PLANT RESPONSE--ON THE INFLUENCE OF TEMPERATURE
Effect of very low temperature--Influence of high
temperature--Determination of death-point--Increased response as
after-effect of temperature variation--Death of plant and abolition
of response by the action of steam.
For every plant there is a range of temperature most favourable to its
vital activity. Above this optimum, the vital activity diminishes, till
a maximum is reached, when it ceases altogether, and if this point be
maintained for a long time the plant is apt to be killed. Similarly, the
vital activity is diminished if the temperature be lowered below the
optimum, and again, at a minimum point it ceases, while below this
minimum the plant may be killed. We may regard these maximum and minimum
temperatures as the death-points. Some plants can resist these extremes
better than others. Length of exposure, it should however be remembered,
is also a determining factor in the question as to whether or not the
plant shall survive unfavourable conditions of temperature. Thus we have
hardy plants, and plants that are affected by excessive variations of
temperature. Within the characteristic power of the species, there may
be, again, a certain amount of individual difference.
These facts being known, I was anxious to determine whether the
undoubted changes induced by temperature in the vital activity of plants
would affect electrical response.
#Effect of very low temperature.#--As regards the influence of very low
temperature, I had opportunities of studying the question on the sudden
appearance of frost. In the previous week, when the temperature was
about 10° C., I had obtained strong electric response in radishes whose
value varied from ·05 to ·1 volt. But two or three days later, as the
effect of the frost, I found electric response to have practically
disappeared. A few radishes were, however, found somewhat resistant, but
the electric response had, even in these cases, fallen from the average
value of ·075 V. under normal temperature to ·003 V. after the frost.
That is to say, the average sensitiveness had been reduced to about
1/25th. On warming the frost-bitten radish to 20° C. there was an
appreciable revival, as shown by increase in response. In specimens
where the effect of frost had been very great, i.e. in those which
showed little or no electric response, warming did not restore
responsiveness. From this it would appear that frost killed some, which
could not be subsequently revived, whereas others were only reduced to a
condition of torpidity, from which there was revival on warming.
I now tried the effect of artificial lowering of temperature on various
plants. A plant which is very easily affected by cold is a certain
species of Eucharis lily. I first obtained responses with the leaf-stalk
of this lily at the ordinary temperature of the room (17° C.). I then
placed it for fifteen minutes in a cooling chamber, temperature -2° C.,
for only ten minutes, after which, on trying to obtain response, it was
found to have practically disappeared. I now warmed the plant by
immersing it for awhile in water at 20° C., and this produced a revival
of the response (fig. 35). If the plant be subjected to low temperature
for too long a time, there is then no subsequent revival.
[Illustration: FIG. 35.--DIMINUTION OF RESPONSE IN EUCHARIS BY LOWERING
OF TEMPERATURE
(_a_) Normal response at 17° C.
(_b_) The response almost disappears when plant is subjected to -2° C.
for fifteen minutes.
(_c_) Revival of response on warming to 20° C.]
I obtained a similar marked diminution of response with the flower-stalk
of Arum lily, on lowering the temperature to zero.
My next attempt was to compare the sensibility of different plants to
the effect of lowered temperatures. For this purpose I chose three
specimens: (1) Eucharis lily; (2) Ivy; and (3) Holly. I took their
normal response at 17° C., and found that, generally speaking, they
attained a fairly constant value after the third or fourth response.
After taking these records of normal response, I placed the specimens in
an ice-chamber, temperature 0° C., for twenty-four hours, and
afterwards took their records once more at the ordinary temperature of
the room. From these it will be seen that while the responsiveness of
Eucharis lily, known to be susceptible to the effect of cold, had
entirely disappeared, that of the hardier plants, Holly and Ivy, showed
very little change (fig. 36).
Another very curious effect that I have noticed is that when a plant
approaches its death-point by reason of excessively high or low
temperature, not only is its general responsiveness diminished almost to
zero, but even the slight response occasionally becomes reversed.
[Illustration: FIG. 36.--AFTER-EFFECT OF COLD ON IVY, HOLLY, AND
EUCHARIS LILY
_a._ The normal response; _b._ Response after subjection to freezing
temperature for twenty-four hours.]
