SMITHSONIAN MISCELLANEOUS COLLECTIONS

VOLUME 92, NUMBER 11

PHOTOTROPIC SENSITIVITY IN RELA-

TION TO WAVE LENGTH
(With Two Plates)

BY
EARL S. JOHNSTON
Division of Radiation and Organisms, Smithsonian Institution

(Publication 3285)

CITY OF WASHINGTON
PUBLISHED BY THE SMITHSONIAN INSTITUTION
DECEMBER 6, 1934
BALTIMORE, UD., V. S. A.
 PHOTOTROPIC SENSITIVITY IN RELATION TO
WAVE LENGTH
By earl S. JOHNSTON
Dk'ision of Radiation and Or(/anisnis, Smithsonian Institution

(With Two Plates)

INTRODUCTION
Asymmetric growth resulting from unilateral stimulus has heen des-
ignated tropism. Growth curvatures following unilateral illumination
are usually classified under the term phototropism. Different plants
respond in different degrees to light, but perhaps those most fre-
quently used in phototropic experiments are the sporangiophores of
Phycomyces and the coleoptiles of Az'ena. In such studies the in-
tensity, the wave length, and the duration of exposure to light each
acts as a contributing factor toward the final result. Just as there
appears to be a threshold of intensity for a given duration of light
exposure, so there are wave lengths which seem to exert no influence
on these growth responses, but with exposures wave lengths
to other
the plants show Not only do different
distinct degrees of sensitivity.
plants vary in their sensitivity, but separate portions of the same plant
respond differently. Recent work on growth substances indicates the
presence 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 shown
by the response of the coleoptiles of Avena sativa. The variety used
is Culberson, C.I. no. 272,, for which the author wishes to thank

Mr. 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 SURVEY
Many of the early experiments on phototropism have been reviewed
by Parr (1918) and the data classified under four general theories:
I. The " intensity " theory originating with
De CandoUe in 1832 and
adhered to in a more form by Wiesner, Darwin,
or less modified
Engleman, Oltmanns, Yerkes, Loeb, and Davenport. 2. The ray-

Smithsonian Miscellaneous Collections, Vol. 92, No. 11

 :

2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92

direction theory advanced by Sachs in 1876 and supported by the

experiments of Strasburger, Davenport, and Canon. 3. The wave-
length theory first investigated by Payer in 1842. 4. The energy
theory first mentioned by Miiller in 1872 in which the maximum
response of cress seedlings shifted in the spectrum for different
energy values of the wave lengths studied.
The basis for much of the recent quantitative work on phototropism
was laid by Blaauw (1909, 1914, 191 5, 1919)- His studies were
perhaps the first serious attempt made
growth re-
to interpret this
sponse in terms of modern physics. Plant responses were studied in
different 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 for
phototropic response of Az'ena seedlings to lie between 4660 and
4780 A, while the red and yellow regions were ineffective. According
to Blaauw (1914), the curvature of a plant resulting from unilateral
illumination is caused by the light-growth responses on the opposite
sides which are illuminated differently. The minimum amount of
radiation required to produce phototropic response was found to be
20 meter-candle-seconds. It also appears from his work that for
equal effects the product of light intensity and time of exposure is a
constant.
It is impossible to evaluate the effect of wave length in manj^ of the
early phototropic experiments because of the lack of accurate physical
data. Some 10 years after the early quantitative studies of Blaauw.
Parr (1918) made a study of the responses of Pilobohis to different
wave lengths and intensities of carefully measured artificial light.
The results of these quantitative studies are best summarized in her
own 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 no
indication of intermediate maxima or minima. (3) The presentation time does
not vary in direct ratio vi^ith the measured value of the energy of the light in
the different regions of the spectrum. (4) The presentation time varies in
inverse ratio to the square roots of the wave frequency. (5) The product of the
square root of the frequency times the presentation time, decreases with the
decrease 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 the
presentation time may be expressed approximately in the Weber-Fechner formula,
if the wave-frequencies be made a function of the constant. (7) The relation
of the spectral energy to the presentation time may also be approximately
expressed in the Trondle formula, the wave-frequencies being made a function
of the constant.
NO. II PHOTOTROPIC SENSITIVITY JOHNSTON 3

About the same time Hurd (1919) showed wave-length effect on

young rhizoids by equalizing the intensity of the light coming through
a series of Wratten filters. Only the blue (4700 to 5200 A) and
violet (4000 to 4700 A) lights produced phototropism, negative in
direction. The other lights at the intensity of 1800 meter-candles had
no effect. However, with a greater intensity the green light (5200 to
5600 A) exerted a negative phototropic effect as well as the blue and
violet.

