The Motor Mechanism of Plants

different, yet they all give practically an identical value for the critical temperature of death. The electromotive and variation of resistance methods also give similar results. Sudden down-movement Opening or closing movement Longitudinal contraction When the leaf of Mimosa is exposed to a continually * rising temperature, it shows an expansive erectile movement which is suddenly converted, at the critical temperature, into a fall of the leaf due to a spasmodic contraction. The thermo-mechanical record is V-shaped, the point of inver¬ sion being very sharp and well-defined. Under standard conditions the inversion takes place at 60 c C., the sudden contraction being the death-spasm of the plant. There is a post-mortem relaxation succeeding the death-contraction. 1 Various other plants, sensitive and ordinary, exhibit the fe characteristic death-spasm at or about 60 0 C. V

In a growing radial organ a longitudinal contraction 4 takes place at the critical temperature. In an anisotropic v growing organ the death-spasm is exhibited by the pre¬ dominant contraction of the more actively growing side of the organ. A sudden excitatory change of galvanometric negativity occurs at the fatal temperature of 6o°. The resistance of the tissue also undergoes a sudden diminution at this critical temperature. In the previous chapter, the occurrence of an intense excita¬ tion at the fatal temperature was demonstrated by the contractile movement of various motor organs. Now there are other tissues, mature stems for example, in which, on account of the presence of woody tissue, any mechanical movement of response is an impossibility. Do such rigid structures exhibit any excitation at the critical death-tem¬ perature ? Again, it may be thought that the spasmodic fall of the Mimosa leaf at the critical temperature is due not to excitation but /to the coagulation cf the protoplasm at that particular temperature. I his supposition fails, however, to explain the lowering of the death-point under fatigue (Experiment 121).

The following method of investigation was devised to prove that excitation is induced even in woody stems at the fatal temperature. The excitation was detected by its transmitted effect. It is well known that, stimulation of a point on the stem of Mimosa gives rise to an impulse which causes the fail of the distant leaves ,* this effect is produced, as shown by the following experiment, where the stimulus is the excitation induced at death. Since the indicating leaves are maintained at normal temperature, their responsive fall cannot possibly be attributed to coagulation.

Experiment 128. Tyufiswtittcdr dcuth-cxcitutiofi oj ipiosci. An isolated shoot of Mimosa bearing a number of leaves is taken and its cut end immersed in a water-bath to a depth of 2 cm. The leaves of a cut shoot, as already explained, regain their normal excitability in the course oi an hour or so. The water of the bath was heated electrically by means of a coil of wire ; the heating current was so regulated that the rise of temperature was at the rate of i° C. per minute. Steady rise of temperature of the water in the bath did not at first produce any noticeable effect ; but on the attainment of the critical temperature, an intense excitatory impulse was evidently generated in the immersed portion of the cut stem, for all the leaves fell in serial succession. That this excitation was due only to the local death of the immersed portion of the stem is proved by the fact that the leaves re-erected themselves in the course of about 20 minutes. Cooling the water of the bath and reheating it once more did not give rise to any further impulse, for the portion immersed was already dead. The stem was then lowered about 2 cm. deeper into the water, and repetition of the experiment caused a new excitatory impulse due to the death of the fresh portion of the living stem, indicated once more by the serial fall of the leaves. This affords conclusive proof of the occurrence of an intense excitation in a woody stem at death.

1 then repeated the experiment with intact seedlings of Mimosa. Experiment 129. Transmission of death-excitation in intact seedlings. — I took a batch of young seedlings of Mimosa and carefully removed them from the soil without injuring the roots. The lower portion of each plant was placed in the bath, the first indicating leaf being at a distance of 15 mm. from the immersed portion of the stem. The transmission of death-excitation was found to occur in four typical cases at 6o'\ 6o°, 59 , and 6o° respectively. These results prove that an excitatory impulse is generated at the fatal temperature, and that the death-point of a noil-motile tissue, such as mature stem, is the same as that of the pulvinus. For establishing a wider generalisation, the ex¬ periment was successfully repeated with other species of sensitive plants, such as Averrhoa Carambola and Biophytum sensitivum.

