 3.
—
Data from the Second Experiment Showing PhototrofyicEffectiveness in the Spectral Region Indicating the Presenceof a Double Maxinnint RelativeWave-length Light intensity ratio phototropicrange (A) (nionochromator/standard) effectiveriess4460-4560 .29 100.34558-4662 .42 69.34685-4805 .41 71-0IMPROVED EXPERIMENTATION AND RESULTSAnother experiment was planned and carried out in which thetechnic was further improved. A motor generator was installedwherein the current used for the light sources was automatically con-trolled. Both the monochromator lamp and the standard lamp wereconnected in series and replaced at the same time when one burned out.These lamps were the Mazda projection type rated at 200 watts, 50volts, with an average life of 50 hours. They were burned at fouramperes. The water jacket around the standard lamp was removedand the filter cooled by a thermosiphon method in which distilledwater was used. In the longer-wave-length regions the light from themonochromator was passed through suitable glass filters to reducethe effect of scattered light of shorter wave lengths affecting the seed-lings. Unfortunately no filters which transmitted an adequate per-centage of light were available for wave lengths of 4500 A or shorterwhen used in connection with these projection lamps.The data from this more accurately controlled experiment are pre-sented in table 4 and shown graphically in figure 4. The maximumphototropic effect occurs at 4400 A, a region about 150 A shorterthan the maximum found in the earlier experiment. A secondarymaximum occurs at approximately wave length 4750 A with theintervening minimum at about 4575 A. From this double maximumthe sensitivity of Avena falls off rapidly to 5000 A on the long-wave-length side, and to 4100 A on the short-wave-length side. It wouldbe interesting to determine if the limit of sensitivity in the case ofAvena continues to fall off on the short-wave-length end of the spec-trum, as some previous work would indicate. At some future dateit may be possible to extend this curve into the violet and ultravioletregions.
12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92Considerable difficulty was experienced in obtaining a satisfactorybalance point in the region of 4800 A. It was necessary to repeatthis part of the experiment several times. All other points gave con-sistent data. It is possible that a slight shift of the seedling, one wayor the other from the center of the light beam, in this particular por-tion of the spectrum was sufficient to account for the difficulty ofobtaining entirely satisfactory data. If this were true, then it wouldindicate a considerable change in sensitivity over a range of only 100angstroms at about wave length 4800 A.Table 4.
—
Data shoimng the Phototropic Effectiveness of Restricted Regionsof the J^isible Spectrum. That for the Hg Line 435S Ais taken as Unity
Wave-lengthrange (A)
NO. 1
1
IMIOTOTROl'IC SENSITIVITY JOHNSTON 13The efficiency value for line 4358 A falls below the curve. This is tobe expected if the points on the curve adjacent to wave length 4358 Acontained scattered light of more phototropic effectiveness. The valuefor the 4047 A line is above the curve. It may be noted that thisradiation was not exactly monochromatic, since an examination withthe spectroscope showed very faintly the presence of lines 4078 A
14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92DISCUSSIONThe use of the plant photometer in determining the sensitivity ofseedHngs to light has in its favor the elimination of the operator'sjudgment at many points during the experiment. The plant itself isused as a null point instrument. After a time interval the plant hasgrown toward or away from the standard light. There is no need forthe operator to estimate the angle of curvature or the exact time atwhich bending begins. Repeated experiments demonstrate that bymoving the standard lamp 0.5 cm toward or away from the plantwhen located at a balance distance of approximately 25 cm, the curva-ture of seedlings can be changed from one direction to the opposite.It is interesting to note that repetition of balance points seldomdiffered from each other by more than 5 percent. Very rarely wasan unorthodox seedling or an apparently nonsensitive seedling found.One possible objection to this method might be raised. Each pointon the phototropic curve is not strictly comparable to the others. Thisarises from the fact that the plant was at a fixed distance from themonochromator. The intensity of the various wave lengths used wasdifferent. The intensity of the standard light was changed to balancethat of the monochromator light. A better method perhaps would beto maintain the standard light at a fixed intensity with respect to theplant and change the monochromator light to balance the standardlight.It is of interest to note that the maximum phototropic responseoccurs at wave length 4400 A. This point lies midway between thegreatest absorption maxima of chlorophyll a and chlorophyll b re-cently measured by Zscheile (1934) by an improved method. It isalso the position of one of the maxima found by Hoover (1934, dataunpublished) for carbon dioxide absorption by young wheat plants.Since phototropic response is an index of growth retardation it wouldat first appear that photosynthesis progresses best at a point in the spec-trum where growth is least. Such is not the case, however, when theother and somewhat greater maximum of carbon dioxide absorptionis considered. This occurs in the region of 6400 A. Here there isno phototropic response and no retardation in growth.The absence of any phototropic effect in the red and infrared, asshown in these experiments as well as by those of other investigators,and the sharp rise in the curve from about 5000 A into the blue, istypical of an electronic photochemical reaction. The photochemicalnature of at least some of the underlying processes involved in photo-tropism is also suggested by the part played by auxins.