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 apparatus
consisted of a long box divided into three compartments. Each end
compartment contained an electric lamp which could be moved toward
or away from the light-filter window in the partition separating it
from the central or plant compartment. Plants which easily respond
in their directional growth to differences in light intensities were em-
ployed in place of the adjustable indicator or photometer screen in
the ordinary Bunsen photometer.
Sonne (1928-1929) determined the necessary amount of energy of
differentwave lengths to produce a minimum phototropic response in
oats. The young plants were so placed that about i cm of their tips
were exposed at different distances from the light of a mono-
chromator for different exposure periods. The visible part of the
spectrum 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 i

second. The energy was measured by a thermo-element. The results

are summarized in table i.

Table i. —Sonne's Data showing Phototropic Sensitivity Determined from the

Amount .of Energy Required to Produce a Minimum Response
in Oats

Wave length
4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92

It will be seen from this table that the amount of energy which
barely causes phototropic curvature varies with the wave length. The
yellow (5700 A) is about 600 times as intense as is the white light
necessary 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 is
thus approximately 10,000 times as efifective phototropically as the
green and 20,000 times that of the yellow. The violet (4050 A) is

also very efifective but only about half that of the blue.

350

Fig. I. —Graphs from Bachmann and Bergann showing the sensitivity of Avena
sativa 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 as
the 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 curvatures
and light-growth reactions are parallel processes. The stronger the
light-growth reaction in a given wave-length region, the greater will
be the phototropic response. The seedlings " choice " in the com-
pensation experiments between two wave-length ranges is always that
which corresponds to the stronger growth reaction.
NO. II PHOTOTROPIC SENSITIVITY JOHNSTON

Bachmann and Bergann 1930) review the early work of Blaauw

(

and correct the energy values of his data for light absorbed by CUSO4
and water filter, surface reflections, and color filter in order to com-
pare his results with those obtained by Bergann. The results of Sonne
and Koningsberger are and compared. These data are
also corrected
represented gra])hically in figure i, which the continuous line is
in

the 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 different
spectral regions were equal. At this point the efficiency of each region
was taken as proportional to its relative energy content. Wratten
fiiters were used in conjunction with a copper chloride filter. The most

sensitive region proved to be in the violet (4000-4300 A). In figure 2

Castle 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 these
results with those obtained from the absorption spectrum of the
photosensitive substance.

PRELIMINARY EXPERIMENTS
The 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 for
evaluating four spectral regions in terms of plant response. The gen-
eral procedure was to place an oat seedling between two different and
oppositely placed lights, and after an interval observe the growth
curvature. If, for example, when the seedling was exposed to blue and
to green lights, a distinct bending was noted toward the blue side,

the lights were so adjusted as to increase the green or decrease

the blue intensity. Another seedling was then used and the process
repeated until a balance point was reached where the effect of one
light neutralized the effect of the other. When this balance point was
determined, a specially constructed thermocouple replaced the plant
and the relative light intensities were measured. From these experi-
ments it was found that no measurable phototropic response was
found for wave lengths longer than 6000 A Wratten no. 24 red
( —
filter), while a noticeable bending was found with the yellow filter

—
(Coming's heat-resisting yellow yellow shade), whose cut-off on
the short-wave-length side was 5200 A. The threshold for wave-
length influence was found to lie somewhere between 5200 and
6000 A. The effects of green and blue light (Wratten filters nos. 61
and 47 respectively) were progressively greater, being in round num-
bers 1,000 for the green and 30,000 for the blue times that of the
yellow.
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 constructed
monochromator illustrated in plate i. Care was taken to eliminate
scattered light and to keep the conditions surrounding the coleoptile
NO. II PHOTOTROPIC SENSITIVITY —JOHNSTON 7

symmetrical, with the exception of the wave-length region being

investigated. A double-walled glass cylinder with water between the
walls slowly rotated about the axis of the coleoptile. Two strips of
paper blackened on the inside and separated i cm from each other
were wrapped about the cylinder in order to shield all but a restricted
region at the tip of the coleoptile from the light. The cylinder was
encased in a light-proof box which contained two oppositely placed
side windows. Through one window, light was passed from the
monochromator, and through the other, light from the standard lamp.
The standard used was a 200-watt, 50-volt projectionMazda lamp
with the filaments in a plane. The standard lamp was enclosed in an
air-cooled brass housing with one small glass window opening toward
the plant. The light from the standard was passed through a number
6.0 Corning line filter, a heat-absorbing glass, and a water cell before