Experiment 130. Transmitted death-excitation in Aver- rhoa . — A detached leaf was used. One of the sensitive leaflets was attached to the Oscillating recording apparatus, it will be shown m a later chapter that leaflets of Averrhoa and Biophytum react to intense excitation not by single but by multiple response. The cut end of the long petiole of the leaf was immersed in a bath and the temperature gradually raised at the standard rate of i° C. per minute. The indicating leaflet was at a distance of 50 mm. from the cut end. When the temperature of the bath reached the

Fig. 128. Transmitted death-excitation at 6o° C. gave rise to multiple response in Averrhoa. critical point, an intense death-excitation was initiated and transmitted to a distance, causing multiple response of the leaflet (fig. 128). This was observed when the temperature of the bath had risen to 62° C. The death-excitation in the immersed portion of the stem must have occurred one or two degrees earlier, for a certain time must have elapsed for the impulse to reach the distant leaflet. Repetition of the experi¬ ment showed no response of the leaflet. But on lowering the petiole 2 cm. into the water the death-excitation of the fresh living portion gave once more the excitatory multiple response. 1 obtained similar results with .Biophytum.

The experiments that have been described prove that an intense excitation occurs at the moment of death brought about by fatal temperature. Does this occur only when the plant is scalded to death, or does it also accompany death caused by other means ? Is there, for example, any excitation at the fatal moment when the tissue is killed by poison ? The question was investigated by subjecting a part of the plant to the action of poison, and noting whether an excitatory impulse was generated at the fatal moment.

Experiment 131. Effect of dilute solution of poison. — I took some seedlings of Mimosa with their roots im¬ mersed in a beaker of water ; the distance of the nearest motile leaf was 2 cm. above the level of immersion. Another beaker was prepared containing 1 per cent, solution of potassium cyanide. The poison was applied to the roots of the plants by substituting for the beaker containing water, that containing the cyanide solution. The solution was slowly absorbed and the sudden and serial fall of leaves indicated the transmitted death-excitation which occurred 212 seconds after the application of the poison, this being the average interval. The solution thus became effective in initiating death after continuous application for about 3-5 minutes. It appeared probable that a stronger poisonous solution would prove effective in a shorter time.

Experiment 132. Effect of a stronger dose of poison . — • The experiment was repeated with a second batch of similar seedlings, the strength of the cyanide solution being increased from 1 to 5 per cent. The average period for initiation of death-excitation by the 5 per cent, dose was found to be 51 seconds, instead of 212 seconds by the 1 percent, solution. The hastening of death is thus seen to be approximately proportional to the strength ct the dose.

Experiment 133. Comparison of virulence of different poisons. — Ihe initiation of death is not merely determined by the strength of the dose but also by the virulence of the poison. In order to compare the poisonous action of potassium cyanide and of mercuric chloride, I repeated the experiment with i per cent, solution of the latter poison, the average period for the initiation of death-excitation was found to be 650 seconds, in place of 212 seconds with 1 per cent, solution of potassium cyanide. The poisonous action of potassium cyanide on the plant may therefore be regarded as three times more virulent than that of mercuric chloride.

A misgiving might arise that the fall of the indicating leaf was due not to the transmission of the death-excitation, but to the direct action of the poison carried to the leaf itself by the movement of sap. This is, however, negatived by the subsequent erection of the leaf after the passage of the impulse ; translocation of the poison would per¬ manently abolish all power of response. The following experiment was devised for preventing even the remote possibility of the transfer of the poison to the indicating

Experiment 134. — The thin long flower-stalks of Biophy- tum are very effective in conducting excitation. When these are stimulated in any way, excitation travels down¬ wards and overflow's into the rosette of leaves which bear numerous pairs of sensitive leaflets, and causes successive closure of the leaflets from the centre outwards. One drop of 5 per cent, solution of KCN was applied to the tip of the flower- stalk, special care being taken that the poison did not leak downwards. In these circumstances there was no possibility of the transport of the poison downwards against the direction of ascent of sap Excitation was nevertheless initiated at the poisoned tip of the flower-stalk, and trans¬ mitted to the leaves below, as shown by the successive

closure of their leaflets. The transmitted excitation was so intense that it gave rise to a series of multiple responses of the leaflet attached to the recorder (fig. 129) . After recovery from multiple pulsation, the leaflet was directly subjected to moderate stimulation at the point marked with a cross. This gave rise to a single response, proving once more that the poison had not reached it. The multiple response Fig. 129. Multiple response of leaflet of Biophytum under transmitted death-excitation due to poisoning.

