The Motor Mechanism of Plants

Fig. 39. Abolition of moto-excitability of pulvinus of Mimosa by excessive absorption of water. Note prolongation of period of recovery and ineffectiveness of stimulations applied at moments marked with thick dots. Subsequent restoration of excitability by application of glycerine. motility I w'as able to trace to the wrater-logged condition of the pulvinus. It should be remembered that water¬ logged muscle aEo loses its power of contraction. Experiment 34. — The effect of excessive absorption of water on response is shown in fig. 39. After taking a pair of normal responses, a drop of water was applied on the pulvinus during recovery from the second stimulation.

Testing stimuli were applied at the usual interval of 15 minutes, the moments of application being represented by thick dots. These stimulations failed to induce any responsive movement. The probable explanation of the phenomenon is az follows : first, abnormal distension of a cell paralyses its power of contraction, as in muscle ; secondly, contraction being possible only by expulsion of sap into neighbouring cells, this cannot take place when all the cortical cells are already in a state of the utmost distension.

Fig. 40. Iiffect of moderate absorption of water. First two response normal. Application oi water, at arrow, induced depression of moto-excitability (Mimosa). 1 next attempted to restore the arrested power of con¬ traction by artificial means. Since glycerine has the power ol abstracting water, a drop of this substance was applied to the pulvinus. Fhis resulted in a quick restoration of the normal contractility, the two responses after the applica¬ tion of glycerine being practically similar to those at the beginning of the series.

Experiment 35. — In another experiment the increase of turgor produced by brief application of water was not so excessive ; but it sufficed to produce great depression of moto-excitability, as shown in fig. 40. The record was continued for 2 hours, when the motility was found to be slowly regained, evidently by the restoration of normal turgor through movement ol sap from the excessively turgid to a less turgid region. The plant is enclosed in a chamber, with a transparent glass or mica cover ensuring uniform illumination. An electric device is employed for varying the temperature of the pl ant-chamber, which is maintained in a proper humid

Fig. 41. Responses taken on a fast-moving plate, at three different temperatures, 220, 270, and 320 C., the lowest below and the highest above. Amplitude of response larger and steepness of curve greater at higher temperature (Mnr 3sa). condition. \n electric current from a battery outside is led through a coil of heating-wire inside the chamber. The heat generated can be regulated by varying the intensity of the current. The temperature inside the chamber can thus be raised to any desired degree, and maintained constant during the period of the experiment.

Experiment 36. — Three records on a last-moving plate were obtained at three temperatures of 22°, 27°, and 32 C. The amplitude of response is seen to be progressively in¬ creased undei the rising temperature. The rate of con¬ traction is more rapid at the higher temperature, as is seen in the increased steepness of the curve (fig. 41). The amplitude of resoonse reaches its maximum between 340 and 350 C., which may therefore be taken as the opti¬ mum ; a further rise of temperature induces a depression, which also occurs on lowering the temperature below the normal (see fig. 54, p. 94). The minimum temperature for the abolition of motor response in Mimosa in the tropics is about 180 C.

The effects of different gases and vapours are described below. For subjecting the plant to the action of different gases and narcotic vapours, the plant is enclosed in a glass chamber with an inlet and an outlet pipe. Gases and vapours can be pumped into the chamber, air being expelled by the exit pipe. In order to observe the after-effect, the chamber can be filled with fresh air. The plant exhibits revival from temporary depression or narcotisation ; but if the narcotic dose has exceeded the safety limit, or if the gas has been too poisonous, death supervenes without possibility of revival

Experiment 37. — The stimulating effect of this gas is clearly visible in fig. 42. The particular leaf was showing signs of fatigue ; the in¬ troduction of ozone into the plant-chamber brought about an immediate ch ange, inducing an enhancement of excitability indicated by the increased amplitude of response. Experiment 38. — In contrast to the stimulating effect of ozone is the large dose of C02, which causes the moto-excit ability of the pulvinus to undergo a depression. It is probable that there is a reserve of oxygen in the tissues for the maintenance of the oxidative processes of organic decomposition, upon which the evolution of energy depends. Hence replace¬ ment of the air in the plant - chamber by undiluted carbonic acid gas may not be immediately followed by total abolition of excitability. The depressing effect of C02 is, however, quite evident in the rapid diminution of the

amplitude of response (fig. 43)- A noticeable feature in the record is the displacement of the base-line upwards, indi¬ cating persistent contraction. When the plant -chamber was refilled with fresh air, the normal excitability was restored. While a large dose of C02 causes depression, a small dose induces an enhancement of excitability. 1 his explains why, under continued action of dilute C02, the response exhibits a preliminary enhancement, followed by a decline ( cf . fig. 70).