#Influence of high temperature, and determination of death-point.#--I next
tried to find out whether a rise of temperature produced a depression of
response, and whether the response disappeared at a maximum
temperature--the temperature of death-point. For this purpose I took a
batch of six radishes and obtained from them responses at gradually
increasing temperatures. These specimens were obtained late in the
season, and their electric responsiveness was much lower than those
obtained earlier. The plant, previously kept for five minutes in water
at a definite temperature (say 17° C.), was mounted in the vibration
apparatus and responses observed. The plant was then dismounted, and
replaced in the water-bath at a higher temperature (say 30° C.) again,
for five minutes. A second set of responses was now taken. In this way
observations were made with each specimen till the temperature at which
response almost or altogether ceased was reached. I give below a table
of results obtained with six specimens of radish, from which it would
appear that response begins to be abolished in these cases at
temperatures varying from 53° to 55° C.
TABLE SHOWING EFFECT OF HIGH TEMPERATURE IN
ABOLISHING RESPONSE
Temperature Galvanometric response
(100 dns. = ·07 V.)
(1) {17° C 70 dns.
{53° " 4 "
(2) {17° " 160 "
{53° " 1 "
(3) {17° " 100 "
{50° " 2 "
(4) {17° " 80 "
{55° " 0 "
(5) {17° " 40 "
{60° " 0 "
(6) {17° " 60 "
{55° " 0 "
[Illustration: FIG. 37.--THE GLASS CHAMBER CONTAINING THE PLANT
Amplitude of vibration which determines the intensity of stimulus is
measured by the graduated circle seen to the right. Temperature is
regulated by the electric heating coil R. For experiments on action
of anæsthetics, vapour of chloroform is blown in through the side
tube.]
#Electric heating.#--The experiments just described were, however, rather
troublesome, inasmuch as, in order to produce each variation of
temperature, the specimen had to be taken out of the apparatus, warmed,
and remounted. I therefore introduced a modification by which this
difficulty was obviated. The specimen was now enclosed in a glass
chamber (fig. 37), which also contained a spiral of German-silver wire,
through which electric currents could be sent, for the purpose of
heating the chamber. By varying the intensity of the current, the
temperature could be regulated at will. The specimen chosen for
experiment was the leaf-stalk of celery. It was kept at each given
temperature for ten minutes, and two records were taken during that
time. It was then raised by 10° C., and the same process was repeated.
It will be noticed from the record (fig. 38) that in this particular
case, as the temperature rose from 20° C. to 30° C., there was a marked
diminution of response. At the same time, in this case at least,
recovery was quicker. At 20° C., for example, the response was 21 dns.,
and the recovery was not complete in the course of a minute. At 30° C.,
however, the response had been reduced to 7·5 divisions, but there was
almost complete recovery in twelve seconds. As the temperature was
gradually increased, a continuous decrease of response occurred. This
diminution of response with increased temperature appears to be
universal, but the quickening of recovery may be true of individual
cases only.
[Illustration: FIG. 38.--EFFECT OF TEMPERATURE ON RESPONSE
The response was abolished at the hot-water temperature of 55° C.]
TABLE SHOWING DIMINUTION OF RESPONSE WITH INCREASING TEMPERATURE
(·01 Volt = 35 divisions)
Temperature Response
20° 21
30° 7·5
40° 5·5
50° 4
65° 3
In radishes response disappeared completely at 55° C., but with celery,
heated in the manner described, I could not obtain its entire abolition
at 60° C. or even higher. A noticeable circumstance, however, was the
prolongation of the period of recovery at these high temperatures. I
soon understood the reason of this apparent anomaly. The method adopted
in the present case was that of dry heating, whereas the previous
experiments had been carried on by the use of hot water. It is well
known that one can stand a temperature of 100° C. without ill effects in
the hot-air chamber of a Turkish bath, while immersion in water at
100° C. would be fatal.
In order to find out whether subjection to hot water would kill the
celery-stalk, I took it out and placed it for five minutes in water at
55° C. This, as will be seen from the record taken afterwards,
effectively killed the plant (fig. 38, w).
[Illustration: FIG. 39.--EFFECT OF RISING AND FALLING TEMPERATURE ON THE
RESPONSE OF SCOTCH KALE]
#Increased sensitiveness as after-effect of temperature variation.#--A
very curious effect of temperature variation is the marked increase of
sensitiveness which often appears as its after-effect. I noticed this
first in a series of observations where records were taken during the
rise of temperature and continued while the temperature was falling
(fig. 39). The temperature was adjusted by electric heating. It was
found that the responses were markedly enhanced during cooling, as
compared with responses given at the same temperatures while warming
(see table). Temperature variation thus seems to have a stimulating
effect on response, by increasing molecular mobility in some way. The
second record (fig. 40) shows the variation of response in Eucharis lily
(1) during the rise, and (2) during the fall of temperature. Fig. 41
gives a curve of variation of response during the rise and fall of
temperature.
