Researches on Irritability of Plants

It will be seen from fig. 167 that at 30° C. the period of a complete pulsation was 80 seconds, the amplitude being 25 mm. On lowering of temperature to 29° C. the period became lengthened to 88 seconds, the amplitude being enhanced to 30 mm. At 28° C. the period was protracted to 96 seconds, the amplitude being enhanced to 35 mm. And finally at 27° C. there was further lengthening of the period to 110 seconds, the amplitude of pulsation being enhanced to 40 mm.

In addition to the changes of period and amplitude there is another noticeable effect induced by variation of tempera- ture. We have seen in the case of Mimosa that the lowering of temperature induces a depression of the leaf, and the rise of temperature an erection of the leaf. Effects similar to Fic. 167.—Effect of lowering of temperature on the amplitude and frequency of Desmodium pulsation. Note the shifting of base- line upwards, indicating fall of leaflet during cooling; observe also the enhancement of systolic contraction:

these are also induced in the leaflet of Desmodium. The result of cooling is thus a slight fall of the leaflet, in conse- quence of which we observe the shifting of the base-line upwards. A still more’ striking effect is the increase in the extent of contraction, by which the systolic limit is enhanced. This will clearly be noticed in the series of records in fig. 167. The rhythmic pulsation of the cardiac tissue is arrested when subjected to a certain low temperature. Similarly the pulsation of Desmodium gyrans is arrested at a sufficiently

low temperature. The critical point is somewhat modified by the tonic condition of the specimen. With vigorous specimens the temperature at which arrest takes place may be as low as 17° C. Fic. 168.—Effect of rapid cooling by ice water applied at the moment marked by arrow. Note arrest at systole and gradual revival on warming, the pulsations exhibiting There are certain interesting points in connection with the arrest brought about by cooling. Two series of records are here given in which the arrest was quickly brought about

Fic. 169.—Magnified record given by a different specimen of arrest of pulsation by cooling and subsequent revival by warming. Note extension of diastole during warming. by the application of cold water to the pulvinule. The leaflet was then allowed to return to the temperature of the room, with the revival of pulsatory activity. The entire process of arrest and subsequent revival is clearly seen in fig. 168. It will be noticed that sudden cooling arrested the pulsation at systole and that on gradual warming the rhythmic activity was revived with the gradual restoration of the original amplitude.

This is still better seen in fig. 169, which gives a mag- nified record of the arrest due to cold, and subsequent restoration of pulsation on return to the temperature of the room. Ice-cold water was applied after the second pulsation. The arrest at systolic contraction is clearly Fic. 170.—Effect of rise of temperature on the amplitude and frequency of pulsations of the heart of frog. (Pembrey and Phillips.) demonstrated. As the temperature was raised there was increasing expansion, in consequence of which the diastolic excursion was continuously increased. The effect of cooling is thus to increase the force of contraction and diminish that of expansion. The effect of warming is the reverse of these.

We have seen that both in the rhythmic animal and vegetable tissues, the period is increased and amplitude enhanced under the lowering of temperature. The converse is the case with rise of temperature. Fig. 170 shows the effect of rising temperature on the pulsation of the heart of the frog. Similar effects are induced in the pulsation of Desmodium. Fig. 171 gives a series of pulsations of the leaflet at the temperatures of 19° C., 23°5° C., and 28°5°C.,

the record in each case being continued for a period of 20 minutes. At a low temperature the pulsation is apt to be somewhat irregular, hence the unequal amplitude in the successive pulsations at 19° C, It is apparent that while at 19° C. there were 34 pulsa- tions, at 235° C. the number had been increased to 43, and at 28°5° C. to 6 pulsations. perature on the pulsation of Des- Fic. 172.—Effect of rise of modium gyvans. Time-marks temperature on a different below indicate intervals of 1 leaflet.

Taking a more vigorous specimen, I obtained records for 12 minutes each at temperatures of 28°5° C., 31°5° C., and 34°5° C. It will be seen that while at 28°5° there were only 4 pulsations, these had become increased to 6} pulsations at 31°5° and to to pulsations at 34°5° C. The other noticeable feature is the marked diminution of amplitude with the rise of temperature (fig. 172). Instead of taking isolated records at different tempera- tures, I next raised the temperature of the plant-chamber very gradually, by careful manipulation of the heating current, and obtained a record with a different specimen. In this way the temperature was raised continuously from

30° C. to 38°5° C. How regularly the frequency of pulsation is increased, and the amplitude diminished, with the rise of temperature is shown in fig. 173. In the record just given there is observable an arrest of pulsation when the specimen was subjected to as high a temperature as 38° C. But by accustoming it to warmth, the plant can resist even higher temperatures. Thus I kept several specimens in a glass-house, the temperature in which at midday was 37° C. These specimens could be exposed to a temperature as high as 45° C. without the arrest of the pulsation. I reproduce here a record (fig. 174) where the specimen was gradually raised from 30° C. to 42° C., and then allowed to cool and return to the tempera- ture of 30°C. It will be seen that the amplitude of pulsa- tion was continuously decreased, yet there was no arrest even at 42° C. On cooling, the amplitude was restored to the original value.

