electromagnetic coils, the free ends of the horseshoe being pointed.
Facing them are the conical holes of the soft iron armature A. This
armature carries two rods which slide through hollow tubes. The distal
ends of the rods support the holder H, carrying the smoked glass plate.
Under normal conditions, the plate-holder is held by suitable springs,
somewhat to the right of, and free from contact with, the writer. A
clockwork C carries a rotating arm, which makes periodic contact with
a pool of mercury contained in the vessel V, once in a minute. On the
completion of the electromagnetic circuit, the armature A is attracted,
the recording glass plate being thereby moved to the left making
contact with the writer. The successive dots in the record thus take
place at intervals of a minute. Only a moderate amount of electric
current is thus consumed in maintaining the oscillation of the plate. A
4-volt storage cell of 20 amperes capacity is quite sufficient to work
the apparatus for several days.
The responsive fall of the leaf of _Mimosa_ is completed in the course
of about two seconds. The leaf remains in the fallen or ‘contracted’
position for nearly fifteen seconds; it then begins to recover slowly.
As the successive dots of the Oscillating Recorder are at intervals
of a minute, the maximum fall of leaf is accomplished between two
successive dots. The dotted response record here obtained exhibits
the recovery from maximum fall under stimulation (_cf._ Fig. 23). The
recovery of the leaf in one minute is less than one-tenth the total
amplitude of the fall, and is proportionately the same in all the
response records. Hence the successive amplitudes of response curves
that are recorded at different hours of the day afford us measures
of the relative variations of excitability of the plant at different
times. This enables us to demonstrate the reality of diurnal variation
of excitability. In my experimental investigations on the subject I
have not been content to take my data from any particular method of
obtaining response, but have employed both types of recorders, the
Resonant and Oscillating. It will be shown that the results given by
the different instruments are in complete agreement with each other.
EFFECTS OF EXTERNAL CONDITIONS ON EXCITABILITY.
Before giving the daily records of periodic variation of excitability,
I will give my experimental results on the influence of various
external conditions in modifying excitability. The conditions which
are likely to affect excitability and induce periodicity are, first,
the effects of light and darkness: under natural conditions the plant
is subjected in the morning to the changing condition from darkness
to light; then to the action of continued light during the day; and in
the evening to the changing condition from light to darkness. A second
periodic factor is the change in the condition of turgidity, which
is at its maximum in the morning, as evidenced by the characteristic
erect position of the petiole. Finally, the plant in the course of day
and night is subjected to a great variation of temperature. I will
now describe the effects of these various factors on excitability. It
should be mentioned here that the experiments were carried out about
the middle of the day, when the excitability, generally speaking, is
found to remain constant.
EFFECTS OF LIGHT AND DARKNESS.
[Illustration: FIG. 16. Effect of cloud. Dotted up-curve indicates
responsive fall, and continuous down-line exhibits slow recovery. First
four responses normal; next three show depression due to diminution
of light brought on by cloud, the duration of which is indicated
by horizontal line below. Last three records show restoration of
excitability brought on by clearing of sky. All records read from left
to right.]
I have frequently noticed that a depression of excitability occurred
when the sky was darkened by passing clouds. This is clearly seen
in the above records obtained with the Resonant Recorder. Uniform
sub-maximal stimuli had been applied to a specimen of _Mimosa_ at
intervals of fifteen minutes. The dotted up-line represents the
responsive fall, and the continuous down-line, the slow recovery. The
first four are the normal uniform responses (Fig. 16). The next three
show the depressing effect of relative darkness due to cloudy weather.
The sky cleared after forty-five minutes, and we notice the consequent
restoration of normal excitability.
_Effect of sudden darkness and its continuation. Experiment 14._--In
the next record (Fig. 17) is shown the immediate and continued action
of darkness. The first two are the normal uniform responses in light.
By means of screens, the plant was next subjected to sudden darkness;
this brought about a marked depression of excitability. Subjection to
sudden darkness thus acts as a stimulus inducing a marked but transient
fall of excitability. Under the continuous action of darkness, however,
the excitability is at first restored and then undergoes a persistent
depression.
[Illustration: FIG. 17. Effect of sudden darkness. Plant subjected to
sudden darkness beyond horizontal line seen below. First two responses
normal. Note sudden depression of excitability, revival and final
depression under continued darkness.]
_Effect of transition from darkness to light: Experiment 15._--Here
we have to deal first with the immediate effect of sudden transition,
and then with the persistent effect of continuous light. In the record
given in Fig. 18 the plant had been kept in the dark and the responses
taken in the usual manner. It was then subjected to light; the sudden
change from darkness to light acted as a stimulus, inducing a transient
depression of excitability. In this connection it is interesting to
note that Godlewski found that in the phenomenon of growth, transition
from darkness to light acted as a stimulus, causing a transient
decrease in the normal rate. The effect of continued light on _Mimosa_
is an enhancement of excitability.
[Illustration: FIG. 18. Effect of change from darkness to light. The
first three records are normal under darkness. Horizontal line below
indicates exposure to light. Note preliminary depression followed by
enhancement of excitability.]
EFFECT OF EXCESSIVE TURGOR.
I have often found that the moto-excitability is depressed under
excessive turgor. Thus the “over-turgid” leaf of _Biophytum sensitivum_
does not exhibit any mechanical response on rainy days.
[Illustration: FIG. 19. Effect of enhanced turgor, artificially
induced. First two responses normal. Application of water, at arrow,
induces depression of moto-excitability.]
_Experiment 16._--The effect of excessive turgor on moto-excitability
may be demonstrated in the case of _Mimosa_ by allowing its main
pulvinus to absorb water. The result is seen in the above record
(Fig. 19), where water was applied on the pulvinus after the second
response. It is seen how a depression of moto-excitability results from
excessive turgor brought on by absorption of water. In such cases,
however, the plant is found to accommodate itself to the abnormal
condition and gradually regain its normal excitability in the course of
one or two hours.
INFLUENCE OF TEMPERATURE.
The moto-excitability of the pulvinus of _Mimosa_ is greatly
modified under the influence of temperature. For the purpose
of this investigation I enclosed the plant in a glass chamber,
raising the temperature to the desired degree by means of electric
heating. Responses to identical stimuli were then taken at different
temperatures. It was found that the effect of heightened temperature,
up to an optimum, was to enhance the amplitude of response. Thus with
a given specimen it was found that while at 22°C. the amplitude of
response was 2.5 mm., it became 22 mm. at 27°C., and 52 mm. at 32°C.
The excitability is enhanced under rising, and depressed under falling
temperature. The moto-excitability of _Mimosa_ is practically abolished
at the minimum temperature of about 19°C.
[Illustration: FIG. 20. Effect of moderate cooling during a period
shown by horizontal line below. Moderate depression followed by quick
restoration.]
_Effect of lowering of temperature: Experiment 17._--A simple way of
exhibiting the effect of lowering of temperature is by artificial
cooling of the pulvinus. This cannot very well be done by application
of a stream of cooled water, because, as we have seen, absorption of
water by the pulvinus is attended by a loss of excitability: diluted
glycerine has, however, no such drawback. This fluid at ordinary
temperature was first applied on the pulvinus, and after an interval of
half an hour records were taken in the usual manner. Cooled glycerine
was then applied and the record taken once more; the results are seen
in Figs. 20 and 21. In the former, the first response was normal at
the temperature of the room, which was 32°C.; the next two exhibit
depression of excitability under moderate cooling; the duration of
application of moderately cooled glycerine is there indicated by the
horizontal line below. On the cessation of application, the normal
temperature was quickly restored, with the restoration of normal
excitability.
In the next record (Fig. 21) is shown the effect of a more intense
cold. It will be noticed that the first effect was a depression, and
subsequently, a complete abolition of excitability. Thick dots in the
record represent applications of stimulus which proved ineffective. It
will also be noticed that even on the cessation of cooling, and the
return of the tissue to normal temperature the induced abolition of
excitability persisted as an after-effect for a considerable time. I
have likewise found that the after-effect of cold in abolishing the
conduction of excitation is also very persistent. These experiments
show that owing to physiological inertia, the variations of
excitability in the plant often lag considerably behind the external
changes which induce them.
[Illustration: FIG. 21. Effect of application of more intense cold.
Note sudden depression followed by abolition of excitability, also
persistent after-effect.]
_Effect of high temperature: Experiment 18_.--It has been shown that
the moto-excitability is enhanced by rising temperature; there is,
however, an optimum temperature above which the excitability undergoes
a depression. This is seen in the following record (Fig. 22), where the
normal response at 32°C. was depressed on raising the temperature to
42°C.; the excitability was, however, gradually restored when the plant
was allowed to regain the former temperature.
[Illustration: FIG. 22. Effect of temperature above optimum. Note
depression of excitability induced by high temperature, and gradual
restoration on return to normal.]
I may now briefly recapitulate some of the important results: darkness
depresses and light exalts the moto-excitability. Excessive turgor
depresses motility. Still more marked is the effect of temperature.
Lowering of temperature depresses and finally abolishes the
moto-excitability: rise of temperature enhances it up to an optimum
temperature, but beyond this point the excitability undergoes
depression. _The change in excitability induced by the variation of
external condition is not immediate; the induced effect, generally
speaking, lags behind the inducing cause._
DIURNAL VARIATION OF EXCITABILITY.
