PLANT RESPONSE--ON THE RELATION BETWEEN STIMULUS AND RESPONSE
Increased response with increasing stimulus--Apparent diminution of
response with excessively strong stimulus.
As already said, in the living tissue, molecular disturbance induced by
stimulus is accompanied by an electric disturbance, which gradually
disappears with the return of the disturbed molecules to their position
of equilibrium. The greater the molecular distortion produced by the
stimulus, the greater is the electric variation produced. The electric
response is thus an outward expression of a molecular disturbance
produced by an external agency, the stimulus.
#Curve of relation between stimulus and response.#--In the curve showing
the relation between stimulus and response in nerve and muscle, it is
found that the molecular effect as exhibited either by contraction or
E.M. variation in muscle, or simply by E.M. variation in nerve, is at
first slight. In the second part, there is a rapidly increasing effect
with increased stimulus. Finally, a tendency shows itself to approach a
limit of response. Thus we find the curve at first slightly convex, then
straight and ascending, and lastly, concave to the abscissa (fig. 30).
In muscle the limit of response is reached much sooner than in nerve. As
will be seen, the range of variation of stimulus in these curves is not
very great. When the stimulus is carried beyond moderate limits, the
response, owing to fatigue or other causes, may sometimes undergo an
actual diminution.
[Illustration: FIG. 30.--CURVES SHOWING THE RELATION BETWEEN THE
INTENSITY OF STIMULUS AND RESPONSE
Abscissæ indicate increasing intensity of stimulus. Ordinates indicate
magnitude of response. (Waller.)]
I have obtained very interesting results, with reference to the relation
between stimulus and response, when experimenting with plants. These
results are suggestive of various types of response met with in animal
tissues.
1. In order to obtain the simplest type of effects, not complicated by
secondary phenomena, one has to choose specimens which exhibit little
fatigue. Having procured these, I undertook two series of experiments.
In the first (_A_) the stimulus was applied by means of the
spring-tapper, and in the second (_B_) by torsional vibration.
[Illustration: FIG. 31
Taps of increasing strength 1:2:3:4 producing increased response in leaf
stalk of turnip.]
(_A_) The first stimulus was given by a fall of the lever through _h_,
the second through 2 _h_, and so on. The response-curves clearly show
increasing effect with increased stimulus (fig. 31).
[Illustration: FIG. 32.--INCREASED RESPONSE WITH INCREASING VIBRATIONAL
STIMULI (CAULIFLOWER-STALK)
Stimuli applied at intervals of three minutes. Vertical line = ·1 volt.]
(_B_) 1. The vibrational stimulus was increased from 2·5° to 5° to 7·5°
to 10° to 12·5° in amplitude. It will be observed how the intensity of
response tends to approach a limit (fig. 32).
TABLE SHOWING THE INCREASED E.M. VARIATION
PRODUCED BY INCREASING STIMULUS
+----------------------------------------+
| Angle of Vibration | E.M.F |
+----------------------------------------+
| 2·5° | ·044 volt |
| 5° | ·075 volt |
| 7·5° | ·090 volt |
| 10° | ·100 volt |
| 12·5° | ·106 volt |
+----------------------+-----------------+
2. The next figure shows how little variation is produced with low value
of stimulus, but with increasing stimulus the response undergoes a rapid
increase, after which it tends to approach a limit (fig. 33, _a_).
[Illustration: FIG. 33.--RESPONSES TO INCREASING STIMULI PRODUCED BY
INCREASING ANGLE OF VIBRATION
(_a_) Record with a specimen of fresh radish. Stimuli applied at
intervals of two minutes. The record is taken for one minute.
(_b_) Record for stale radish. There is a reversed response for the
feeble stimulus of 5° vibration.]
3. As an extreme instance of the case just cited, I have often come
across a curious phenomenon. During the gradual increase of the stimulus
from a low value there would be apparently no response. But when a
critical value was reached a maximum response would suddenly occur, and
would not be exceeded when the stimulus was further increased. Here we
have a parallel to what is known in animal physiology as the 'all or
none' principle. With the cardiac muscle, for example, there is a
certain minimal intensity which is effective in producing response, but
further increase of stimulus produces no increase in response.