I have shown that under excessive cooling the force of expansion is reduced, in consequence of which there is an arrest towards systole. With excess of heat, on the other hand, the reverse effect takes place. On account of increase of force of expansion, or diminution of force of contraction, the systolic limit is progressively diminished. When the Fic. 174.—Effect of continuous rise from 30° C. to 42° C. and return to 30° C. Note progressive diminution of systolic contraction during warming, and its increase during cooling.

specimen is allowed to cool, the reverse effect 1s exhibited by gradual enhancement of systolic contraction. Similar effects are seen to take place in a record (fig. 175) obtained with a different specimen. The temperature may sometimes be raised to 45° C. without inducing any arrest of pulsation. A tendency is now observed towards contraction, as shown by the general shifting of the pulsations upwards. This movement of contraction goes on till there is a complete arrest brought about by heat rigor.

The effect of lowering of temperature on the rhythmic pulsation of Desmodium gyrans is similar to that on the pulsation of frog’s heart. Lowering of temperature enhances the amplitude but reduces the frequency of pulsation of both.. The pulsation of Desmodium leaflet is arrested at the minimum temperature of about 17° C. Arrest takes place at systole ; gradual warming revives the pulsation, which undergoes a staircase increase with enhancing diastolic expansion.

Rise of temperature induces enhanced frequency and diminished amplitude of pulsation. During rise of temperature to about 43° C. there is a tendency of arrest towards diastole. The systolic con- traction undergoes continuous diminution during rise of temperature. During the fall of temperature there is a gradual enhance- ment of systolic contraction. The temperature maximum at which arrest of pulsation takes place may be as high as 45°C. Above this temperature there is a tendency to contraction and permanent arrest under heat-rigor.

Application of gaseous or liquid reagents—Modifying influence of, tonic condition of specimen, strength, and duration of application—Effect of sugar solution—Effect of alcohol—Action of carbonic-acid gas— Effects of anesthetics, ether, and chloroform—Action of carbon disulphide—Effect of copper sulphate solution—Effect of potassium cyanide solution—Antagonistic actions of acids and alkalis on the pulsations of the heart and of Desmodium—Similarities of reaction in rhythmic tissues, animal and vegetal.

WE will now study the effects of various chemical agents on the pulsating activity of the Desmodium leaflet. These may be applied either externally or internally. As regards external application, a liquid reagent may be applied directly on the pulvinule ; gases and vapours, on the other hand, are made to circulate in the plant-chamber. We may secure internal application by forcing in the solution at the cut end of the petiole, by means of hydrostatic pressure (cf. fig. 141).

The characteristic effects of different reagents are seen exhibited in the induced change of period or of amplitude of pulsation. Another noticeable effect often observed is the transposition of the systolic or diastolic limit of pulsa- tion. Certain agents may thus induce an arrest at systole, others at diastole. The effect of a given agent may be modified by various conditions, such as internal or external -application, the strength of the solution, and the duration of application.

Thus the same agent which in a strong solution induces depression, may in a very dilute solution cause an exaltation. The result also is dependent on the tonic condition of the tissue. A specimen in which the amplitude of pulsation is the maximum possible cannot exhibit any further increase under a stimulating agent. But with a less vigorous spe- cimen the effect of the same agent is manifested by a marked Fic. 176.—Stimulating action of dilute sugar solution applied at the moment marked by an arrow.

enhancement of amplitude of pulsation. Again, a vigorous plant may survive a given dose of a toxic agent ; while a less vigorous specimen will succumb under the same treat- ment. Finally we have the interesting phenomenon in Fic. 177.—Effect of external application of alcohol. Preliminary enhancement followed by depression, the period of pulsation being prolonged. virtue of which the plant accommodates itself to a changed unfavourable condition. Certain chemical agents act as stimulants, inducing an enhancement of amplitude of pulsation. As an example of

this I may mention dilute sugar solution. The specimen experimented on was but moderately vigorous. After taking four normal pulsations, which are seen to be uniform (fig. 176), a 2 per cent. solution of sugar was applied inter- nally at the moment marked by an arrow. No immediate effect was noticeable, but after an interval of a single Fic. 178.—Effect of internal application of 15 per cent. solution of alcohol. Fic. '179.—Effect of dilute carbonic-acid gas ;. enhancement of amplitude and slowing of the period.

pulsation the stimulating character of the agent became evident by the resulting staircase enhancement of pulsation. The effect of this reagent in dilute solutions is in general to induce an enhancement of response. Stronger solu- tion, however, induces a depression which may culminate in arrest of pulsation. In fig. 177 a slight preliminary exaltation of amplitude, followed by depression, may be noticed. The period of succeeding pulsations is found to become prolonged.