entering the rotating cylinder surrounding the plant. The number

6.0 Corning line filter transmitted wave lengths from about 4400 A

to 5800 A and from 7000 A to 12800 A of the light transmitted by the

water filter. The radiation intensity of the standard was 0.37 micro-
watts/cm- at a distance of 25 cm. This value, of course, varied with
different lamps and also with the same lamp as it aged. photo- A
graphic red lamp was used behind the small rear window of the plant
box for properly placing a coleoptile at the beginning of each expo-
sure. Previous experiments showed the coleoptile to be insensitive
for all practical purposes to this particular light. The monochromator
lamp was located outside the phototropic room, which was a small
room with no outside walls located in the west basement of the Smith-
sonian building. Very little daily temperature fluctuation occurred
in this room because of its ideal location.
Coleoptiles of oats, Avena sativa Culberson, were used in all these
experiments. The seeds were germinated at approximately 25° C.
between glass plates covered with moist filter paper. The plates were
so placed in moisture chambers that the seedlings grew vertically. A
careful selection of the seedlings was made for straightness when
they had attained a length of 2 to 4 cm. One was then transferred to
a small Erlenmeyer flask fitted with a cork stopper. It was supported
by means of a little cotton in a small hole of the stopper. The flask
was filled with distilled water so that the roots were entirely im-
mersed. With the cross hairs in a small telescope as a guide, the
seedling was adjusted to a vertical position within the glass cylinder
located between the two lights.
The general experimental procedure was to illuminate the coleoptile
on its two opposite sides, preferably the narrow edges, and after a
8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92

time interval to note the resulting growth curvature. If the light

adjustment 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 the
used seedling discarded for a new one. As the balance point was
approached the exposure time necessarily increased. Finally on mov-
ing the standard light back and forth through a distance of i cm, the
plants could be made to curve repeatedly toward one light then toward
the other. The balance was taken to be the midpoint between
point
these two positions. Care was always used not to expose the fresh
seedlings to any light but red in the preliminary handling. Priestley

(1926) has shown that light afifects normal and etiolated shoots very
diiferently. The amount of light required to induce phototropic curva-
ture in normal light-grown shoots is greater, and must be continued
longer, than that required to bring a similar curvature in etiolated
shoots.
After a balance point had been determined and tested by using
several seedlings, a specially constructed thermocouple was inserted
into 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 of
bismuth-tin alloy, each about 25 microns in diameter. The alloy was
made up of 95 percent bismuth and 5 percent tin. Utmost care was
needed in measuring the light intensities since the plants were found
to be much more sensitive to the light than the best physical instru-
ments available. It should be remembered, however, that the seedling
integrates the effect of radiation over a relatively long period, while
the thermocouple responds in a few seconds.
The results of this experiment are presented in table 2. The ratio

of the intensity of the monochromator light to that of the standard

light is given in the third column for corresponding wave-length
ranges shown in the first column. Where filters were used in combina-
tion with themonochromator 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 be
made were for the range 5040 to 5160 A. In the last column of the
table the relative phototropic effectiveness of the different wave-
length ranges is given. The ratio 29.10 was arbitrarily taken as
unity.

With unilateral illumination through the monochromator and a

number '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 region

of phototropism. In order to determine this point more accurately a
mercury arc in pyrex glass was substituted for the Mazda lamp of the
monochromator. and by passing this light through a number "jy
Wratten filter, a seedling was unilaterally illuminated by the 5461
mercury line. In five such tests, each lasting from two to several
hours, two gave positive bending and three no bending. With reason-

Table 2. Data from the Preliminary Experiment Shozving Phototropic

Effectiveness of Restricted Regions of the Spectrum. That for
Wave-length Region 3040-3160 A is Taken as Unity
Filter "
Relative
Wave-length used with Light intensity ratio phototropic
range (A) monochromator (Monochromator/standard) effectiveness
7250-7700 W 88
6900-7300 W 88
6550-6950 CLF 2
6250-6600 CLF 2
5940-6270 CLE 3
5670-5950 TR
5430-5670 W 22; CLF 5.1
5200-5430 W 77
5040-5160 CLF 6.1 29.10 i.o
4940-5070 CLF 6.1 2.49 1 1.

