# Researches on Irritability of Plants/Chapter 8

CHAPTER VIII
DEATH-SPASM IN PLANTS
A plant may be killed by subjecting it to a certain maximum temperature. The exact moment at which death is initiated is difficult to determine, since there has been found no certain and immediate criterion of death. One method by which the occurrence of death may be determined is by the abolition of that electric response which is characteristic of the living condition. A plant as long as it is alive gives in answer to a stimulus an electric response of galvanometric negativity. On the occurrence of death this particular response disappears. I find that the electric response is abolished when the plant has been subjected for a time to a temperature of about 60° C.
If the plant is subjected to a gradual rise of temperature, there would arrive a time when the death-change will begin to occur. In the animal an early symptom of death is the setting in of rigor mortis. We shall find that in plants also a death-spasm, analogous to the death-throe of the animal, occurs at a critical moment.
In order to obtain an automatic record which would ​indicate the beginning of death-change, I first took a specimen of Mimosa and subjected it to a gradual rise of temperature in a water-bath. The leaf was attached to the recording-lever in the usual manner. The recording apparatus employed was of the oscillating type, where the plate oscillates to and fro by an electro-magnetic contrivance, thus producing a series of dots in the response-curve. In the present investigation the electro-magnetic circuit is completed for a brief period, at every degree rise of temperature in the bath. Successive dots thus represent intervals of temperature of 1° C. The ordinate of the curve indicates expansive or contractile movement of the leaf: down-curve representing the expansion, and up-curve the contraction.
The temperature of the bath is continuously raised by the application of gas or spirit flame. For certain reasons, to be presently explained, it is necessary to raise the temperature gradually and continuously, without any sudden variation. There should also be no mechanical disturbance of water in the bath during heating, as that would disturb the leaf and vitiate the record. These difficulties are overcome by constructing the heating-bath of two vessels, one placed within the other. Heating the water of the outer vessel raises the temperature of the water in the inner in a very even manner, and without any mechanical disturbance.
It is necessary to subject the plant to gradual rise of temperature in order to protect it from excitation. Any sudden variation, due either to lowering or raising of temperature, causes excitatory movement of the leaf. This is seen in the following records (fig. 57), obtained with Mimosa. The first response is of excitation due to application of a drop of ice-cold water on the pulvinus; the second response is due to the very opposite treatment of application of a drop of hot water. In both cases we obtain the excitatory fall of the leaf.
The effect of temperature as such is, however, very definite: gradual rise of temperature inducing progressive ​erection of the leaf; gradual lowering of temperature, on the other hand, inducing progressive depression of the leaf. Thus the effect of temperature, as such, is expansion with rise and contraction with fall. These opposite effects of erection and fall are progressive and slow. Excitatory reaction, on the other hand, is sudden and always attended by the contractile fall of the leaf.
The Mimosa used for experiment may be an entire plant; or, if more convenient, a cut branch containing a leaf may be employed. The result obtained is the same in both cases. It is found that during continuous rise of
Fig. 57.—Excitatory response of Mimosa induced by sudden application of either cold water (C) or hot water (H).
temperature the leaf is erected till it reaches a critical temperature at which the expansion is converted into a spasmodic excitatory contraction. The curve is thus v-shaped, the turning-point of the thermo-mechanical curve being very sharp and definite. Under constant conditions, the critical point of inversion is also very definite. The sudden inversion marks the initiation of the death-change.
Here it is necessary to bear in mind certain conditions for the securing of definite results. It is obvious that death will ensue if a plant be placed in an unfavourable environment as regards temperature for a prolonged period. But as such a temperature would only cause the death of the plant by indirect and cumulative action, it cannot be said to ​constitute the death-point. For precision in such a determination it is necessary to discover a temperature which is of itself efficient to initiate an abrupt death-change. On the other hand, there must be a certain latent period after the expiration of which the change would be outwardly manifested. An interval will elapse, moreover, during which the tissue is attaining the temperature of the bath. If the rate of rise of temperature be too rapid, then, owing to the lag caused by the two factors, by the time the death-spasm commences the recorded temperature may have gone beyond the actual death-point.
There are thus two points which are somewhat antagonistic. In the first place, in order to obtain the immediate point of death it is necessary that the plant should undergo an exposure which is not too prolonged. Nevertheless, to make due allowance for the latent period and for attainment of the surrounding temperature, the rate of rise of temperature must be gradual. In the case of tissues which are not too thick, the latter condition is sufficiently fulfilled by a rate of rise of 1° C. per minute. For the precise determination of the death-point the rate of rise of temperature must be specified. It must also be borne in mind that after the initiation of the death-change a certain time must elapse before the whole mass of tissue in the interior is killed. With a thick mass of tissue, owing to its inefficient thermal conductivity, the attainment of the surrounding temperature and occurrence of death throughout the tissue will be a protracted process.
The definite rate of rise of temperature may be simply secured by moving the heating flame nearer to, or further from, the bath. With thin organs, such as the pulvinus of Mimosa, I find that a spasmodic contraction takes place at or very near 60° C., when the rate of rise of temperature is approximately 1° C. per minute. This is seen in fig. 58; the record was commenced at 25° C., and the successive dots in the record are at intervals of 1° C. The down-curve indicates the expansive erection of leaf. As soon as the ​temperature had reached 60° C. there was an abrupt inversion, and the spasmodic contraction tool»: place at a very rapid rate. The successive dots in the rip-portion of the curve are at intervals of .2 of a. degree. The point of inversion, as we shall see, indicates the death-point, and the curve giving the death-record we shall call the death-curve. It should be remembered that the particular
electric response characteristic of living condition of the tissue is found to disappear after the tissue had been subjected to the temperature of 60° C.