Local stimulation of the leaflet at cross x gave rise to a single therefore indicated the intense death-excitation initiated at a distance. The results obtained with different organs of plants under diverse methods of investigation concur in proving that an intense excitation occurs at the death of the tissue, whether this is brought about by poisoning or by a rise of tempera¬ ture to the la cal degree. 1 describe Li the t'.vo following chapters the results of another method of inquiry which gave identical value ior the fatal temperature.

At the moment of death, violent excitation is developed in the tissue. The excitation at death has been shown to be con¬ ducted to a distance, causing excitatory fall of the indicating leaves in sensitive plants. Thus, on locally raising the temperature of a portion of stem, an excitatory impulse was found to be generated there at a definite critical tem¬ perature. The death-points of motile and non-motile tissues are found to be the same. Death-excitation, locally initiated by the action of poison, is also transmitted to a distance. Ihe duration of application for producing death-excitation is found to depend on the strength of the dose and on the virulence of the poison.

It was shown in previous chapters that the spasmodic con¬ traction of living cells, accompanied by a spasm of electric negativity, gives a definite indication of irreversible change associated with death. It appeared probable that additional modes of response might be available for the immediate detection of the initiation of death in the tissue of plants. The following investigation was undertaken to ascertain whether an}/ sudden change in weight occurs when the tissue immersed in a water-bath is raised to the fatal degree of temperature.

1 he plant-specimen, suspended by a waxed cocoon- thread from the right pan of a balance, was immersed in water, its weight in water being exactly balanced by suitable counterpoise placed on the left pan (fig. 130). The thin brass vessel containing the water in which the specimen is immersed is ciou hie- walled, and an electric coil for heating is arranged below it. By regulating the electric current, the rate of rise of temperature was adjusted at i° C. per minute. The heated water between the two walls of the vessel raises the temperature of the water in the interior so uniformly that any disturbing action of convection currents on the immersed specimen is eliminated. Should the specimen .ose weight during rise of temperature, the long index of the balance will move to the right (down-curve) ;

an increase in weight, on the other hand, will be indicated by a movement of the index to the left (up-curve). The record is taken on a smoked-glass plate allowed to fall at a uniform rate of 10 cm. in 40 minutes, which is Fig. 130. The Balance for recording apparent variation in weight The immersed specimen suspended from right pan r counter¬ poised by weight placed on left pan L. Electric heating arrangement of bath not shown, k, press-key for making dotted record ; the mechanism of the striker is shown on the right.

the duration of a single experiment. The bent tip of the writing-lever is at a distance of 2 mm. from the recording- plate. The record in the form of successive dots is obtained by the employment of a striker ; this can be made periodi¬ cally to press the writing- lever against the smoked-glass surface, thus making a dotted record. The striker consists of a thin horizontal rod ; a second rod at right angles to the striker passes through a guide tube, inside of which is a spiral spring by which the striker is kept at a short distance from the writing- lever. A string from the end of the second rod passes over a pulley, and terminates in a press- key K. An observer watches the thermometer in the bath and presses the key so as to make a dot for every degree rise of temperature ; the interval between two successive dots thus represents a rise of i° C.

The water in the bath is freed from absorbed air by previous boiling. The air which may have remained entangled in the tissue is also removed by placing the specimen, immersed in a beaker of water, under the receiver of an air-pump. The air-bubbles are removed by repeated production of vacuum and restoration of normal pressure. The specimen, generally speaking, is heavier than water, and sinks when suspended in the water ; when it is lighter, a little weight has to be attached to it to make it sink.