I will now give automatic records of responses taken once every hour
for twenty-four hours. They prove conclusively the diurnal variation
of excitability in _Mimosa_. After studying in detail the variations
characteristic of particular times of the day, I will endeavour to
correlate them with the effects brought on by the periodic changes of
the environment.
_Experiment 19._--As a typical example I will first give a record
obtained in the month of February, that is, say, in spring. From this
it will not be difficult to follow the variations which take place
earlier in winter or later in summer.
[Illustration: FIG. 23. Record for twenty-four hours, exhibiting
diurnal variation of excitability (spring specimen). The displacement
of base-line is due to nyctitropic movement.]
The record given in Fig. 23 was commenced at 5 P.M. and continued to
the same hour next day. The first thing noticeable is the periodic
displacement of the base-line. This is due to the nyctitropic movements
of the leaf. It should be remembered that the up-movement of the leaf
is represented by down-curve, and _vice versâ_. After the maximum fall
of the leaf, which in this case was attained at 9 P.M., there followed
a reverse movement: the highest erection, indicative of maximum turgor,
was reached at 6 A.M. The leaf then fell slowly and reached a middle
position at noon. The extent of the nyctitropic movement varies in
individual cases; in some it is slight, in others very large. The
erectile movement began, as stated before, at about 9 P.M.; in some
cases, however, it may occur as early as 6 P.M.
In following the characteristic variations of response occurring
throughout the day, we find that while they are practically uniform
between the hours of 5 and 6 P.M., a continuous decline is manifested
after setting in of darkness (7 P.M.); the fall of excitability
continues even after sunrise (6-30 A.M.), response being practically
abolished at 8 A.M. The excitability is then gradually restored in
a staircase manner, the maximum being reached after 12 noon. After
attaining this, the excitability remains more or less constant till the
evening. It will be noticed that the amplitude of response at 5 P.M.
on the second day was the same as the corresponding response on the
previous day.
The results of this and numerous other records taken in spring may be
summarized as:--
1. The maximum excitability of _Mimosa_ is attained between 1 and
3 P.M., and remains constant for several hours. In connection
with the constancy of response at this period, it should be
remembered that when the response is at its maximum a slight
increase of excitability cannot further enhance the amplitude of
response.
2. The excitability, generally speaking, undergoes a continuous
decline from evening to morning, the response being practically
abolished at or about 8 A.M.
3. From 8 A.M. to 12 noon, the excitability is gradually enhanced
in a staircase manner, till the maximum excitability is reached
after 1 P.M.
I have obtained numerous records in support of these conclusions,
some of which are reproduced in the following figures. In these cases
responses to uniform stimuli at intervals of half an hour were taken at
different parts of the day, the recorder employed being of the Resonant
type.
_Mid-day record: Experiment 20._--The record of daily periodicity
previously given shows that the excitability reaches its maximum after
12 noon, and that it remains constant at the maximum value for several
hours. This fact is fully borne out in the following record obtained
with a different specimen (Fig. 24). The responses were taken here from
noon to 3 P.M., once every half-hour.
[Illustration: FIG. 24. Mid-day record from noon to 3 P.M. exhibiting
uniform excitability. Responses taken once every half-hour.]
_Evening record: Experiment 21._--The record given in Fig. 23 shows
that the amplitude of response falls continuously after 6 P.M. It
might be thought that the diminished amplitude in the first part may
be due to the natural nyctitropic fall of the leaf. The range of the
pulvinar movement being limited, it is clear that the extent of the
responsive fall must become smaller on account of the natural fall of
the leaf during the first part of the night. That this is not the whole
explanation of the decline of response in the evening will be clear
from certain facts which I will presently adduce. It was stated that
the leaf of _Mimosa_ exhibits nyctitropic fall from 6 to 9 P.M., after
which there is a reverse movement of erection. In certain specimens,
however, the erectile movement commenced as early as 6 P.M. It is
obvious that in these latter cases diminution of amplitude of response
cannot be due to the reduction of the range of movement of the leaf.
In Fig. 25 is given a series of records from 6 to 10 P.M. obtained
with a leaf in which erectile movement had commenced early in the
evening. Though the full range of responsive movement was in this case
available, yet the amplitude of successive responses is seen to undergo
continuous diminution.
[Illustration: FIG. 25. Evening record from 6 to 10 P.M., showing
gradual depression of excitability.]
_Record in the morning: Experiment 22._--The excitability is, as we
have seen, nearly abolished about 8 A.M., after which there is a
gradual restoration. This gradual enhancement of excitability to a
maximum in the course of the forenoon is seen well illustrated in the
record below (Fig. 26).
[Illustration: FIG. 26. Morning record from 8 A.M. to 12 noon,
exhibiting gradual enhancement of excitability.]
The record of daily periodicity given in Fig. 23 may be regarded as
a typical example. Modifications may, however, be observed which are
traceable to individual peculiarities. As an example of this, I give
a record (Fig. 27) obtained with a specimen in which nyctitropic
movement was very pronounced. The periodic variation of excitability
exhibited here is practically the same as shown by other specimens.
The interesting variation is in the character of the recovery from
stimulus; the leaf was falling from 6 to 9 P.M.; owing to the shifting
of the base-line upwards the recovery appears to be incomplete. After 9
P.M. the leaf was erected, at first slowly, then at a very rapid rate.
The consequent fall of the base-line late at night is very abrupt;
hence there is an apparent over-shooting in the line of recovery.
[Illustration: FIG. 27. Record of diurnal variation of excitability; it
exhibits marked nyctitropic movement.]
EFFECT OF TEMPERATURE ON VARIATION OF EXCITABILITY.
So far I have merely described the observed diurnal variation of
excitability. We may next inquire whether there is any causal relation
between the change of external conditions and the observed variation of
excitability. It has been shown that the moto-excitability is greatly
influenced by temperature. In order to find in what manner the diurnal
variation of excitability was influenced by the daily variation of
temperature, I took special care to secure by means of the thermograph
a continuous record of temperature variations. The table which follows
shows the relation between the hours of the day, temperature, and
amplitude of response, in a typical case of diurnal variation of
excitability.
TABLE II.--SHOWING THE RELATION BETWEEN HOUR OF THE DAY, TEMPERATURE,
AND EXCITABILITY. (SPRING SPECIMEN.)
+--------------------------------+--------------------------------+
| _Hours _Tempera- _Amplitude | _Hours _Tempera- _Amplitude |
| of ture._ of | of ture._ of |
| day._ Response._ | day._ Response._ |
+--------------------------------+--------------------------------+
| 5 p.m. 28° C. 28 mm. | 5 a.m. 20° C. 5 mm. |
| 6 " 25.5° " 28 " | 6 " 20.5° " 4.2 " |
| 7 " 24.5° " 27 " | 7 " 21° " 3.5 " |
| 8 " 23° " 23.5 " | 8 " 22° " 2.5 " |
| 9 " 22° " 21.5 " | 9 " 24° " 0 " |
|10 " 21° " 18 " |10 " 26° " 6 " |
|11 " 20.5° " 15 " |11 " 26.5° " 15.5 " |
|12 " 20° " 13 " |12 " 28° " 22.5 " |
| 1 a.m. 20° " 10 " | 1 p.m. 28° " 26 " |
| 2 " 20° " 8 " | 2 " 28.5° " 28 " |
| 3 " 20° " 7.5 " | 3 " 28.5° " 28 " |
| 4 " 19.5° " 6 " | 4 " 29° " 28 " |
+--------------------------------+--------------------------------+
From the data given in the table, two curves have been obtained. One of
these shows the relation between the hours of the day and temperature;
the other exhibits the relation between the hours of the day and the
excitability as gauged by the amplitude of response (Fig. 28). It
will be seen that there is, broadly speaking, a marked resemblance
between the two curves, which demonstrate the predominant influence of
temperature on diurnal variation of excitability.
[Illustration: FIG. 28. The continuous curve shows the relation between
the hour of the day and temperature. The dotted curve exhibits relation
between the hour of the day and excitability.]
EFFECT OF PHYSIOLOGICAL INERTIA.
It has been shown (page 59) that owing to physiological inertia, the
change of excitability, generally speaking, lags behind the inducing
cause. This fact finds striking illustration in the lag exhibited by
the curve of excitability in reference to the temperature curve. The
minimum temperature was attained at about 4 A.M., but the excitability
was not reduced to a minimum till four hours later and again there is a
marked fall of temperature after 5 P.M., but the excitability did not
become depressed till two hours later.
There is again the factor of variation of light, the effect of which is
not so great as that of temperature. The periods of maximum of light
and temperature are, however, not coincident.
We may now discuss in greater detail the diurnal variation of
excitability in _Mimosa_, taking the typical case, the record of which
is given in Fig. 23. The temperature here is seen to remain almost
constant, and at an optimum, from 1 to 5 P.M., the condition of light
is also favourable. Hence the excitability is found to be constant,
and at its maximum between these hours. The temperature begins to fall
after 6 P.M., and there is, in addition, the depressing action of
gathering darkness. Owing to the time-lag, the fall of excitability
does not commence immediately at 6 P.M., but an hour afterwards,
and continues till the next morning. During this period we have the
cumulative effect of twelve hours’ darkness and the increasing
depression due to cold, the temperature minimum occurring at 4 A.M.
On account of the combined effects of these various factors, and
phenomenon of lag, the period of minimum excitability is in general
reached about 8 A.M. In certain other cases this may occur earlier.
After the attainment of this minimum, the excitability is gradually
and continuously increased, under the action of light and of rising
temperature, till the maximum is reached in the afternoon.