a'he normal irritability of plants is greatly depressed by the vitiated air of a town. The* effect of this on certain plants is most marked. For examp ](t, Biophy turn sensitivum grows as a weed seven miles outside Calcutta, and exhibits great sensitiveness to all modes of stimulation. But after removal to town it loses its irritability in a short time and succumbs in the course of a few weeks. I find that some of the impurities present in the town air, such as traces of H2S and of S02, are very deleterious to the normal irritability of the plant.

Experiment 39. — The presence of sulphuretted hydrogen in the air of towns is perhaps the most harm Til. This is illustrated in fig. 44. The intro¬ duction of this gas into the plant -chamber caused the period of recovery to become greatly protracted. The abolition of excitability is evidenced by the fact that successive stimula¬ tions at the noints marked with thick dots proved to be quite ineffective. The action of this gas is so poisonous that restoration to fresh air did not bring about any revival. The plant was subsequentlv found to have died from the poisonous effect of the gas.

Effect of Ether Experiment 40. — The con¬ tinued action of ether induces a Fig. 45. ] . fleet of Ether (Mimosa), depression of excitability, as is seen in the reduction of the am¬ plitude ol successive responses. J he depressing effect of this narcotic passes ©ff on the readmission of fresh air (fig. 45). Fig. 44. Total abolition of excitability and death of piant under the action of Sulphuretted Hydrogen (Mimosa). Experiment 41. — The vapour oi chloroform acts as a very strong narcotic, and any excess of its application proves fatal. In fig. 46 is shown the effect of a large dose of chloro¬ form on the plant. This not only produced a total abolition of excitability, but brought about a sudden spasm, which was the spasm of death, seen as a line shooting upward. After this, the blowing off of the vapour failed to revive the plant.

the relation of the dose to the reaction. An agent which in large doses proves to be toxic is found to be stimulating when given in minute quantities. The absorption of chloroform vapour being slow, the im¬ mediate effect is that of a small dose, shown by an enhancement of the amplitude of response. Continued application produces, however, the depressing action of a large dose (see fig. 71). Fig. 46. Abolition of excitability under Chloroform (Mimosa).

ments on the action of other gases and vapours. Ammonia was found to depress excitability, which was slowly restored after removal of the vapour. The vapour ot carbon disulphide produced depression, the response showing increasing relaxation. W hen the application had not been too prolonged, fresh air revived the normal excitability. Nitrogen dioxide is, on the other hand, extremely The effects of various drugs in solution will be described in the next chapter.

Excitability is increased under rise of temperature up to an optimum, which is about 340 C. ; depression sets in above this optimum. Lowering of temperature induces depression and the excitability disappears at the critical point, which is about 180 C. in the tropics. Chemical agents induce parallel modifications of respoir e in plant and in animal tissues. Asphyxiation by C02 causes a great depression of excita¬ bility, which is restored on the readmission of fresh air. Small doses of C02 induce a transient enhancement of excitability.

Sulphuretted hydrogen causes abolition of excitability and the death of the plant. V'apour of ether exerts a moderate narcotic action, in¬ ducing a temporary abolition of excitability, which is restored on the readmission of fresh air. Chloroform is a stronger narcotic ; excess of it causes the death of the plant. A minute dose of a toxic agent is often found to have a stimulating action. 'Infe most suitable plant for researches on irritability is Mimosa pudica, which can be obtained in all parts of the world. An impression unfortunately prevails that the excitatory reaction of the plain can be obtained onl^ in ^ §ummer' and under favourable circumstances, thus militating I against its extensive use in physiological in\ estigation. This misgiving is, however, without any foundation, for I found no difficulty in demonstrating even the most delicate experi¬ ments with Mimosa before the meeting of the American Association for the Advancement of Science held in the United States about Christmas in 1914. The prevailing outside temperature at the time was considerably oeiow freezing-point. With foresight and care it is not at all! difficult to maintain in a hothouse a large number of these plants in a sensitive condition ah the year round. -