TABLE SHOWING THE VARIATION OF RESPONSE IN SCOTCH KALE DURING THE RISE
AND FALL OF TEMPERATURE
Temperature Response Response
[Temperature rising] [Temperature falling]
19° C. 47 dns. --
25° " 24 " -- ^
30° " ¦ 11 " 23 dns. ¦
50° " ¦ 8 " 16 " ¦
70° " v 7 " --
----->
[Illustration: FIG. 40.--RECORDS OF RESPONSES IN EUCHARIS LILY DURING
RISE AND FALL OF TEMPERATURE
Stimulus constant, applied at intervals of one minute. The temperature
of plant-chamber gradually rose on starting current in the heating
coil; on breaking current, the temperature fell gradually.
Temperature corresponding to each record is given below.
Temperature rising: (1) 20°, (2) 20°, (3) 22°, (4) 38°, (5) 53°,
(6) 68°, (7) 65°.
Temperature falling: (8) 60°, (9) 51°, (10) 45°, (11) 40°, (12) 38°.]
#Point of temperature maximum.#--We have seen how, in cases of lowered
temperature, response is abolished earlier in plants like Eucharis,
which are affected by cold, than in the hardier plants such as Holly and
Ivy. Plants again are unequally affected as regards the upper range. In
the case of Scotch kale, for instance, response disappears after ten
minutes of water temperature of about 55° C., but with Eucharis fairly
marked response can still be obtained after such immersion and does not
disappear till it has been subjected for ten minutes to hot water, at a
temperature of 65° C. or even higher. The reason of this great power of
resistance to heat is probably found in the fact that the Eucharis is a
tropical plant, and is grown, in this country, in hot-houses where a
comparatively high temperature is maintained.
[Illustration: FIG. 41.--CURVE SHOWING VARIATION OF RESPONSE IN EUCHARIS
WITH THE RISE AND FALL OF TEMPERATURE]
#The effect of steam.#--I next wished to obtain a continuous record by
which the effects of suddenly increased temperatures, culminating in the
death of the plant, might be made evident. For this purpose I mounted
the plant in the glass chamber, into which steam could be introduced. I
had chosen a specimen which gave regular response. On the introduction
of steam, with the consequent sudden increase of temperature, there was
a transitory augmentation of excitability. But this quickly disappeared,
and in five minutes the plant was effectively killed, as will be seen
graphically illustrated in the record (fig. 42).
[Illustration: FIG. 42.--EFFECT OF STEAM IN KILLING RESPONSE
The two records to the left exhibit normal response at 17° C. Sudden
warming by steam produced at first an increase of response, but five
minutes exposure to steam killed the plant (carrot) and abolished
the response.
Vibrational stimulus of 30° applied at intervals of one minute; vertical
line = ·1 volt.]
It will thus be seen that those modifications of vital activity which
are produced in plants by temperature variation can be very accurately
gauged by electric response. Indeed it may be said that there is no
other method by which the moment of cessation of vitality can be so
satisfactorily distinguished. Ordinarily, we are able to judge that a
plant has died, only after various indirect effects of death, such as
withering, have begun to appear. But in the electric response we have an
immediate indication of the arrest of vitality, and we are thereby
enabled to determine the death-point, which it is impossible to do by
any other means.
It may be mentioned here that the explanation suggested by Kunkel, of
the response being due to movement of water in the plant, is inadequate.
For in that case we should expect a definite stimulation to be under all
conditions followed by a definite electric response, whose intensity and
sign should remain invariable. But we find, instead, the response to be
profoundly modified by any influence which affects the vitality of the
plant. For instance, the response is at its maximum at an optimum
temperature, a rise of a few degrees producing a profound depression;
the response disappears at the maximum and minimum temperatures, and is
revived when brought back to the optimum. Anæsthetics and poisons
abolish the response. Again, we have the response undergoing an actual
reversal when the tissue is stale. All these facts show that mere
movement of water could not be the effective cause of plant response.