EFFECT OF SEASON.
It was said that temperature exerted a predominant influence in
inducing variation of excitability. We may, therefore, expect that
the diurnal period would be modified in a certain way according to
the season. In winter the night temperature falls very low; hence the
depression of excitability is correspondingly great, and results in
the complete abolition of excitability. The after-effect of intense
cold is seen in the condition of inexcitability persisting for a very
long period in the morning. In summer the prevailing high temperature
modifies the diurnal periodicity in a different manner. When the night
is warm, the fall of excitability is slight. In the day, on the other
hand, the temperature may rise above the optimum, bringing about a
depression. In such a case the excitability in the earlier part of the
evening may actually be greater than in the middle of the day. These
modifications are shown in a very interesting way in the following
record (Fig. 29) taken at the end of April. The temperature of Calcutta
at this season often rises above 100°F. or 38°C. Table III also
exhibits, in the case of the summer specimen, the relation between the
hours of the day, temperature, and excitability.
[Illustration: FIG. 29. Diurnal variation of excitability exhibited by
summer specimen.]
An inspection of the record given in Fig. 29 shows that the amplitude
of response was enhanced after 4 P.M. The temperature up to that time
was unusually high (38°C.), and there was in consequence a depression
of excitability. After that hour there was a mitigation of heat, the
temperature returning towards the optimum. Hence we find that the
maximum excitability was attained between the hours 4 and 6 P.M. The
minimum temperature at night was higher in the present case than that
of the experiment carried out in February; in the former the minimum
was 25.5°C., while in the latter it was 19.5°C. On account of this
difference the night record in summer shows a fall of excitability
which is far more gradual than that obtained in spring. The
excitability is here not totally abolished in the morning, but reaches
a minimum after 8 A.M.; the sensitiveness is then gradually enhanced
in a staircase manner.
TABLE III--SHOWING THE RELATION BETWEEN HOURS OF THE DAY, TEMPERATURE,
AND EXCITABILITY. (SUMMER SPECIMEN.)
+--------------------------------+--------------------------------+
| _Hours _Tempera- _Amplitude | _Hours _Tempera- _Amplitude |
| of ture._ of | of ture._ of |
| day._ Response._ | day._ Response._ |
+--------------------------------+--------------------------------+
| 1 p.m. 38° C. 22 mm | 1 a.m. 26° C. 21.5 mm. |
| 2 " 38° " 23 " | 2 " 26° " 20 " |
| 3 " 38° " 24.5 " | 3 " 25.5° " 18.5 " |
| 4 " 37° " 28 " | 4 " 25.5° " 17 " |
| 5 " 35.5° " 29 " | 5 " 25.5° " 16 " |
| 6 " 33° " 27 " | 6 " 26° " 15 " |
| 7 " 31° " 26 " | 7 " 27° " 14 " |
| 8 " 30° " 26 " | 8 " 29° " 13 " |
| 9 " 29° " 25 " | 9 " 30.5° " 11 " |
|10 " 27° " 24.5 " |10 " 33° " 16 " |
|11 " 27° " 24 " |11 " 35° " 17 " |
|12 " 26.5° " 22.5 " |12 " 37° " 21 " |
+--------------------------------+--------------------------------+
SUMMARY.
The moto-excitability of _Mimosa_ was gauged every hour of the day
and night, by the amplitude of the response to a testing stimulus.
This is effected by means of automatic devices which excite the plant
periodically by an absolutely constant stimulus, and record the
corresponding mechanical response.
From the record thus obtained, it was found that the excitability
of the plant is not the same throughout the day, but undergoes a
variation characteristically different at different times of the day.
In a typical case in spring the excitability attained its maximum
value after 1 P.M. and remained constant for several hours. There was
then a continuous fall of excitability, the minimum being reached at
about eight in the morning. The plant at this time was practically
insensitive. The moto-excitability was then gradually enhanced in a
staircase manner till it again reached a maximum next afternoon.
The effect of sudden darkness was found to induce a transient
depression, followed by revival of excitability. The effect of
persistent darkness was to induce a depression.
Exposure to light from darkness caused a transient depression, followed
by an enhancement of excitability.
Excessive turgor induced a diminished response.
Lowering of temperature induced a depression of excitability,
culminating in an abolition of response. The after-effect of excessive
cold was a prolonged depression of excitability.
Excitability was enhanced by rising temperature up to an optimum; above
this point a depression was induced.
Owing to physiological inertia the change of excitability induced by
variation of external condition lags behind the inducing cause.
The diurnal variation of excitability is primarily due to diurnal
variation of temperature. The effect is modified in a minor degree by
variation of light.
V.--RESPONSE OF PETIOLE-PULVINUS PREPARATION OF _MIMOSA PUDICA_
_By_
SIR J. C. BOSE,
_Assisted by_
SURENDRA CHANDRA DAS, M.A.
The most suitable plant for researches on irritability of plants is
_Mimosa pudica_, which can be obtained in all parts of the world.
An impression unfortunately prevails that the excitatory reaction
of the plant can be obtained only in summer and under favourable
circumstances; this has militated against its extensive use in
physiological experiments, but the misgiving is without any foundation;
for I found no difficulty in demonstrating even the most delicate
experiments on _Mimosa_ before the meeting of the American Association
for the Advancement of Science held during Christmas of 1914. The
prevailing outside temperature at the time was considerably below
the freezing point. With foresight and care it should not be at all
difficult to maintain in a hot-house a large number of these plants in
a sensitive condition all the year round.
In order to remove the drawback connected with the supply of sufficient
material, I commenced an investigation to find whether a detached leaf
preparation could be made as effective for the study of irritability
as the whole plant. Here we have at the central end of the leaf the
pulvinus, which acts as the contractile organ; the conducting strand
in the interior of the petiole, on the other hand, is the vehicle for
transmission of excitation. The problem to be solved is the rendering
of an isolated petiole-and-pulvinus of _Mimosa_ as efficient for
researches on irritability as the nerve-and-muscle preparation of a
frog. On the success of this attempt depended the practical opening
out of an extended field of physiological investigation which would be
unhampered by any scarcity of experimental material.
In connection with this it is well to note the surprising difference in
vegetative growth as exhibited by plants grown in soil and in pots. A
pot-specimen of _Mimosa_ produces relatively few leaves, but one grown
in the open ground is extremely luxuriant. As an instance in point,
I may state that for the last five months I have taken from a plant
grown in a field about 20 leaves a day for experiment, without making
any impression on it. A large box containing soil would be practically
as good as the open ground, and the slower rate of growth in a colder
climate could be easily made up by planting half a dozen specimens. The
protection of the plants from inclemencies of weather can be ensured by
means of a glass cover with simple heat-regulation by electric lamps,
in place of an expensive green-house.
Returning to the question of the employment of an isolated leaf, which
I shall designate as a petiole-pulvinus preparation, instead of the
entire plant, the first attempts which I made proved unsuccessful. The
cut leaf kept in water would sometimes exhibit very feeble response, at
other times all signs of excitability appeared to be totally abolished.
It was impossible to attempt an investigation on the effect of changing
environment on excitability when the normal sensitiveness itself
underwent so capricious a change
These difficulties were ultimately overcome from knowledge derived
through systematic investigation on the relative importance of
the different parts of the motor apparatus, on the immediate and
after-effect of section on the excitability of the leaf, and on the
rate of decay of this excitability on isolation from the plant. The
experience thus gained enabled me to secure long-continued and uniform
sensibility under normal conditions. It was thus possible to study the
physiological effects of changing external conditions by observing the
responsive variation in the isolated petiole-pulvinus preparation. I
propose to deal with the different aspects of the investigation in the
following order:--
1. The effect of wound or section in modification of normal
excitability.
2. The change of excitability after immersion in water.
3. Quantitative determination of the rate of decay of excitability
in an isolated preparation.
4. Effect of amputation of the upper half of pulvinus.
5. Effect of removal of the lower half.
6. Influence of the weight of leaf on rapidity of responsive fall.
7. The action of chemical agents.
8. Effect of “fatigue” on response.
9. The influence of constant electric current on recovery.
10. The action of light and darkness on excitability.
The isolated petiole-pulvinus preparation is made by cutting out a
portion of the stem bearing a single lateral leaf. The four diverging
sub-petioles may also be cut off. In order to prevent rapid drying
the specimen has to be kept in water. Preparations made in this way
often appeared to have lost their sensibility. I was, however, able to
trace this loss to two different factors: first, to the physiological
depression due to injury caused by section, and, second, to the sudden
increase of turgor brought on by excessive absorption of water. I shall
now proceed to show that the loss of sensibility is not permanent, but
is capable of restoration.
EFFECT OF WOUND OR SECTION IN MODIFICATION OF NORMAL EXCITABILITY.
In connection with the question of effect of injury, it is to be borne
in mind that after each excitation the plant becomes temporarily
irresponsive and that the excitability is fully restored after the
completion of protoplasmic recovery. A cut or a section acts as a very
intense stimulus, from the effect of which the recovery is very slow.
If the stem be cut very near the leaf, the excitation of the pulvinus
is very intense, and the consequent loss of excitability becomes more
or less persistent. But if the stem be cut at a greater distance, the
transmitted excitation is less intense, and the cut specimen recovers
its excitability within a moderate time. I have also succeeded in
reducing the excitatory depression by previously benumbing the tissue
by physiological means. The isolated specimen can be made still more
compact by cutting off the sub-petioles bearing the leaflets; the
preparation now consists of a short length of stem of about 2 cm. and
an equally short length of primary petiole, the motile pulvinus being
at the junction of the two.
