Researches on Irritability of Plants

exhibit the after-effect on the removal of the gas. It will thus be understood how important it is to maintain the external conditions constant for so long a period as two hours. The method of maintaining the testing stimulus constant has already been explained. With special care the temperature of the plant-chamber can also be kept uniform. The other factor which is liable to variation is the intensity of light. I have often noticed a fluctuation

Fic. 44.—Effect of sudden darkness on excitability of Mimosa. First three responses, normal; four succeeding responses due to effect of darkness. Line below indicates period of darkness. Vibration frequency of writer, five times per second. in the uniformity of responses which was traceable to a passing cloud. I soon found that a sudden change in the intensity of light induces a marked variation of motile excitability in Mimosa. Thus on bringing a highly sensi- tive plant to a dark room its excitability is found to dis- appear. This abolition of excitability is generally speaking temporary, since the plant often regains its sensitiveness after about an hour, though still kept in the dark. The fact that under normal conditions it is the sudden diminu- tion of light rather than darkness that induces depression

of excitability, is borne out by the fact that the plant is fully sensitive at night. In order to demonstrate the variation of excitability induced by sudden diminution of light, I first took a set of three normal responses in diffuse daylight. The plant- chamber was then suddenly darkened by means of an opaque screen. It will be noticed (fig. 44) that the next two responses were nearly abolished; the excitability of the plant was however beginning to be restored after 45 minutes’ exposure to darkness. After an hour in darkness the excitability was fully restored, the response here being even larger than in light.

In order to guard against the disturbing effect of varia- tion of light it is advisable to carry out the following experiments in an open veranda, the plant being kept in a chamber with frames of ground glass. In this way the plant is maintained under diffuse light of fairly uniform intensity. Another peculiarity I noticed in Mimosa was a depression of excitability on rainy days. This effect I was afterwards able to trace to the absorption of water by the pulvinus. The variation of motile excitability by absorption of water is very clearly exhibited in the accompanying record (fig. 45). A pair of normal uniform responses were first taken. A drop of water was then applied on the pulvinus, when the leaf was recovering from the second stimulus. It will be noticed that the period of recovery became very much protracted in consequence of absorption of water. The usual time for complete recovery is about 15 minutes. In the present case it was prolonged to 45 minutes. Testing stimuli were applied at the usual intervals of 15 minutes, the moments of application being represented by thick

dots. It will be seen that there is an abolition of excitability, stimuli which were formerly effective becoming now quite ineffective. I next tried to find out whether it were possible to restore the lost excitability by artificial means. Guided by the consideration that glycerin has the power of abstracting water, I applied a drop of strong glycerin to the pulvinus. It will be noted that this had the effect of quickly restoring the motile excitability of the pulvinus. The two responses

Fic. 45.—Abolition of motile excitability of pulvinus by absorp- tion of water. Note prolongation of period of recovery and ineffectiveness of stimuli applied at moments marked with thick dots. Subsequent restoration of excitability by applica- tion of glycerin. after the application of glycerin are practically similar to the normal responses at the beginning of the series. Iam unable to say whether the restoration of excitability was here due entirely to the abstraction of water. One might think that continuous abstraction of water would induce a continuous variation of excitability—probably an enhance- ment reaching a maximum followed by a decline. I find, however, that the application of glycerin restores the normal excitability, and that generally speaking this remains constant even under the continued action of the reagent. This is a fortunate circumstance for those particular investi-

gations where it is required to make an electrolytic contact with the pulvinus without inducing any change in its motile excitability. I shall now proceed to describe the effects of various gases and vapours on the excitability of Mimosa. The plant is enclosed in a small glass chamber, the different gases being made to stream in and out through entrance and exit tubes. The various effects induced may be classi- fied as (1) stimulating, (2) depressing, and (3) toxic. The exaltation of excitability induced by stimulating agents is exhibited by the enhancement of amplitude of response. The effect of depressing agents is seen in the diminution of amplitude of response ; in this class may be included agents which have slight narcotic action. In all these cases the removal of the gas is attended by the restoration of normal excitability of the plant. A curious fact noticeable in this connection is the phenomenon of accommodation. Under the action of a slightly depressing agent, there is induced a diminution of excitability. But the plant may accommo- date itself to the change, in consequence of which the excitability is more or less restored to the original condition. It should also be borne in mind that the character of the reaction is modified to a certain extent by the tonic con- dition of the plant, a plant in a vigorous condition being better able to withstand unfavourable circumstances than one in a weak condition.

Lastly, there are gaseous agents which are toxic in their action; their application is attended by rapid loss of excitability and death of the plant. I will now describe in detail the effects of various gases, beginning with those which stimulate and ending with others which cause the death of the plant. The stimulating effect of this gas is clearly seen in fig. 46. The particular leaf, before the application of ozone, was showing signs of fatigue, as evidenced by the gradual diminution of the heights of successive responses.

The introduction of ozone brought, however, an immediate change ; the induced enhancement of excitability is seen in the growing amplitude of successive responses till a limit was reached. The effect of undiluted carbonic-acid gas is a depression of excitability. This is seen in the present record (fig. 47), where on the application of this gas the amplitudes of suc- cessive responses are seen to undergo a decline. Another noticeable fact is the incompleteness of recovery after each

excitation. The plant-chamber was next refilled with fresh air, and we observe the restoration of normal excitability. The immediate effect of dilute vapour of alcohol is sometimes a transient enhancement of excitability. But continued action of the vapour induces a depression. In the accompanying record (fig. 48) there was little immediate effect ; but after an application of 15 minutes there was induced a depression of response; another effect also

noticeable is the alternating character of the response that took place after the application of alcohol. The vapour of ether induces a depression of excitability as seen in the diminution of amplitude of response. The first effect of dilute ether-vapour is often a short-lived exaltation. The narcotic effect of this agent on Mimosa is feeble compared with that induced by chloroform. The depressing effect of ether passes off on readmission of fresh air (fig. 40).