The effect of internal application of alcohol is very similar to that of external application. In fig. 178 is seen a Fic. 180.—Strong carbonic-acid gas, inducing arrest. Line below indicates duration of application. Slow revival of pulsation on substitution of fresh air. Fic. 181.—Effect of internal application of water charged with carbonic acid. This gas when diluted with air causes an enhancement of amplitude, though the period becomes longer (fig. 179).

Application of undiluted gas, however, induces an arrest. If fresh air be now substituted, there is produced a slow revival of pulsation (fig. 180). In another experiment I obtained a record of the effect of internal application of water charged with carbonic acid. The pulsating activity was found slowed down, the ampli- tude of the pulsation also undergoing a diminution. The depressing effect is, however, seen to pass away gradually (fig. 181). This agent was applied in the form of vapour, which was slowly blown into the plant-chamber. If the vapour be

Fic. 182.—Effect of vapour of ether; depression and subse- quent arrest. Pulsations revived on blowing off the vapour. much diluted with air, then the first effect of ether is to induce a transient exaltation, followed by depression and arrest of pulsation. If the leaflet be subjected to strong vapour, or if the application be prolonged, then the arrest of pulsation proves to be permanent. But if diluted vapour is employed and fresh air substituted immediately after the arrest, then there is a slow revival of pulsation. This can be seen in fig. 182, where after the application of ether an arrest took place after three rapidly diminishing pulsations. On blowing off the ether vapour, the pulsation is seen to revive slowly after a period of 20 minutes.

The effect of this reagent on the pulsation of Desmodium is similar to that of ether. It is, however, far more toxic in its reaction, a slight excess in the application being attended by permanent arrest of pulsation. In the experiment of which a record is given (fig. 183), diluted chloroform vapour was introduced into the chamber. This is observed at Fic. 183.—Effect of vapour of chloroform. Immediate excitatory effect followed by depression and arrest. On blowing off the anesthetic, pulsations were revived after half an hour, repre- sented by a gap in the record.

first to have an excitatory effect in the first two pulsations. The amplitude of the third pulsation became much reduced, Fic. 184.—Arrest of pulsation by vapour of carbon disulphide, and revival after readmission of fresh air. Duration of application indicated by the horizontal line under the record. and an arrest ensued at the fourth pulsation after the appli- blown away and fresh air substituted in the chamber. But the arrest persisted for half an hour. After this interval there was a revival, and the pulsation attained for a time an amplitude even greater than the normal.

The vapour of this reagent also arrests the pulsating activity of the leaflet. The record (fig. 184) shows the quick arrest after a single pulsation. Substitution of fresh air is seen again to revive the pulsation. As an example of toxic agents we may take solutions of substances like copper sulphate. When a strong solution of Fic. 185.—Effect of internal application of CuSO, solution in inducing arrest of pulsation. this substance is applied directly on the pulvinule, an arrest takes place within a short time. The effect is delayed when the solution is applied at the cut end of the petiole. A record of the effect of internal application of copper sulphate solution is given in fig. 185.

We saw in Chapter XII. that the toxic effect of potassium cyanide in abolishing the conductivity of the tissue was far more pronounced than that induced by copper sulphate. The poisonous action of cyanide is equally powerful in abolishing the rhythmic activity of the Desmodium leaflet. Fic. 186.—Quick arrest of pulsation at systole by the action of KCN solution. This is well observed in fig. 186, where the pulsation is seen to be quickly arrested at systole.

We have hitherto seen the remarkable similarities of the effect of various chemical agents on the rhythmic activi- ties of the animal and vegetal tissues. A very striking Fic. 187.—Arrest of pulsation of the heart of frog in diastole by the action of dilute lactic acid. (Gaskell.) Record to be read from right to left in this and following figures. characteristic is the antagonistic reactions of acid and alkali induces in the heart an atonic reaction, in consequence of which there is induced an arrest of pulsation in the relaxed or diastolic condition (fig. 187). The action of dilute alkaline

solution is the very reverse, inducing tonic contraction and arrest in systole (fig. 180). I find these effects repeated in an astonishing manner in Fic. 188.—Arrest of pulsation of Desmodium in diastole, by the action of dilute lactic acid. Fic. 189.—Arrest of pulsation of heart in systole, by the action of dilute NaHO. (Gaskell.) Fic. 190.—Arrest of pulsation of Desmodium in systole, by the action of dilute NaHO. the pulsation of Desmodium. The internal application of dilute solution of lactic acid is seen to induce an arrest in a state of diastolic relaxation (fig. 188). The application of