4. From an inspection of the records of responses which are given, it
will be seen that the slope of a curve which shows the relation of
stimulus to response will at first be slight, the curve will then ascend
rapidly, and at high values of stimulus tend to become horizontal. The
curve as a whole becomes, first slightly convex to the abscissa, then
straight and ascending, and lastly concave. A far more pronounced
convexity in the first part is shown in some cases, especially when the
specimen is stale. This is due to the fact that under these
circumstances response is apt to begin with an actual reversal of sign,
the plant under feebler than a certain critical intensity of stimulus
giving positive, instead of the normal negative, response (fig. 33,
_b_).
#Diminution of response with excessively strong stimulus.#--It is found
that in animal tissues there is sometimes an actual diminution of
response with excessive increase of stimulus. Thus Waller finds, in
working with retina, that as the intensity of light stimulus is
gradually increased, the response at first increases, and then sometimes
undergoes a diminution. This phenomenon is unfortunately complicated by
fatigue, itself regarded as obscure. It is therefore difficult to say
whether the diminution of response is due to fatigue or to some
reversing action of an excessively strong stimulus.
From fig. 33, _b_, above, it is seen that there was an actual reversal
of response in the lower portion of the curve. It is therefore not
improbable that there may be more than one point of reversal.
In physical phenomena we are, however, acquainted with numerous
instances of reversals. For example, a common effect of magnetisation
is to produce an elongation of an iron rod. But Bidwell finds that as
the magnetising force is pushed to an extreme, at a certain point
elongation ceases and is succeeded, with further increase of magnetising
force, by an actual contraction. Again a photographic plate, when
exposed continuously to light, gives at first a negative image. Still
longer exposure produces a positive. Then again we have a negative.
There is thus produced a series of recurrent reversals. In photographic
prints of flashes of lightning, two kinds of images are observed, one,
the positive--when the lightning discharge is moderately intense--and
the other, negative, the so-called 'dark lightning'--due to the reversal
action of an intensely strong discharge.
In studying the changes of conductivity produced in metallic particles
by the stimulus of Hertzian radiation, I have often noticed that whereas
feeble radiation produces one effect, strong radiation produces the
opposite. Again, under the continuous action of electric radiation, I
have frequently found recurrent reversals.[13]
#Diminution of response under strong stimulus traced to fatigue.#--But
there are instances in plant response where the diminution effect can be
definitely traced to fatigue. The records of these cases are extremely
suggestive as to the manner in which the diminution is brought about.
The accompanying figures (fig. 34) give records of responses to
increasing stimulus. They were made with specimens of cauliflower-stalks,
one of which (_a_) showed little fatigue, while in the other (_b_)
fatigue was present. It will be seen that the curves obtained by joining
the apices of the successive single responses are very similar.
[Illustration: FIG. 34.--RESPONSES TO INCREASING STIMULUS OBTAINED WITH
TWO SPECIMENS OF STALK OF CAULIFLOWER
In (_a_) fatigue is absent, in (_b_) it is present.]
In one case there is no fatigue, the recovery from each stimulus being
complete. Every response in the series therefore starts from a position
of perfect equilibrium, and the height of the single responses increases
with increasing stimulation. But in the second case, the strain is not
completely removed after any single stimulation of the series. That
recovery is partial is seen by the gradual shifting of the base line
upwards. In the former case the base line is horizontal and represents a
condition of complete equilibrium. Now, however, the base line, or line
of modified equilibrium, is tilted upwards. Thus even in this case if we
measure the heights of successive responses from the line of absolute
equilibrium, they will be found to increase with increasing stimulus.
Ordinarily, however, we make no allowance for the shifting of the base
line, measuring response rather from the place of its previous recovery,
or from the point of modified equilibrium. Judged in this way, the
responses undergo an apparent diminution.
FOOTNOTES:
[13] See 'On Electric Touch,' _Proc. Roy. Soc._ Aug. 1900.