[Illustration: FIG. 30--The Resonant Recorder, with petiole-pulvinus
preparation. (From a photograph.)]
For the restoration of sensitiveness, and to meet working conditions,
the lower end of the cut stem is mounted on a T-tube, with
funnel-attachment and exit-tube, as shown in Fig. 30. The other two cut
ends--of the stem and of the petiole--may be covered with moist cloth
or may be closed with collodion flexile to prevent rapid evaporation
and drying up of the specimen. A slight hydrostatic pressure maintains
the specimen in a moderately turgid condition. A preparation thus
made is insensitive at the beginning, but if left undisturbed it
slowly recovers its excitability. The history of the depression of
excitability after shock of preparation and its gradual restoration
is graphically illustrated by a series of records made by the plant
(Fig. 31).
The petiole-pulvinus preparation thus made offers all facilities for
experiment. Owing to its small size it can be easily manipulated; it
can be enclosed in a small chamber and subjected to varying conditions
of temperature and to the action of different vapours and gases. Drugs
are easily absorbed at the cut end, and poison and its antidote can
be successively applied through the funnel without any disturbance
of the continuity of record. In fact, many experiments which would
be impossible with the entire plant are quite practicable with the
isolated leaf.
The arrangement for taking records of response is seen in Fig. 30,
which is reproduced from a photograph of the actual apparatus. For
recording the response and recovery of the leaf under stimulation,
I use my Resonant Recorder fully described in the ‘Philosophical
Transactions’ (1913). The petiole is attached to one arm of the
horizontal lever. The writer, made of fine steel wire with a bent tip,
is at right angles to the lever, and is maintained by electromagnetic
means in a state of to-and-fro vibration, say, ten times in a second.
The record, consisting of a series of dots, is free from errors arising
from friction of continuous contact of the writer with the recording
surface. The successive dots in the record at definite intervals of a
tenth of a second also give the time-relations of the response curve.
On account of its small size, the petiole-pulvinus preparation offers
great facilities for mounting in different ways suitable for special
investigations. Ordinarily, the cut stem with its lower end enclosed
in moist cloth is supported below. A very suitable form of stimulus is
that of induction shock from a secondary coil, the intensity of which
is capable of variation in the usual manner by adjusting the distance
between the primary and the secondary coils. The motile pulvinus, P,
may be excited directly. For investigations on velocity of transmission
of excitation, stimulus is applied on the petiole at some distance
from the pulvinus, by means of suitable electrodes. Excitation is now
transmitted along the intervening length of petiole, the conducting
power of which will be found appropriately modified under the action
of chemical and other agents. In this normal method of mounting, the
more excitable lower half of the pulvinus is below; excitatory reaction
produces the fall of the petiole, gravity helping the movement. The
preparation may, however, be mounted in the inverted position, with
the more excitable lower half of the pulvinus facing upwards. The
excitatory movement will now be the erection of the petiole, against
gravity.
Under natural conditions the stem is fixed, and it is the petiole which
moves under excitation. But a very interesting case presents itself
when the petiole is fixed and the stem free. Here is presented the
unusual spectacle of the plant or the stem “wagging” in response to
excitation.
THE CHANGE OF EXCITABILITY AFTER IMMERSION IN WATER.
The isolated specimen can be kept alive for several days immersed in
water. The excitability of the pulvinus, however, undergoes great
depression, or even abolition, by the sudden change of turgor brought
on by excessive absorption of water. The plant gradually accommodates
itself to the changed condition, and the excitability is restored in a
staircase manner from zero to a maximum.
In studying the action of a chemical solution on excitability,
the solution may be applied through the cut end or directly on
the pulvinus. The sudden variation of turgor, due to the liquid,
always induces a depression, irrespective of the stimulating or
the depressing action of the drug. The difficulty may be eliminated
by previous long-continued application of water on the pulvinus and
waiting till the attainment of uniform excitability which generally
takes place in the course of about three hours. Subsequent application
of a chemical solution gives rise to characteristic variation in the
response.
QUANTITATIVE DETERMINATION OF THE RATE OF DECAY OF EXCITABILITY IN AN
ISOLATED PREPARATION.
[Illustration: FIG. 31--Variation of excitability after section. (1)
Immediate effect; (2) variation of excitability in a second specimen
during 50 hours: (_a_) response 4 hours after section; (_b_) response
after 24 hours; (_c_) after 49 hours. Up-line of record represents
responsive fall of the leaf, down-line indicates recovery from
excitation.]
_Variation of excitability after section: Experiment 23._--In order
to test the history of the change of excitability resulting from the
immediate and after-effect of section, I took an intact plant and
fixed the upper half of the stem in a clamp. The response of a given
leaf was now taken to the stimulus of an induction shock of 0.1 unit
intensity, the unit chosen being that which causes a bare perception
of shock in a human being. The specimen was vigorous and the response
obtained was found to be a maximum. The stem bearing the leaf was cut
at the moment marked in the record with a cross, and water was applied
at the cut end. The effect of section was to cause the maximum fall of
the leaf, with subsequent recovery. After this, successive responses
to uniform stimuli at intervals of 15 minutes show, in (1) of Fig. 31,
that a depression of excitability has been induced owing to the shock
caused by section. In course of an hour, however, the excitability
had been restored almost to its original value before the section.
This was the case with a vigorous specimen, but with less vigorous
ones a longer period of about three hours is required for restoration.
In certain other cases the response after section exhibits alternate
fatigue; that is to say, one response is large and the next feeble,
and this alternation goes on for a length of time. The isolated
specimen, generally speaking, attains a uniform sensibility after a few
hours, which is maintained, with very slight decline under constant
external conditions, for about 24 hours. On the third day the fall
of excitability is very rapid, and the sensibility declines to zero
in about 50 hours after isolation [Fig. 31 (2)]. We may describe
the whole cycle of change as follows: by the shock of operation the
isolated preparation is rendered insensitive for nearly an hour, the
excitability is then gradually restored almost to its normal value
before operation. Under constant external conditions, this excitability
remains fairly constant for about 24 hours after which depression sets
in. The rate of fall of excitability becomes very rapid 40 hours after
the operation, being finally abolished after the fiftieth hour. It
is probable that in a colder climate the fall of excitability would
be much slower. The most important outcome of this inquiry is the
demonstration of the possibility of obtaining persistent and uniform
sensibility in isolated preparations. On account of this, not only is
the difficulty of supply of material entirely removed but a very high
degree of accuracy secured for the investigation itself.
EFFECT OF AMPUTATION OF UPPER HALF OF PULVINUS.
_Experiment 24._--The determination of the _rôle_ played by different
parts of the pulvinus in response and recovery is of much theoretical
importance. Our knowledge on this subject is unfortunately very
scanty. The generally accepted view is that on excitation “the actual
downward curvature of the pulvinus is partly due to a contraction of
the walls of the motor cells consequent upon the decrease of turgor,
but is accentuated by expansion of the insensitive adaxial half of the
pulvinus--which was strongly compressed in the unstimulated condition
of the organ--and also by the weight of the leaf.”[H] According to
Pfeffer, after excitation of the organ, “the original condition of
turgor is gradually reproduced in the lower half of the pulvinus, which
expands, raising the leaf and producing compression of the upper half
of the pulvinus, which aids in the rapid curvature of the stimulated
pulvinus.”[I]
[H] Haberlandt, ‘Physiological Plant Anatomy,’ 1914, p. 570. English
Translation, Macmillan & Co.
[I] PFEFFER--‘Physiology of Plants,’ vol. 3, pp. 75 and 76. English
Translation, Clarendon Press.
It was held, then, that the rapidity of the fall of leaf under stimulus
is materially aided (1) by the expansion of the upper half of the
pulvinus, which is normally in a state of compression, and (2) by the
weight of the leaf. So much for theory. The experimental evidence
available regarding the relative importance of the upper and lower
halves of the pulvinus is not very conclusive. Lindsay attempted to
decide the question by his amputation experiments. He showed that when
the upper half was removed the leaf carried out the response, but rigor
set in when the lower half was amputated. Pfeffer’s experiments on the
subject, however, contradicted the above results. He found that “after
the upper half of the pulvinus was carefully removed, no movement
was produced by stimulation, whereas when the lower half is absent a
weakened power of movement is retained.” Pfeffer, however, adds, “since
the operation undoubtedly affects the irritability, it is impossible
to determine from such experiments the exact part played by the active
contraction of the lower half of the pulvinus.”[I]
The cause of uncertainty in this investigation is twofold. First,
it arises from the unknown change in irritability consequent on
amputation; and, secondly, from absence of any quantitative standard
by which the effect of selective amputation of the pulvinus may
be measured. As regards the first, I have been able to reduce the
depressing action caused by injury to a minimum by benumbing the tissue
before operation, through local application of cold, and also allowing
the shock-effect to disappear after a rest of several hours. As regards
the physiological gauge of efficiency of the motor mechanism, such
a measure is afforded by the relation between a definite testing
stimulus and the resulting response with its time-relations, which
is secured by my Resonant Recorder with the standardised electrical
stimulator.
[Illustration: FIG. 32--Effect of amputation of upper half of pulvinus.
Upper record gives normal response before amputation, and the lower,
response after amputation. (Successive dots at intervals of 0.1 sec).
Apex-time 11 sec, in both.]