The effect of vapour of carbon disulphide is similar to that of ether. It induces a depression during the introduc- tion of vapour into the plant-chamber ; the induced depres- sion, however, passes off on restoration by means of fresh air (fig. 50). Contrary to my anticipation, coal gas proved to be but moderately depressing in its action. I have kept the plant surrounded by this gas for more than two hours without the abolition of its excitability. The effect of the gas is very different when it contains impurities such as sulphuretted hydrogen. I give a record (fig. 51) which

Fic. 51.—Effect of coal gas: note irregularity of response after introduction. exhibits the depressing effect of coal gas, and the gradual restoration of normal excitability on admission of fresh air. The vapour of chloroform acts as a very strong narcotic. In the record here given (fig. 52) the response became very much reduced immediately after application; the power of recovery was also abolished. Subsequent application of stimulus did not result in any sign of response. Even on blowing off the vapour there was no restoration of excitability for a very consider- able period. In the present case the period of total insen- sibility lasted for six hours, after which the excitability was slowly restored.

The vapour of ammonia is found to cause an abolition of Chloroform excitability in a very short time. On the introduction of ammonia there is produced an excitatory fall. This may be avoided, however, by introducing this vapour immediately after the excitation induced by the testing stimulus. In the record here given (fig. 53), the first two are the normal responses. Introduction of ammonia is seen to induce an abolition of excitability, three successive stimula- tions, represented by thick dots, at the usual intervals of 15 minutes proving to be quite ineffective. On blowing off the vapour the excitability is seen to be very gradually restored. If stronger vapour of ammonia be employed, then the loss of excitability lasts for several hours.

The effect of this gas is not merely depressing but extremely toxic. It can be seen from the record that the introduction of this gas caused the period of recovery to be very protracted. The abolition of excitability is evidenced by the fact that successive stimulations at the usual interval of 15 minutes, proved to be quite ineffective (fig. 54). The action of this gas was so poisonous that restoration of fresh air did not bring about any revival. The plant was subse-

Fic. 53.—Abolition of excitability under the action of ammonia. Fic. 54.—Total abolition of excitability and death of plant under the action of sulphuretted hydrogen, quently found to have died under the poisonous effect of this gas. dioxide, on the other hand, is extremely poisonous. Intro- duction of this gas was attended by an immediate excitatory fall, which was repeated twice. After this the plant became perfectly insensitive (fig. 55) ; the gas had in reality killed it.

Equally fatal is the effect of sulphur dioxide. Intro- duction of the gas was attended by an immediate excitatory Fic.55.—Toxic effect of Fic. 56.—Abolition of ex- nitrogen dioxide : applica- citability and death of tion of gas at x induced plant by the action of excitation, followed by loss sulphur dioxide. of excitability and death of plant. fall of the leaf, after which it became quite insensitive (fig. 56). Restoration of fresh air did not revive the plant, which succumbed completely to the toxic action of the gas.

There is in general a temporary depression of excitability in Mimosa under sudden diminution of intensity of light. Absorption of water induces a depression or abolition of the motile excitability of pulvinus. Excitability is restored under application of glycerin. Carbonic-acid gas and vapour of alcohol induce a moderate depression of excitability, which is fully restored on admission of fresh air. Depression of excitability is also induced under the action of coal gas, and vapour of carbon disulphide.

The vapour of ether exerts a moderate narcotic action. The effect of vapour of chloroform is very pronounced, loss of excitability under its action being prolonged. Sulphuretted hydrogen, nitrogen dioxide, and sulphur dioxide abolish the excitability and bring about the death of the plant. Criterion of the death of plant—Abolition of electric response at death— Mechanical spasm of death—Water-bath for uniform rise of tempera- ture—Excitatory effect of sudden cooling or heating—Erection of leaf with rising, and depression of leaf with falling, temperature— Thermo-mechanical inversion at the death-point — Necessity for specification of rate of rise of temperature—Death record of Mimosa— Abolition of response after death-spasm—Constancy of death-point exhibited by different specimens—Death records of Desmodium gyvansand Vicia Fava—Death-spasm in ordinary plants—The electric-

spasm of death—Lowering of death-point by fatigue and by poisonous solution. A PLANT may be killed by subjecting it to a certain maxi- mum 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 characteris- tic of the living condition. A plant as long as it is alive gives in answer to a stimulus an electric response of galvano- metric negativity. On the occurrence of death this parti- cular 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 vigor mortis. We shall find that in plants also a death-spasm, analogous to the death-throe of the animal, occurs at a critical moment. 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 tempera- ture 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 tem- perature gradually and continuously, without any sudden variation. There should also be no mechanical disturb- ance 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 1aising of tem- perature, 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; gvadual 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 Fic. 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 environ- ment 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 deter- mination 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 mani- fested. 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 anta- gonistic. 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 tempera- ture must be gradual. In the case of tissues which are not too thick, the latter condition is sufficiently fulfilled by arate of rise of 1°C. per minute. For the precise determina- tion 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 took place at a very rapid rate. The successive dots in the up-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 Fic. 58.—Death-curve of Mimosa. Successive dots in down or expansive part of curve represent rise of temperature of 1° C. Spasmodic contraction causing inversion of curve takes place at 60° C.

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 death-point, say to 45° C., we get a continuous responsive expansion ; when cooled the leaf recovers, to a greater or less extent, its