If the sudden contraction that takes place at 60° C. should prove to be the death-spasm, then this should be the last response given by the plants. If we raise the temperature of the plant short of the dearth-point, say to 45° C., we get a continuous responsive expansion; when cooled the leaf recovers, to a greater or less extent, its ​former position. If the temperature be raised again, there is once more a growing erection, and when the death-point is reached there is a sudden spasmodic contraction.
But if the specimen once passes through the temperature at which the spasm takes place, then there should be an abolition of all further response, proving the sudden contraction at 60° C. to be the spasm of death. Thus, after obtaining the sudden inversion of the curve at 60° C. in the last experiment, the plant was kept at that temperature for 15 minutes. Cold water was now substituted in the bath, and the record was taken once more of the effect of rise
Fig. 59.—Abolition of response to warming or cooling in specimen which had passed the death-point.
and fall of temperature. A record is reproduced (fig. 59) which exhibits the result. In the lower curve is shown the record of effect of rise of temperature from 45° to 65° C., and in the upper the effect of cooling from 60° to 45° C. It is seen that while in the last experiment the plant exhibited a spasmodic contraction at 60° C., there is no trace of such an effect in the present case. The very slight movement observable in the two curves is the physical effect of heating and cooling, quite negligible compared with the physiological erectile movement due to warming and the subsequent spasmodic contractile movement heralding the initiation of death-change.
​In order to discover how constant is the death-point, I repeated the experiment with numerous other specimens. We have seen that the pulvinated organs present in the leaves of Desmodium gyrans and the bean plant (Vicia Faba) exhibit responsive movement under excitation. In fig. 60 is depicted the death record taken under standard conditions with the leaf of Desmodium. The record was commenced at 35° C.; it is seen that thermo-mechanical inversion took place at 61° C.
Fig. 60.—Record showing death-point of Desmodium at 61° C.
The next figure (fig. 61) shows the record with the leaf of bean plant. Here the responsive movements are very large. The inversion is seen to take place at 60° C.
The occurrence of death-spasm may also be shown by means of ordinary plants. If we take a hollow tubular organ, such as the hollow leaf-stalk of gourd or hollow flower-stalk of any other plant, cut it in the form of a spiral and subject it to the rising temperature of the bath, there is noticed at first an expansive movement of uncurling of spiral. On reaching the death-point, however, the former movement is suddenly reversed to one of curling.
Flowers like French marigold exhibit death-spasm by sudden movement of opening or closure.
By employing the electric mode of investigation I ​found that there is induced an electric-spasm at the onset of death. When the temperature is rising, a given point of the plant-tissue exhibits increasing galvanometric positivity, till at the critical temperature there is a sudden electric inversion into galvanometric negativity. The
electric-curve of death is of the same type as the thermo-mechanical curve. With specimens of Musa and Amaranth the death-point was found to be 59.5° C.
As the death-spasm is a form of physiological response we should expect the curve of death to undergo modification under physiological variation. One such modification would lie in the translocation of the point of inversion, or the displacement of the death-point. Thus age has some influence, the death-point of very young specimens being lower than that of mature ones. I shall demonstrate ​the influences of other agencies, such as fatigue or poisonous drugs, in the displacement of the death-point.
I have already given a record which showed the death-point of the leaf of bean plant to be 60° C. under normal conditions. Employing a similar specimen, fatigue was induced in it by means of tetanising electric-shocks; the death record was then taken in the usual manner. It
| Fig. 62.—Lowering of death-point under fatigue; death-spasm took place at 37° C. | Fig. 63.—Effect of poison in lowering the death-point. |
will be seen (fig. 62) that in this particular case, on account of fatigue, the death-point was lowered from the normal 60° C. to 37° C., that is to say, by as much as 23° C. The lowering of the death-point, I find, is determined by the extent of fatigue.
In order to discover the effect of poisonous solutions on the death-point, I subjected a specimen of the bean leaf to dilute copper-sulphate solution and took its thermo-mechanioal record (fig. 63). The effect of the poisonous agent is clearly demonstrated by an appropriate lowering of the death-point, in this case from the normal 60° C. to 42° C., or by 18° C.
Summary
The electric response of galvanometric negativity is characteristic of the living condition of the vegetable tissue. Dead plants do not exhibit this characteristic electric-response.
When a plant is subjected for a time to a temperature of 60° C. its electric response disappears, such abolition being indicative of the death of the plant.
A leaf of Mimosa subjected to abrupt variation of temperature—either sudden cooling or sudden warming—exhibits excitatory reaction. But if the temperature be gradually raised, there is a progressive erectile movement of the leaf; gradual cooling induces a depression of the leaf.
When the leaf of Mimosa is continuously raised in temperature, then at a critical point the erectile expansive movement is suddenly converted into one of spasmodic contraction. This inversion takes place under standard conditions at or about 60° C. After this the response of the plant is permanently abolished.
Various other plants, sensitive and ordinary, exhibit this characteristic death-spasm at or about 60° C.
In taking an electric record it is found that an electric-spasm also takes place at the critical temperature, which is very near 60° C.
The death-point of the plant is lowered under physiolological depression. Thus under fatigue induced by tetanising electric-shocks, the death-point was lowered from the normal 60° C. to 37° C.
Poisonous reagents also lower the death-point. In a particular case poisonous solution of copper sulphate lowered the death-point by 18° C.