NO. II PHOTOTROPIC SENSITIVITY JOHNSTON 1
5
Went and his school have shown- that small pieces of agar and gela-tine impregnated with this growth-promoting substance when placedasymmetrically on decapitated coleoptiles bring about a growthcurvature with the small agar or gelatine block above the convex por-tion of the coleoptile. The amount of bending can be influenced byexposing the tips to light before impregnating the agar or gelatineblocks. It would appear that light either prevents the formation ofthe auxins or destroys their activity. Furthermore, Kogl (1933) andKogl, Haagen-Smit. and b:rxleben (1933) show this growth-promoting substance to be an unsaturated acid of the formulaC18H32O5, which loses its growth-promoting activity on oxidation.Recently Flint (1934) has called attention to a very interestingrelationship between light and the germination of lettuce seed. Cer-tain varieties fail to germinate unless exposed while in a moist condi-tion to a small amount of light. In his preliminary work it is shownthat light of wave lengths shorter than about 5200 A inhibits germina-tion, while that longer than about 5200 A brings about changes result-ing in germination. Furthermore, he has shown that normal or non-light-sensitive seeds could be made light-sensitive by subjecting themin a moist condition to strong blue light. These seeds would notgerminate until exposed to light of wave lengths longer than 5200 A.All of this work is very suggestive of a common photochemicallyresponsive growth-promoting substance in these lettuce seeds and inthe coleoptiles of oats. Light in the visible spectrum of wave lengthshorter than 5200 A exerts an inhibiting influence on the oat seedling.Likewise this same wave-length region exerts a decided inhibitingaction on the germination of these lettuce seeds. However, an expo-sure to light of longer wave length is necessary for the germinationof the light-sensitive seeds, even though the exposure is of as short aduration as one minute. This stimulating effect of the red was notnoted in the phototropic experiments. All that can be said is that redlight did not exert an inhibiting action. The seedlings were handled inred light, so that if a stimulating action were present, it could not bedetected, since no corresponding experiments were tried in totaldarkness. SUMMARYThe influence of radiation of dififerent wave lengths on photo-tropism is briefly reviewed and discussed.Experiments are described in which the plant photometer wasused to determine the sensitivity of the coleoptile of Avena salivato the dififerent wave-length regions of the visible spectrum.
l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92The phototropic sensitivity curve rises sharply from 4100 A to amaximum at 4400 A. It then drops off to a minimum at about 4575 Aand again rises to a secondary maximum in the region 4700 to 4800 A.The fall is very rapid from this |3oint to 5000 A, from where it tapersoff very gradually to the threshold on the long-wave-length side atabout 5461 A.Phototropism, because of its photochemical nature, its relation toauxins and the fact that it is a specific light-growth reaction, placesin the hands of the experimenter an important tool for investigatingthe fundamental relationship of plant growth processes to light.LITERATURE CITEDBachmann, Fr., and Bergann, Fr.1930. t)ber die Werkigkeit von Strahlen verschiedener Wellenlange fiir diephototropische Reizung von Avena safiva. Planta, Arch. wiss. Bot.,vol. 10, pp. 744-755-Bergann, Friedrich.1930. Untersuchungen iiber Lichtwachstuni, Lichtkriimmug und Lichtabfallbei Avena sativa mit Hilfe monochroniatischen Lichtes. Planta,Arch. wiss. Bot., vol. 10, pp. 666-743.Blaauw, a. H.1909. Die Perzeption des Lichtes. Rec. Trav. bot. neerl., vol. 5, pp. 209-372.1914. Licht und Wachstum. I. Zeitschr. Bot., vol. 6, pp. 641-703.1915. Licht und Wachstum. II. Zeitschr. Bot., vol. 7, pp. 465-532.1919. Licht und Wachstum. III. Die Erklarung des Phototropismus. Med.Landbouwhoogeschool, Wageningen, vol. 15, pp. 89-204.Castle, E. S.1931. The phototropic sensitivity of Phycomyces as related to wave length.Journ. Gen. Physiol., vol. 14, pp. 701-711.Flint, Lewis H.1934. Light in relation to dormancy and germination in lettuce seed. Sci-ence, vol. 80, pp. 38-40.HuRD, Annie May.191 9. Some orienting effects of monochromatic lights of equal intensities onfucus spores and rhizoids. Proc. Nat. Acad. Sci., vol. 5, pp. 201-206.Johnston, Earl S.1926. A plant photometer. Plant Physiol., vol. i, pp. 89-90.1931. A quantitative determination of phototropic response to wave length.(Paper presented at meeting of the Amer. Soc. Plant Physiol.,New Orleans, La., Dec. 29.)Johnston, Earl S., Brackett, F. S., and Hoover, W. H.1931. Relation of phototropism to the wave length of light. Plant Physiol.,vol. 6, pp. 307-313.KoGL, Fritz.1933. On plant growth hormones {auxin A and auxin B). Rep. BritishAssoc. Adv. Sci., vol. 1933, pp. 600-609.
NO. II PHOTOTROPIC SENSITIVITY JOHNSTON jyKoGL, F., Haagen-Smit, a. J., and Erxleben, H.1933- t)ber ein Phytohormon der Zellstreckung. Reindarstellung des Auxinsaus menschlichem Harn. Zeitschr. Physiol. Chem., vol. 214, pp.241-261.Langley, S. p.1884. Researches on solar heat and its absorption by the earth's atmosphere.U. S. War Dep., Prof. Papers Signal Service, no. 15, 242 pp.Washington.Parr, Rosalie.1918. The response of Pilohohis to light. Ann. Bot., vol. ^2, pp. 177-205.Priestley, J. H.1926. Light and growth. III. An interpretation of phototropic growth curva-tures. New Phytol., vol. 25, pp. 213-226.Sonne. Carl.1928-1929. Weitere Mitteilungen Tiber die Abhangikeit der lichtbiologischenReactionen von der Wellenlange des Lichts. Strahlentherapie, vol.31, pp. 778-785.Zscheile, F. Paul, Jr.1934. An impoved method for the purification of chlorophyll a and b; quan-titative measurement of their absorption spectra; evidence for theexistence of a third component of chlorophyll. Bot. Gaz., vol. 95,pp. 529-562.


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Phototropic Curvature of an Oat Seedling ResultingFROM A Difference in Wave Lengths of Light IlluminatingIT FROM Opposite Sides