In carrying out this investigation I first took the record of normal
response of an intact leaf on a fast moving plate. A second record,
with the same stimulus, was taken after the removal of the upper half
of the pulvinus, having taken the necessary precautions that have been
described. Comparison of the two records (Fig. 32) shows that the only
difference between them is in the exhibition of slight diminution of
excitability due to operation. But, as regards the latent period and
the quickness of attaining maximum fall, there is no difference between
the two records before and after the amputation of the upper half. The
upper part of the pulvinus is thus seen practically to have little
influence in hastening the fall.
EFFECT OF REMOVAL OF THE LOWER HALF.
_Experiment 25._--The shock-effect caused by the amputation of the
lower half was found to be very great, and it required a long period of
rest before the upper half regained its excitability. The excitatory
reaction of the upper half is by contraction, and the response is,
therefore, the lifting of the petiole. Thus, in an intact specimen,
excitation causes antagonistic reactions of the two halves. But the
sensibility of the upper half is very feeble and the rate of its
contractile movement, relatively speaking, very slow. The record of
the response of the upper half of the pulvinus, seen in Fig. 33, was
taken with an Oscillating Recorder, where the successive dots are at
intervals of 1 sec.: the magnification employed was about five times
greater than in recording the response of the lower half (Fig. 32). The
intensity of stimulus to evoke response had also to be considerably
increased. Taking into account the factors of magnification and the
intensity of stimulus for effective response, the lower half I find to
be about 80 times more sensitive than the upper. Thus, under feeble
stimulus the upper half exerts practically no antagonistic reaction.
The excitatory response of the upper half is also seen to be very
sluggish.
[Illustration: FIG. 33.--Response after amputation of lower half of
pulvinus. (Successive dots at intervals of a second; vertical lines
mark minutes.) Apex-time, 40 secs.]
INFLUENCE OF THE WEIGHT OF LEAF ON RAPIDITY OF RESPONSIVE FALL.
_Experiment 26._--It is obvious that the mechanical moment exerted by
the weight of the leaf must help its responsive fall under excitation.
But the relative importance of the factors of active contraction
of the lower half of the pulvinus and of the weight, in the rate
of the responsive down-movement, still remains to be determined. A
satisfactory way of solving the problem would lie in the study of
the characteristics of response-records taken under three different
conditions: (1) When the leaf is helped in its fall by its weight; (2)
when the action of the weight is eliminated; and (3) when the fall
has to be executed against an equivalent weight. An approximation to
these conditions was made in the following manner. We may regard the
mechanical moment to be principally due to the weight of the four
sub-petioles applied at the end of the main petiole. In a given case
these sub-petioles were cut off, and their weight found to be 0.5 grm.
The main petiole was now attached to the right arm of the lever, and
three successive records were taken: (1) With no weight attached to
the petiole; (2) with 0.5 grm. attached to its end; and (3) with 0.5
grm. attached to left arm of the lever at an equal distance from the
fulcrum. In the first case, the fall due to the excitatory contraction
will practically have little weight to help it; in the second case,
it will be helped by a weight equivalent to those of the sub-petioles
with their attached leaflets; and in the third case, the fall will be
opposed by an equivalent weight. We find that in these three cases
there is very little difference in the time taken by the leaf to
complete the fall (Fig. 34).
[Illustration: FIG. 34.--Effect of weight on rapidity of fall. N,
without action of weight; W, with weight helping; and A, with weight
opposing.]
It has been shown that the presence or absence of the upper half of
the pulvinus makes practically no difference in the period of fall;
it is now seen that the weight exerts comparatively little effect. We
are thus led to conclude that in determining the rapidity of fall,
the factors of expansive force of the upper half of the pulvinus and
the weight of the leaf are negligible compared to the active force of
contraction exerted by the lower half of the pulvinus.
ACTION OF CHEMICAL AGENTS.
In connection with this subject it need hardly be said that the various
experiments which I had previously carried out with the intact plant
can also be repeated with the isolated preparation. I will only give
here accounts of experiments which are entirely new.
The chemical solution may be applied directly to the pulvinus, or it
may be absorbed through the cut end, the absorption being hastened
by hydrostatic pressure. The normal record is taken after observing
precautions which have already been mentioned. The reaction of a
given chemical agent is demonstrated by the changed character of the
record. The effect of the drug is found to depend not merely on its
chemical nature, but also on the dose. There is another very important
factor--that of the tonic condition of the tissue--which is found to
modify the result. The influence of this will be realised from the
account of an experiment to be given presently, where an identical
agent is shown to produce diametrically opposite effects on two
specimens, one of which was in a normal, and the other in a sub-tonic,
condition. The experiments described below relate to reactions of
specimens in a normal condition.
[Illustration: FIG. 35.--Stimulating action of hydrogen peroxide.]
_Hydrogen Peroxide: Experiment 27._--This reagent in dilute solution
exerts a stimulating action. Normal records, were taken after
long-continued application of water on the pulvinus. The peroxide, as
supplied by Messrs. Parke Davis & Co., was diluted to 1 per cent., and
applied to the pulvinus; this gave rise to an enhancement of response.
Re-application of water reduced the amplitude to the old normal value
(Fig. 35).
[Illustration: FIG. 36.--Incomplete recovery under the action BaCl_{2}
and transient restoration under tetanisation at T.]
_Barium Chloride: Experiment 28._--The action of this agent is
very characteristic, inducing great sluggishness in recovery. The
preparation had been kept in 1-per cent. solution of this substance for
two hours. After this the first response to a given test-stimulus was
taken; the response was only moderate, and the recovery incomplete. The
sluggishness was so great that the next stimulation, represented by a
thick dot (Fig. 36), was ineffective. Tetanising electric shock at T,
not only brought about response, but removed for the time being the
induced sluggishness. This is seen in the next two records, which were
taken under the old test-stimulus. There is now an enhanced response
and a complete recovery. Beneficial effect of tetanisation disappeared,
however, on the cessation of stimulus. This is seen in the next two
records which were taken after two hours. The amplitude of response was
not only diminished, but the recovery also was incomplete.
[Illustration: FIG. 37.--Antagonistic action of alkali and acid. Arrest
of response in contraction under NaOH (↑), restoration and final arrest
in expansion under lactic acid. (↑)]
_Antagonistic actions of Alkali and Acid: Experiment 29._--Alkali and
acid are known to exert antagonistic actions on the spontaneous beat
of the heart; dilute solution of NaOH arrests the beat of the heart
in systolic contraction, while dilute lactic acid arrests the beat in
diastolic expansion. I have found identical antagonistic reactions
in the pulsating tissue of _Desmodium gyrans_, the telegraph plant.
It is very interesting to find that these agents also exert their
characteristic effects on the response of _Mimosa_ in a manner which
is precisely the same. This is seen illustrated in Fig. 37, where the
application of NaOH arrested the response in a contracted state; after
this, the antagonistic effect of dilute lactic acid is seen first, in
its power of restoring the excitability; its continued application,
however, causes a second arrest, but this time in a state of relaxed
expansion.
_CuSO_4 Solution._--This agent acts as a poison, causing a gradual
diminution of amplitude of response, culminating in actual arrest at
death. Certain poisons, again, exhibit another striking symptom at the
moment of death, an account of which will be given in a separate paper.
EFFECT OF “FATIGUE” ON RESPONSE.
[Illustration: FIG. 38.--“Fatigue” induced by shortening intervening
period of rest.]
With _Mimosa_, after each excitation the recovery becomes complete
after a resting period of about 15 min. With this interval of rest the
successive responses for a given stimulus are equal, and are at their
maximum.
_Experiment 30._--When the resting interval is diminished the recovery
becomes incomplete, and there is a consequent diminution of amplitude
of response. There is thus an increased fatigue with diminished period
of rest. This is illustrated in Fig. 38, where the first two responses
are at intervals of 15 min.; the resting interval was then reduced to
10 min., the response undergoing a marked diminution. Conversely, by
increasing the resting interval, first to 12 and then to 15 min., the
extent of fatigue was reduced and then abolished.
THE INFLUENCE OF CONSTANT ELECTRIC CURRENT ON RECOVERY.
[Illustration: FIG. 39.--Action of constant current in removal of
fatigue by hastening recovery; N, curve of response in fatigued
specimen; C, after passage of current.]
_Experiment 31._--From the above experiment it would appear that
since the incompleteness of recovery induces fatigue, hastening of
recovery would remove it. With this idea I tried various methods for
quickening the recovery of the excited leaf. The application of a
constant electric current was found to have the desired effect. Two
electrodes for introduction of current were applied, one on the stem
and the other on the petiole, at some distance from the pulvinus. In
order to avoid the excitatory effect of sudden application, the applied
current should be increased gradually; this was secured by means of a
potentiometer slide. In my experiment a current having an intensity of
1.4 micro-ampère was found to be effective. Responses at intervals of
10 min., as we have seen, exhibit marked fatigue. Two responses were
recorded on a fast-moving plate, N before, and C after, the application
of the current. It will be seen (Fig. 39) how the application of
current has, by hastening the recovery, enhanced the amplitude of
response and brought about a diminution of fatigue. In connection with
this, I may state that the tonic condition is, in general, improved as
an after-effect of the passage of current. This is seen in some cases
by a slight increase in excitability; in others, where the responses
had been irregular, the previous passage of a current tends to make the
responses more uniform.
ACTION OF LIGHT AND DARKNESS ON EXCITABILITY.