4810-4930 CLF 6.1 0.68 42.8

4670-4800 CLF 6.1 0.54 53.9
4550-4670 CLF 7 0.29 100.3
4450-4550 0.27'^ 107.8
4410-4500 0.34" 85.6
4360-4450 0.36" 80.8
4280-4360 0.41" 71.0
4210-4280 0.47 67.0
4130-4220 0.84 34.6
4070-4135 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 monochromator
light was varied by changing the resistance in its lamp circuit until
a balance point was
obtained.

able certaintyit can be concluded that under these particular experi-

mental conditions the threshold wave-length efi'ect is at or very near

5461 A.
When the phototropic efifectiveness
is plotted against wave length,

a curve is shown in figure 3, with its maximum at about

obtained as
wave length 4550 A. The horizontal lines represent the wave-length
ranges for which balance points were determined. Points where filters
were used in addition to the monochromator are represented as circles.
 1

lO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92

There is a slight suggestion of two other maxima, one on each side

of the peak. It could not be determined from these data whether or

not 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 constant
voltage, there was some fluctuation during the exposure of the seed-

L \. - U ^

4tO0 4200 4^00

L_—4600
4500
1

4700
1 1

4900
1

5000
— ^1
SIOO 5200
I

5300
I <=>—

Fig. 3. — Phototropic
sensitivity curve of preliminary experiment (continuous
line). The
ordinates are relative sensitivity values, the abscissae, wave lengths
in angstroms, and the horizontal bars indicate the wave-length ranges of the
balance points. Circles indicate points obtained with filters combined with the
monochromator. Points more accurately determined are indicated by crosses
and connected by dash lines.

lings and during the intensity measurements. Also, in some of the

work the standard lam]> as well as its filter cell was cooled by tap
water. This resulted in an accumulation of iron on the glass surfaces
during the time required for determining the balance points. These
uncontrolled 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 time
the lamps were connected to a battery of storage cells and the current
held 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 the

midpoint of each band plotted in figure 3. Here a distinct break in
ascent of the curve is shown.

Table 3. Data from the Second Experiment Showing Phototrofyic

Effectiveness in the Spectral Region Indicating the Presence
of a Double Maxinnint
Relative
Wave-length Light intensity ratio phototropic
range (A) (nionochromator/standard) effectiveriess

4460-4560 .29 100.3

4558-4662 .42 69.3
4685-4805 .41 71-0

IMPROVED EXPERIMENTATION AND RESULTS

Another experiment was planned and carried out in which the
technic was further improved. A motor generator was installed
wherein the current used for the light sources was automatically con-
trolled. Both the monochromator lamp and the standard lamp were
connected in series and replaced at the same time when one burned out.
These lamps were the Mazda projection type rated at 200 watts, 50
volts, with an average life of 50 hours. They were burned at four
amperes. The water jacket around the standard lamp was removed
and the filter cooled by a thermosiphon method in which distilled
water was used. In the longer-wave-length regions the light from the
monochromator was passed through suitable glass filters to reduce
the 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 shorter
when 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 maximum
phototropic effect occurs at 4400 A, a region about 150 shorter A
than the maximum found in the earlier experiment. secondary A
maximum occurs at approximately wave length 4750 A with the
intervening minimum at about 4575 A. From this double maximum
the 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 would
be interesting to determine if the limit of sensitivity in the case of
Avena continues to fall off on the short-wave-length end of the spec-
trum, as some previous work would indicate. At some future date
it may be possible to extend this curve into the violet and ultraviolet
regions.
 —

12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92

Considerable difficulty was experienced in obtaining a satisfactory

balance point in the region of 4800 A. It was necessary to repeat
this part of the experiment several times. All other points gave con-
sistent data. It is possible that a slight shift of the seedling, one way

or the other from the center of the light beam, in this particular por-
tion of the spectrum was sufficient to account for the difficulty of
obtaining entirely satisfactory data. If this were true, then it would

indicate a considerable change in sensitivity over a range of only 100

angstroms at about wave length 4800 A.

Table 4. Data shoimng the Phototropic Effectiveness of Restricted Regions

of the J^isible Spectrum. That for the Hg Line 435S A
is taken as Unity

Wave-length
range (A)
NO. 1 IMIOTOTROl'IC SENSITIVITY JOHNSTON 13

The efficiency value for line 4358 A falls below the curve. This is to

be expected if the points on the curve adjacent to wave length 4358 A

contained scattered light of more phototropic effectiveness. The value
for the 4047 A line is above the curve. It may be noted that this
radiation was not exactly monochromatic, since an examination with
the spectroscope showed very faintly the presence of lines 4078 A
14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92

