Researches on Irritability of Plants/Chapter 7
CHAPTER VII
EFFECTS OF DIFFERENT GASES ON EXCITABILITY OF MIMOSA
In order to investigate the effects of various gases in modifying the excitability of Mimosa, a series of responses, more or less uniform, is first obtained under uniform stimuli, at intervals of 15 minutes. The given gas is now introduced into the plant-chamber, and another series of responses are once more obtained by the action of the same stimuli as before. The variation of amplitude of responses then gives an indication of the excitatory or depressing action of the agent.
In carrying out the experimental investigation in this manner, we proceed on the assumption that the stimuli applied are invariable, and that the external conditions are maintained constant, with the sole exception of the change induced by the introduction of the given gas. In order to complete a single investigation a period of nearly two hours is often necessary, which is the time required to take eight responses at intervals of 15 minutes. Of these, the first two give the normal responses, the next four the modified responses under the influence of the gas, and the last two 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
Fig. 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 sensitive plant to a dark room its excitability is found to disappear. 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 diminuntion of light rather than darkness that induces depression of excitability, is borne out by the fact that the plant is fully sensitive at night.
Effect of Sudden Darkness
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 variation 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.
Effect of Absorption of Water
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
after the application of glycerin are practically similar to the normal responses at the beginning of the series. I am 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 enhancement 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 investigations 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 classified 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 accommodate 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 condition 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.
Ozone
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.
Fig. 46.—Stimulating action of ozone.
Fig. 47.—Effect of carbonic-acid gas.
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.
Carbonic-acid Gas
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 successive responses are seen to undergo a decline. Another noticeable fact is the incompleteness of recovery after each
Fig. 48.—Effect of vapour of alcohol; note alternating character of response after application.
excitation. The plant-chamber was next refilled with fresh air, and we observe the restoration of normal excitability.
Vapour of Alcohol
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.
Ether
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
Fig. 49.—Effect of ether.
depressing effect of ether passes off on readmission of fresh air (fig. 49).
Carbon Disulphide
The effect of vapour of carbon disulphide is similar to that of ether. It induces a depression duringr the introduction of vapour into the plant-chamber; the induced depression, however, passes off on restoration by means of fresh air (fig. 50).
Coal Gas
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
Fig. 50.—Effect of carbon disulphide.
Fig. 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.
Chloroform
Ammonia
The vapour of ammonia is found to cause an abolition of 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 stimulations, 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.
Sulphuretted Hydrogen
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 subsequently
Fig. 53.—Abolition of excitability under the action of ammonia.
Fig. 54.—Total abolition of excitability and death of plant under the action of sulphuretted hydrogen.
found to have died under the poisonous effect of this gas.
Nitrogen Dioxide
Nitrogen monoxide or laughing-gas has but little effect: there may, however, be a slight excitatory action. Nitrogen dioxide, on the other hand, is extremely poisonous. Introduction 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.
Sulphur Dioxide
Equally fatal is the effect of sulphur dioxide. Introduction of the gas was attended by an immediate excitatory
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.
Summary
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.
Ozone enhances the excitability of Mimosa.
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.
Ammonia induces a marked abolition of excitability.
Sulphuretted hydrogen, nitrogen dioxide, and sulphur dioxide abolish the excitability and bring about the death of the plant.