In taking continuous records of responses I was struck by the marked
change of excitability exhibited by the intact plant under variation
of light. Thus the appearance of a cloud was quickly followed by
an induced depression, and its disappearance by an equally quick
restoration of excitability. This may be explained on the theory that
certain explosive chemical compounds are built up by the photosynthetic
processes in green leaves, and that the intensity of response depends
on the presence of these compounds. But the building up of a chemical
compound must necessarily be a slow process, and it is difficult on the
above hypothesis to connect the rapid variation of excitability with
the production of a chemical compound, or its cessation, concomitant
with changes in the incident light.
[Illustration: FIG. 40.--Stimulating action of light, and depressing
action of darkness. Horizontal line below represents period of
darkness.]
_Experiment 32._--In order to find out whether photo-synthesis had
any effect on excitability, I placed an intact plant in a dark
room and obtained from it a long series of responses under uniform
test-stimulus. While this was being done the green leaflets were
alternately subjected to strong light and to darkness, care being taken
that the pulvinus was shaded all the time. The alternate action of
light and darkness on leaflets induced no variation in the uniformity
of response. This shows that the observed variation of excitability
in _Mimosa_ under the alternate action of light and darkness is not
attributable to the photo-synthetic processes.
I next took a petiole-pulvinus preparation from which the sub-petioles
bearing the leaflets had been cut off, and placed it in a room
illuminated by diffused daylight. The normal responses were taken,
the temperature of the room being 30°C. The room was darkened by
pulling down the blinds, and records were continued in darkness. The
temperature of the room remained unchanged at 30°C. It will be seen
from records given in Fig. 40, that in darkness there is a great
depression of excitability. Blinds were next pulled up and the records
now obtained exhibit the normal excitability under light. The sky
had by this time become brighter, and this accounts for the slight
enhancement of excitability. This experiment proves conclusively that
light has a direct stimulating action on the pulvinus, independent of
photo-synthesis.[J]
[J] See also Bose and Das--‘Physiological Investigations with
Petiole-Pulvinus preparations of _Mimosa pudica_.’ Proc. Roy. Soc. B.
Vol. 89, 1916.
SUMMARY.
On isolation of a petiole-pulvinus preparation, the shock of operation
is found to paralyse its sensibility. After suitable mounting the
excitability is restored, and remains practically uniform for nearly
24 hours. After this a depression sets in, the rate of fall of
excitability becomes rapid 40 hours after the operation, sensibility
being finally abolished after the fiftieth hour.
Experiments carried out on the effect of weight, and the influence of
selective amputation of the upper and lower halves of the pulvinus,
show that in determining the rapidity of fall of leaf, the assumed
factors of the expansive force of the upper half of the pulvinus and
the weight of the leaf are negligible compared to the force of active
contraction exerted by the lower half of the pulvinus. The excitability
of the lower half is eighty times greater than that of the upper.
Chemical agents induce characteristic changes in excitability.
Hydrogen peroxide acts as a stimulant. Barium chloride renders the
recovery incomplete: but tetanisation temporarily removes the induced
sluggishness. Acids and alkalis induce antagonistic reactions,
abolition of excitability with alkali taking place in a contracted, and
with acid in an expanded condition of the pulvinus.
The responses exhibit fatigue when the period of rest is diminished.
The passage of constant current is found to remove the fatigue.
Response is enhanced on exposure to light, and diminished in darkness.
Light is shown to exert a direct stimulating action on the pulvinus,
independent of photo-synthesis.
VI.--ON CONDUCTION OF EXCITATION IN PLANTS
_By_
SIR J. C. BOSE.
The plant _Mimosa_ offers the best material for investigation on
conduction of excitation. With regard to this question the prevailing
opinion had been that in plants like _Mimosa_, there is merely a
transmission of hydro-mechanical disturbance and no transmission of
true excitation comparable with the animal nerve. I have, however, been
able to show that the transmission in the plant is not a mechanical
phenomenon, but a propagation of excitatory protoplasmic change. This
has been proved by the arrest of conduction by the application of
various physiological blocks. Thus local application of increasing cold
retards, and finally abolishes the conducting power. The conducting
tissue becomes paralysed for a time as an after-effect of application
of cold; the lost conducting power may, however, be quickly restored
by tetanising electric shocks. The conducting power of an animal nerve
is arrested by an electrotonic block, the conductivity being restored
on the cessation of the current. I have succeeded in inducing similar
electrotonic block of conduction in _Mimosa_. Conductivity of a
selective portion of petiole may also be permanently abolished by local
action, of poisonous solution of potassium cyanide.[K]
[K] BOSE--“An Automatic Method for the Investigation of Velocity of
Transmission of Excitation in _Mimosa_.” ‘Phil. Trans.’ ‘B, Vol. 204
(1913) and also “Irritability of Plants.” Longman’s Green & Co. (1913),
p. 132.
Having thus established the physiological character of the transmitted
impulse in plants I shall now proceed to give some of the principal
results of my earlier and recent investigations on the effects of
various agencies on conduction of excitation in plants.
Apart from any question of hydro-mechanical transmission, it is
important to distinguish two different modes of transmission of
excitation. In a motile tissue contraction of a cell causes a physical
deformation and stimulation of the neighbouring cell. Examples of this
are furnished by the cardiac muscle of the animal, the pulvinus of
_Mimosa_, and the stamen of _Berberis_. This mode of propagation may
better be described as a _convection_ of excitation.
The _conduction_ of excitation, as in a nerve, is a different process
of transmission of protoplasmic change. The conducting tissue in this
case does not itself exhibit any visible change of form. In the plant
the necessary condition for transmission of excitation to a distance
is that the conducting tissue should be possessed of protoplasmic
continuity in a greater or less degree. This condition is fulfilled
by vascular bundles. There being greater facility of transmission
along the bundles than across them, the velocity in the longitudinal
direction is very much greater than in the transverse.
For accurate determination of velocity of transmission the testing
stimulus should be quantitative and capable of repetition. Abnormal
high velocity has been observed in _Mimosa_ by applying crude and
drastic methods of stimulation, by a transverse cut or a burn. This
is apt to give rise to a very strong hydro-dynamic disturbance,
which travelling with great speed, delivers a mechanical blow on the
responding pulvinus. Such hydro-dynamic transmission is not the same as
physiological conduction.
In the primary petiole of _Mimosa_ the highest velocity under electric
stimulation I find to be about 30 mm. per second. This velocity is
considerably lower than the velocity in the nerve of higher animals,
but higher than in the lower animals. As an example of the latter,
mention may be made of the velocity of 10 mm. per second in the nerve
of _Anodon_ and 1 mm. per second in the nerve of _Eledone_.
PREFERENTIAL DIRECTION OF CONDUCTION.
_Experiment 33._--The conduction of excitatory impulse takes place
in both directions. This can be demonstrated by taking a petiole of
_Biophytum sensitivum_ or of _Averrhoa carambola_. These petioles are
provided with a series of motile leaflets. Stimulation at the middle
point of the petiole gives rise to two waves of excitation, one of
which travels towards the central axis of the plant, and the other away
from it. The centrifugal velocity is greater than the centripetal as
will be seen from the following results:
+---------------+---------------------------+--------------------+
| | Velocity in | |
| _Biophytum_ | centrifugal direction | 2.9 mm per second.|
| | " centripetal " | 2 mm " " |
| _Averrhoa_ | " centrifugal " | 0.5 mm " " |
| | " centripetal " | 0.26 mm " " |
+---------------+---------------------------+--------------------+
EFFECT OF TEMPERATURE.
Variation of temperature has a marked effect on the velocity of
transmission of excitation. Lowering of temperature diminishes the
velocity, culminating in an arrest. Rise of temperature, on the
other hand, enhances the velocity. This enhancement is considerable
in specimens in which the normal velocity is low, but in plants in
optimum condition, the velocity being already high, cannot be further
enhanced. The following tabular statement gives results of effects of
temperature on velocity of transmission in _Mimosa_ and _Biophytum_:--
TABLE IV.--EFFECT OF TEMPERATURE ON VELOCITY OF TRANSMISSION.
+---------------------------+--------------+--------------------+
| Specimen. | Temperature. | Velocity. |
+---------------------------+--------------+--------------------+
|_Mimosa_ (winter specimen) | 22°C | 3.6 mm. per second.|
| | 28°C | 6.3 mm. " " |
| | 31°C | 9.0 mm. " " |
| | | |
|_Biophytum_ | 30°C | 3.7 mm. " " |
| | 35°C | 7.4 mm. " " |
| | 37°C | 9.1 mm. " " |
+---------------------------+--------------+--------------------+
EFFECT OF SEASON.
The velocity of transmission is very much lower in winter than in
summer. In the petiole of _Mimosa_, the velocity in summer is as high
as 30 mm. per second; in winter it is reduced to about 4 mm. The
lowering of velocity in winter is partly due to the prevailing low
temperature and also to the depressed state of physiological activity.
EFFECT OF AGE.
In a _Mimosa_ plant, different leaves will be found of different age.
Of these the youngest will be at the top. Lower down, we obtain a fully
grown young leaf, and near the base, leaves which are very old. The
investigation deals with the effect of age on the conducting power of
the petiole.
_Comparison of conducting power in different leaves: Experiment
34._--Selecting three leaves from the same plant we apply an identical
electric stimulus at points 2 cm. from the three responding pulvini.