Experiment 135. — For the first experiment of the series I took a fruit of Carissa Carandas. The record (fig. 131; shows a change in weight during the rise of temperature. A loss in weight occurred between 45 0 and 60 after which there was an abrupt inversion of the curve, the up-movement of which indicated a sudden increase in weight. Experiment 1^6. — f he fact that sudden change in weight is a final irreversible death-response was proved by taking a record of the effect of variation of temperature on a specimen which had already been killed by exposure to the fatal temperature. This did not show any such variation

in weight at the critical temperature as did the living specimen. The death-curve of variation in weight exhibits a striking similarity to the thermo-mechanical death-curve of Mimosa and other plants ( cf . fig. 121) The loss in weight in one case corresponds with the expansive movement in the other. Again, the sudden increase in weight at death in the present case has a correspondence with the spasmodic contraction of the pulvinus of Mimosa. The significance of this similarity will be pointed out later.

In describing the thermo-mechanical curve in a previous chapter, it was explained that the death-point is lowered by Fig. 131. Record of variation in weight of Carissa fruit under Note abrupt inversion of the curve at 6o° C., indicating a sudden a few degrees when the specimen is suffering from recent injury. In the present method, also, the point of inversion (sudden increase in weight) is often lowered by a few degrees on account of injury. A semi-conducting bulky structure was found to exhibit the mechanical death- inversion at one or two degrees above the normal 60 0 C. ; a similar variation is also observed in the record of such bulky objects by the method of variation in weight. , 1

This abrupt change in weight at death is not only exhibited by fruits but also by other organs, such as the stem, the petiole, and the root. Certain specimens exhibit it more strikingly than others. I will, in the course of this chapter, give typical examples of death-response of various organs. The death-contraction of Mimosa is subsequently reversed into a post-mortem relaxation (p. 204). Similarly, the Reversal began to take place at 57°C., there being a marked increase in apparent weight at 6o° C.

sudden increase in weight at death gradually disappears, and the curve returns to the original position. Before turning to the records of other organs, I give an additional record obtained with another fruit. Experiment 137* Death-record of the fruit of Solanum. — The down-curve of Solanum, indicating loss in weight, wTas arrested and gradually reversed after 56° ; at 6o° the rate of increase in weight was very pronounced, as seen in the sudden erection of the curve (fig. 132).

Experiment 138.— Fig. 133 shows the record obtained with the stem of Basella. A sudden increase in weight occurred at. the temperature of 61 The stem was thick, hence the death-point • was slightly higher than usual. I obtained similar results with the stem of the Gourd. Fig. 133. Death-record of s*em of Basella. Sudden inversion at 6i° C. Experiment 139. Death-record of Arum indicum. — A smail piece" was cut off the young petiole of Arum indicum

Fic. 1 34. Death-record of petiole of Arum indicum. Sudden increase in weight at 6i° C. and allowed a long period of rest to recover from the shock of operation. It was then suspended from one arm of the balance in the usual manner. The record obtained is similar to that of fruits and stems, the point of death- inversion and sudden increase 111 weight being 61 0 C. (fig- *34)- Experiment 140. — The specimen employed was a small piece of Beetroot. Rise of temperature produced an increasing loss in weight which culminated at 6i° C., after which the curve exhibited an abrupt inversion indicative of a sudden increase in weight. I also obtained similar results with Radish.

The characteristic variation in weight under change of temperature— an increasing loss followed by a sudden gain at the critical temperature— was thus exhibited by various organs of plants. The question now arises. What is the underlying cause which gives rise to this characteristic variation in v/eight ? and further, Why should the curve of variation in weight bear such a striking resemblance to the thermo-mechanical curve t In both there is a sudden inversion of the curve at the fatal temperature. The records obtained by these different methods must evidently refer to a definite reaction which is fundamental.

In regard to the sudden increase in weight at the critical temperature, it has been shown that a dead tissue exhibits no such change (Experiment 136). The phenomenon must therefore be essentially physiological. Nevertheless, a physical factor is present, namely, thermal expansion of the immersed tissue and resulting increase of buoyancy during rise of temperature. How is the purely physio¬ logical factor to be discriminated from the physical ? The