DISCUSSION
The use of the plant photometer in determining the sensitivity of
seedHngs to light has in its favor the elimination of the operator's
judgment at many points during the experiment. The plant itself is

used as a null point instrument. After a time interval the plant has
grown toward or away from the standard light. There is no need for
the operator to estimate the angle of curvature or the exact time at
which bending begins. Repeated experiments demonstrate that by
moving the standard lamp 0.5 cm toward or away from the plant
when 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 seldom
differed from each other by more than 5 percent. Very rarely was
an unorthodox seedling or an apparently nonsensitive seedling found.
One possible objection to this method might be raised. Each point
on the phototropic curve is not strictly comparable to the others. This
arises from the fact that the plant was at a fixed distance from the
monochromator. The intensity of the various wave lengths used was
different. The intensity of the standard light was changed to balance
that of the monochromator light. A better method perhaps would be
to maintain the standard light at a fixed intensity with respect to the
plant and change the monochromator light to balance the standard
light.

It is of interest to note that the maximum phototropic response

occurs at wave length 4400 A. This point lies midway between the
greatest absorption maxima of chlorophyll a and chlorophyll b re-
cently measured by Zscheile (1934) by an improved method. It is

also the position of one of the maxima found by Hoover (1934, data
unpublished) for carbon dioxide absorption by young wheat plants.
Since phototropic response is an index of growth retardation it would
at first appear that photosynthesis progresses best at a point in the spec-
trum where growth is least. Such is not the case, however, when the
other and somewhat greater maximum of carbon dioxide absorption
is considered. This occurs in the region of 6400 A. Here there is
no phototropic response and no retardation in growth.
The absence of any phototropic effect in the red and infrared, as
shown 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, is

typical of an electronic photochemical reaction. The photochemical

nature 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

Went and his school have shown- that small pieces of agar and gela-
tine impregnated with thisgrowth-promoting substance when placed
asymmetrically on decapitated coleoptiles bring about a growth
curvature with the small agar or gelatine block above the convex por-
tion of the coleoptile. The amount of bending can be influenced by
exposing the tips to light before impregnating the agar or gelatine
blocks. It would appear that light either prevents the formation of
the auxins or destroys their activity.
Furthermore, Kogl (1933) and
Kogl, Haagen-Smit. and b:rxleben (1933) show this growth-
promoting substance to be an unsaturated acid of the formula
C18H32O5, which loses its growth-promoting activity on oxidation.
Recently Flint (1934) has called attention to a very interesting
relationship 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 shown
that 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 by subjecting them
light-sensitive
in a moist condition to strong blue light. These seeds would not
germinate until exposed to light of wave lengths longer than 5200 A.
All of this work is common photochemically
very suggestive of a
responsive growth-promoting substance in these lettuce seeds and in
the coleoptiles of oats. Light in the visible spectrum of wave length
shorter than 5200 A exerts an inhibiting influence on the oat seedling.
Likewise this same wave-length region exerts a decided inhibiting
action on the germination of these lettuce seeds. However, an expo-
sure to light of longer wave length is necessary for the germination
of the light-sensitive seeds, even though the exposure is of as short a
duration as one minute. This stimulating effect of the red was not
noted in the phototropic experiments. All that can be said is that red
light did not exert an inhibiting action. The seedlings were
handled in
red light, so that a stimulating action were present,
if it could not be
detected, since no corresponding experiments were tried in total
darkness.

SUMMARY
The influence of radiation of dififerent wave lengths on photo-
tropism is briefly reviewed and discussed.
Experiments are described in which the plant photometer was
used to determine the sensitivity of the coleoptile of Avena saliva
to the dififerent wave-length regions of the visible spectrum.
l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92

The phototropic sensitivity curve rises sharply from 4100 A to a

maximum at 4400 A. It then drops off to a minimum at about 4575 A
and again rises to a secondary maximum 4700 to 4800 A.
in the region
The fall is very rapid from this |3oint to 5000 A, from where it tapers
off very gradually to the threshold on the long-wave-length side at
about 5461 A.
Phototropism, because of its photochemical nature, its relation to
auxins and the fact that it is a specific light-growth reaction, places
in the hands of the experimenter an important tool for investigating
the fundamental relationship of plant growth processes to light.

LITERATURE CITED
Bachmann, Fr., and Bergann, Fr.
1930. t)ber die Werkigkeit von Strahlen verschiedener Wellenlange fiir die
phototropische Reizung von Avena safiva. Planta, Arch. wiss. Bot.,
vol. 10, pp. 744-755-

Bergann, Friedrich.
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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92, NO. 11, PL. 2

Phototropic Curvature of an Oat Seedling Resulting

FROM A Difference in Wave Lengths of Light Illuminating
IT FROM Opposite Sides