The electric connections are so made that the same tetanising shock
is applied on the three petioles, very young, fully grown, and very
old. The secondary coil is gradually pushed in till the leaves exhibit
responsive fall. The fully grown leaf was the first to respond, the
velocity of transmission being 23 mm. per second. The secondary coil
had to be pushed nearer the primary through 6 cm. before excitation
could be effectively transmitted through the young petiole; for the
oldest leaf still stronger stimulus was necessary, since in this
case the secondary had to be pushed through an additional distance
of 4 cm. for effective transmission of excitation. I also determined
the relative values of the minimal intensity of stimulus, effective
in causing transmission of excitation in the three cases. Adopting as
before the intensity of electric stimulus which causes bare perception
in a human being as the unit, I find that the effective stimulus for a
fully grown young petiole is 0.3 unit, while the very young required
2.5 units, and the very old 5 units. Hence it may be said that the
conducting power of a very young is an eighth, and of the very old
one-sixteenth of the conductivity of the fully grown young specimen.
It will thus be seen that the conducting power of a very young petiole
is feebler than in a fully grown specimen. The conducting tissue, it
is true, is present, but the power of conduction has not become fully
developed. This power is, as we shall see later, conferred by the
stimulus of the environment. In a very old specimen the diminution of
conducting power is due to the general physiological decline.
EFFECT OF DESICCATION ON CONDUCTING TISSUES.
I have already shown that transmission in the plant is a process
fundamentally similar to that taking place in the animal nerve; it
has also been shown that the effects of various physical and chemical
agents are the same in the conducting tissues of plant and of animal.
[Illustration: FIG. 41--Action of glycerine in enhancing the speed and
intensity of transmitted excitation. Stimulus applied at the vertical
line. Successive dots in record are at intervals of 0.1 sec.]
_Effect of application of glycerine: Experiment 35._--It is known that
desiccation, generally speaking, enhances the excitability of the
animal nerve. As glycerine, by absorption of water, causes partial
desiccation, I tried its effect on conduction of excitation in the
petiole of _Mimosa_. Enhancement of conducting power may be exhibited
in two ways: first, by an increase of velocity of transmission; and,
secondly, by an enhancement of the intensity of the transmitted
excitation, which would give rise to a greater amplitude of response
of the motile indicator. In Fig. 41 are given two records, N, before,
and the other after the application of glycerine on a length of petiole
through which excitation was being transmitted. The time-records
demonstrate conclusively the enhanced rate of transmission after the
application of glycerine. The increased intensity of transmitted
excitation is also seen in the enhanced amplitude of response seen in
the more erect curve in the upper record.
INFLUENCE OF TONIC CONDITION ON CONDUCTIVITY.
Different specimens of _Mimosa_ are found to exhibit differences in
physiological vigour. Some are in an optimum condition, others in an
unfavourable or sub-tonic condition. I shall now describe certain
characteristic differences of conductivity exhibited by tissues in
different conditions.
_Effect of intensity of stimulus on velocity of transmission._--In a
specimen at optimum condition, the velocity remains constant under
varying intensities of stimulus. Thus the velocity of transmission in
a specimen was determined under a stimulus intensity of 0.5 unit; the
next determination was made with a stimulus of four times the previous
intensity, _i.e._, 2 units. In both these cases the velocity remained
constant. But when the specimen is in a sub-tonic condition, the
velocity is found to increase with the intensity of the stimulus. Thus
the velocity of conduction of a specimen of _Mimosa_ in a sub-tonic
condition was found to be 5.9 mm. per second under a stimulus of 0.5
unit; with the intensity raised to 2.5 units, the velocity was enhanced
to 8.3 mm. per second.
_After-effect of stimulus._--In experimenting with a particular
specimen of _Mimosa_ I found that on account of its sub-tonic
condition, the conducting power of the petiole was practically absent.
Previous stimulation was, however, found to confer the power of
conduction as an after-effect. It is thus seen that stimulus canalises
a path for conduction.
The effect of excessive stimulus in a specimen in an optimum condition
is to induce a temporary depression of conductivity; the effect of
strong stimulus on a sub-tonic specimen is precisely the opposite,
namely, an enhancement of conductivity. I give below accounts of two
typical experiments carried out with petiole-pulvinus preparation of
_Mimosa_. Excessive stimulation in these cases was caused by injury.
[Illustration: FIG. 42.--Effect of injury, depressing rate of
conduction in normal specimen; (1) record before, and (2) after injury.
(Dot-intervals, 0.1 sec.).]
_Action of Injury on Normal Specimens: Experiment 36._--A cut stem
with entire leaf was taken, and stimulus applied at a distance of
15 mm. from the pulvinus. From the normal record (1) in Fig. 42 the
velocity of transmission was found to be 18.7 mm. per sec. The end of
the petiole beyond the point of application of the testing stimulus was
now cut off, and record of velocity of transmission taken once more.
It will be seen from record (2) that the excessive stimulus caused by
injury had induced a depression in the conducting power, the velocity
being reduced to 10.7 mm. per sec. Excessive stimulation of normal
specimens is thus seen to depress temporarily the conducting power.
_Action of Injury on Sub-tonic Specimens: Experiment 37._--I will now
describe a very interesting experiment which shows how an identical
agent may, on account of difference in the tonic condition of the
tissue, give rise to diametrically opposite effects. In demonstrating
this, I took a specimen in a sub-tonic condition, in which the
conducting power of the tissue was so far below par, that the
test-stimulus applied at a distance of 15 mm. failed to be transmitted
(Fig. 43). The end of the petiole at a distance of 1 cm. beyond the
point of application of test-stimulus was now cut off. The after-effect
of this injury was found to enhance the conducting power so that the
stimulus previously arrested was now effectively transmitted, the
velocity being 25 mm. per sec. This enhanced conducting power began
slowly to decline, and after half an hour the velocity had declined
to 4.1 mm. per sec. The end of the petiole was cut once more, and the
effect of injury was again found to enhance the conducting power, the
velocity of transmission being restored to 25 mm. per sec.
[Illustration: FIG. 43.--Effect of injury in enhancing the conducting
power of a sub-normal specimen; (1) Ineffective transmission becoming
effective at (2) after section; (3) decline after half an hour, and (4)
increased conductivity after a fresh cut.]
SUMMARY.
There are two different types of propagation of excitation: by
convection, and by conduction. In the former the excited cell undergoes
deformation and causes mechanical stimulation of the next; example
of this type is seen in the stamen of _Berberis_. The conduction of
excitation consists, on the other hand, of propagation of excitatory
protoplasmic change. The transmission in the petiole of _Mimosa_ is a
phenomenon of conduction.
This conduction takes place along vascular elements. The conductivity
is very much greater in the longitudinal than in the transverse
direction.
Rise of temperature enhances, and fall of temperature lowers, the
rate of conduction. Excitation is transmitted in both directions; the
centrifugal velocity is greater than the centripetal.
Dessication of conducting tissue by glycerine enhances the conducting
power. Local application of cold depresses or arrests the conduction.
Application of poison permanently abolishes the power of conduction.
Conductivity is modified by the effect of season, being higher in
summer than in winter.
The power of conduction is also modified by age. In young specimens the
conducting power is low, the conductivity is at its maximum in fully
grown organs; but a decline of conductivity sets in with age.
The tonic condition of a tissue has an influence on conductivity. In
an optimum condition, the velocity is the same for feeble or strong
stimulus. Excessive stimulation induces a temporary depression of the
conducting power.
The effects are different in a sub-tonic tissue: velocity of
transmission increases with intensity of stimulus; after-effect of
stimulus is to initiate or enhance the conducting power. The conducting
path is canalised by stimulus.
VII.--ON ELECTRIC CONTROL OF EXCITATORY IMPULSE
_By_
SIR J. C. BOSE.
I have in my previous works[L] described investigations on the
conduction of excitation in _Mimosa pudica_. It was there shown that
the various characteristics of the propagation of excitation in the
conducting tissue of the plant are in every way similar to those in
the animal nerve. Hence it appeared probable that any newly found
phenomenon in the one case was likely to lead to discovery of a similar
phenomenon in the other.
[L] BOSE--“Comparative Electro-Physiology” (1907). Longmans, Green and
Co.
As the transmission of excitation is a phenomenon of propagation of
molecular disturbance in the conducting vehicle, it appeared that the
excitatory impulse could be controlled by inducing in the conducting
tissue two opposite ‘molecular dispositions’, using that term in the
widest sense. The possibility of accomplishing this by the directive
action of an electric current had attracted my attention for many years.
METHOD OF CONDUCTIVITY BALANCE.
I have previously carried out an electric method of investigation,
dealing with the influence of electric current on conductivity. The
method of Conductivity Balance which I devised for this purpose[M] was
found very suitable. Isolated conducting tissues of certain plants
were found to exhibit transmitted effect of excitatory electric
change of galvanometric negativity, which at the favourable season of
the year was of sufficient intensity to be recorded by a sensitive
galvanometer. A long strand of the conducting tissue was taken and two
electric connections were made with a galvanometer, a few centimetres
from the free ends. Thermal stimulus was applied at the middle, when
two excitatory waves with their concomitant electric changes were
transmitted outwards. By suitably moving the point of application of
stimulus nearer or further away from one of the two electric contacts,
an exact balance was obtained. This was the case when the resultant
galvanometer deflection was reduced to zero. If now an electrical
current be sent along the length of the conducting tissue, the two
excitatory waves sent outwards from the central stimulated point will
encounter the electric current in different ways; one of the excitatory
waves will travel with, and the other against the direction of the
current. If the power of transmitting excitation is modified by the
direction of an electric current then the magnitudes of transmitted
excitations will be different in the two cases, with the result of the
upsetting of the Conductivity Balance. From the results of experiments
carried out by this method on the effect of feeble current on
conductivity, the conclusion was arrived at that _excitation is better
conducted against the direction of the current than with it_. In other
words, the influence of an electric current is to confer a preferential
or selective direction of conductivity for excitation, the tissue
becoming a better conductor in an electric up-hill direction compared
with a down-hill.
[M] _Ibid_, p. 478.
The results were so unexpected that I have for long been desirous
of testing the validity of this conclusion by independent method of
inquiry. I shall presently give full account of the perfected method,
and the various difficulties which had to be overcome to render it
practical. Before doing this I shall describe a simple method which I
have devised for demonstrating the principal results.
CONTROL OF TRANSMITTED EXCITATION IN _AVERRHOA BILIMBI_.
The petiole of _Averrhoa bilimbi_ has a large number of paired
leaflets, which, on excitation, undergo downward closure. Feeble
stimulus is applied at a point in the petiole, and the transmission of
excitation is visibly manifested by the serial fall of the leaflets.
The distance to which the excitation reaches is a measure of normal
power of conduction. Any variation of conductivity, by the passage of
an electric current in one direction or the other is detected by the
enhancement or diminution of the distance through which excitation is
transmitted. I shall describe the special precautions to be taken in
carrying out this investigation.
[Illustration: FIG. 44.--Diagram of experimental arrangement for
control of transmitted excitation in _Averrhoa bilimbi_. For
explanation see text.]
Electric stimulus of induction shock of definite intensity and
duration is supplied at the middle of the petiole at EE′ (Fig. 44).
The leaflets to the left of E, are not necessary for the purpose of
this experiment and therefore removed. The intensity of the induction
shock may be varied in the usual manner by removing the secondary coil
nearer or farther from the primary. The duration of the shock is always
maintained constant. On application of electric stimulus excitation
is transmitted along the petiole, the distance of transmission
depending on the intensity of stimulus. With feeble stimulus two pairs
of leaflets may undergo an excitatory fall; with stronger stimulus
the transmission is extended to the end of the petiole, and all the
leaflets exhibit movements of closure. We shall now study the modifying
influence of a constant current on conduction of excitation. C is
an electric cell, R the reversing key by which the electric current
could be sent from right to left or in the opposite direction. When
the current is sent from right to the left, the excitatory impulse
initiated at EE′ travels against the direction of the current in an
‘up-hill’ direction. When the current is reversed it flows in the
petiole from left to right and the transmitted impulse travels with the
current or in a ‘down-hill’ direction.
Two complications are introduced on the completion of the electric
circuit of the constant current: the first, is the distributing effect
of leakage of the induction current used for excitation, and second,
the polar variation of excitation induced by the constant current.
_Leakage of induction current._--Before completing the constant current
circuit, the alternating induction current goes only through the path
EE′. On completion of the constant current circuit, the alternating
induction current not only passes through the shorter path EE′ but also
by the circuitous path of the constant current circuit. The escaping
induction current would thus excite all the leaflets directly and
not by its transmitted action. This difficulty is fully overcome by
the interposition of a choking coil which will be described below.
A simpler, though less perfect, device may be employed to reduce and
practically eliminate the leakage. This consists of a loop, L, of
silver wire placed outside EE′. The leakage of induction current is
thus diverted along this path of negligible resistance in preference to
the longer circuit through the entire petiole, which has a resistance
of several million ohms.
_Polar action of current on excitability._--It is well known that an
electric current induces a local depression of excitability at the
point of entrance to the tissue, or at the anode, and an enhancement
of excitability at the point of exit, or at the cathode. But the
excitability is unaffected at a point equally distant from anode
and cathode. This is known as the indifferent point. The exciting
electrodes EE′ are placed at the indifferent point. But when the
current enters on the right side, the terminal leaflets to the right
have their excitability depressed by the proximity of anode, but
the leaflets near the electrodes EE′, being at a distance from the
anode are not affected by it. Moreover it will be shown that the
enhanced conductivity conferred by the directive action of the current
overpowers any depression of excitability in the terminal leaflets due
to the proximity of the anode. I shall, for convenience, designate
the transmission as ‘up-hill’, when excitation is propagated against
the direction of the constant electric current, and ‘down-hill’ when
transmitted with the direction of the current.
_Transmission of excitation ‘Up-hill’: Experiment 38._--I shall give
here an account of an experiment which may be taken as typical. I took
a vigorous specimen of _Averrhoa bilimbi_, and applied a stimulus whose
intensity was so adjusted that the propagated impulse brought about
a fall of only two pairs of leaflets. This gave a measure of normal
conduction without the passage of the current. The constant electric
current was now sent from right to left. A necessary precaution is to
increase the current gradually by means of a suitable potentiometer
slide, to its full value. The reason for this will be given later. The
intensity of the constant current employed was 1.4 micro-ampères. Now
on exciting the petiole by the previous stimulus, the conducting power
was found to be greatly enhanced. The excitatory impulse now reached
the end of the petiole, and caused six pairs of leaflets to fall.
_Transmission of excitation ‘Down-hill’: Experiment 39._--In
continuation of the previous experiment, the constant electric current
was reversed, its directions being now from left to right. Transmission
of excitation was now in a down-hill direction. On applying the
induction shock stimulus of the same intensity as before, the
conducting power of the petiole was found to be abolished, none of the
leaflets exhibiting any sign of excitation. This modification of the
conducting power persists during the passage of the constant current.
On cessation of the current the original conducting power is found
to be restored. It will thus be seen that the power of conduction is
capable of modification, and that the passage of an electric current of
moderate intensity induces enhanced power of conduction in an ‘up-hill’
and diminished conductivity in a ‘down-hill’ direction.
ELECTRIC CONTROL OF NERVOUS IMPULSE IN ANIMALS.
In my ‘Researches on Irritability of Plants’ I have shown how
intimately connected are the various physiological reactions in the
plant and in the animal, and I ventured to predict that the recognition
of this unity of response in plant and animal will lead to further
discoveries in physiology in general. This surmise has been fully
justified, as will be seen in the following experiments carried out on
the nerve-and-muscle preparation of a frog. It is best to carry out
the experiments with vigorous specimens; this ensures success, even in
long continued experiments, which can then be repeated with unfailing
certainty for hours. It is also an advantage to use a large frog for
its relatively great length of the nerve.
_Directive action of current on conduction of excitation in a
nerve-and-muscle preparation: Experiment 40._--A preparation was made
with a length of the spine and two nerves leading to the muscles. The
specimen is supported in a suitable manner, and electric connections
made with the toes, one for the entrance and the other for exit of
the constant current. The current thus entered, say, by the left
toe ascended the muscle and went up the nerve on the left side, and
descended through the other nerve on the right side along the muscle
and thence to the right toe. Before the passage of the constant
electric current the spinal nerve was stimulated by an induction
shock of definite intensity. The nervous impulse was conducted by the
two nerves, one to the left and the other to the right, and caused
a feeble twitch of the respective muscles. A feeble current of 1.5
micro-ampère was sent along the nerve-and-muscle circuit, ascending
by the left and descending by the right side. It will be seen that
excitation initiated at the spine is propagated ‘against’ the electric
current on the left side, and ‘with’ the current on the right side.
On repetition of previous electric stimulus the effect of directive
action of current was at once manifested by the left limb being
thrown into a state of strong tetanic contraction, whereas the right
limb remained quiescent. By changing the direction of the constant
current the induced enhancement of conductivity of the nerve was
quickly transferred from the left to the right side, the depression
or arrest of conduction being simultaneously transferred to the left
side. Turning the reversing key one way or the other brought about
_supra_ or _non_-conducting state of the nerve, and this condition was
maintained throughout the duration of the current.
I shall next describe a more perfect method for obtaining quantitative
results both with plant and animal. In order to demonstrate the
universality of the phenomenon, I next used _Mimosa pudica_ instead of
_Averrhoa_, for experiments on plants.
For determination of normal velocity of transmission of excitation
and the induced variation of that velocity, I employed the automatic
method of recording the velocity of transmission of excitation in
_Mimosa_, where the excitatory fall of the motile leaf gave a signal
for the arrival of the excitation initiated at a distant point. In this
method the responding leaf is attached to a light lever, the writer
being placed at right angles to it. The record is taken on a smoked
glass plate, which during its descent makes an instantaneous electric
contact, in consequence of which a stimulating shock is applied at a
given point of the petiole. A mark in the recording plate indicates
the moment of application of stimulus. After a definite interval the
excitation is conducted to the responding pulvinus, when the excitatory
fall of the leaf pulls the writer suddenly to the left. From the
curve traced in this manner the time-interval between the application
of stimulus and the initiation of response can be found, and the
normal rate of transmission of excitation through a given length of
the conducting tissue deduced. The experiment is then repeated with
an electric current flowing along the petiole with or against the
direction of transmission of excitation. The records thus obtained
enable us to determine the influence of the direction of the current
on the rate of transmission. I shall presently describe the various
difficulties which have to be overcome before the method just indicated
can be rendered practical.
The scope of investigation will be best described according to the
following plan[N]:--
[N] For fuller account _see_ Bose--‘The influence of Homodromous and
Heterodromous Electric Current on Transmission of Excitation in Plant
and Animal.’ Proc. R. S. B., Vol. 88, 1915.