In order to remove the drawback connected with the supply of sufficient material, I carried out an investigation to ascertain if a cut shoot would prove to be as efficient for the study of irritability as the whole plant : to ascertain, in fact, if an isolated petiole-? nd-pulvinus preparation of Mimosa would not be as efficient for researches on irricability as the nerve- and- muscle preparation or a dog. The petiole-pulvinus preparation is made by isolating a piece of a stem bearing a single leaf. The apex of the ^r.enij and the four diverging sub-petioles of the leaf may also be cut off. The upper cut end of the piece of stem, and the cut.

lame number nf Q-neei *IG* 47- '-i'he petiole-pulvinus preparation idige numoer OI ^peci- attached to the Resonant Recorder. from the same plant, thus removing the difficulty of scarcity of experimental material. The petiole-pulvinus preparation, moreover, offers greater facilities for experi¬ ment. Owing to its small size it can be easily manipulated ; it can be enclosed in a small chamber and subjected to varying conditions of temperature, and to the action of different vapours and gases. Various drugs and chemical solutions can be successively applied at the cut end of the stem without any disturoance of the continuity of the record. Many experiments which would be impossible with the entire plant are in fact quite practicable with the petiole-pulvinus preparation.

For experiments of short duration the experimental preparations can be made still more compact by discarding end of the petiole are covered with a collodion flexile to prevent drying up. of the stem may also be covered with a piece of moist cloth ; but for ex¬ periments of long dura¬ tion it is advisable to mount it in a T-tube, with a funnel and exit- tube for the introduc¬ tion and removal of water or any chemical solution. The petiole is attached to the Resonant Recorder

the U-tube, and tying a piece of moist cloth over the lower enci of the stem. Under natural conditions the stem is fixed, and it is the petiole which moves under excitation. But a very interesting case presents itself when the petiole is fixed and the stem free. Here is presented the unusual spectacle of the piece of stem ' wagging ' in response to excitation. The petiole-pulvinus preparation made in this way is often found to have lost its sensibility. This loss 1 was able to trace to two different factors. First, to the water-logged condition of the pulvinus by excessive absorption ot water. The plant, aowever, gradually accommodates itself to the changed condition, the normal turgor and excitability being restored in the course of one or two hours. Secondly, the severe injury caused by the cutting of the stem induces a temporary loss of excitability. I will presently speak of the condition under which the normal excitability can be restored.

An important factor for experimental success is the choice of a suitable leaf, for both excitability and con¬ ductivity are modified by age, as explained below. Much confusion has arisen from the wrong supposition that conductivity and moto-excitability are closely associated with each other in Mimosa. The mechanisms are, actually, quite distinct ; I have shown elsewhere 1 that the conducting nervous tissue passes inwards from the petiole via the pul- vinus into the stem. Owing to this particular distribution of the conducting tissue, a strong stimulus applied to one of the sub -petioles gives rise to a centripetal impulse which, under normal conditions, produces excitatory contraction of the pulvinus of its own leaf and causes the subsequent fall of other leaves, above and below, on the stem. Conversely, a strong stimulus applied on the stem below one of the

leaves gives rise to a centrifugal impulse which produces the contractile fall of the nearest leaf, and the subsequent closure of the leaflets of the four attached sub-petioles. Now it is more or less an accident that the motor tissue of the pulvinus should surround the conducting nervous tissue for an insignificant part of its length. If a particular agent paralyses the moto-excitability, it does not necessarily follow that the nerve in the interior has also become paralysed. Though excessive absorption of water causes a temporary abolition of the moto-excitability, it does not affect the conductivity to any appreciable extent. This important fact is clearly established by the results of the following experiments :

Experiment 42. — The pulvinus of an intermediate leaf was rendered immotile by excessive absorption of water. Strong stimulation was applied on one of its four sub¬ petioles. The impulse was transmitted through the pulvinus to the stem without causing a contractile fall of the leaf. The centripetal impulse, however, reached the other leaves on the stem, both above and below, and caused their fall. This is analogous to the transmission of a wave of excitation through voluntary muscle-fibre in a state of water- rigor, the transmitted excitation causing contraction in the normal portion of the muscle.

Experiment 43. — A centrifugal impulse was generated by the application of stimulus to the stem 5 mm. below the water-logged pulvinus. The impulse traversed the length o; the pulvinus and the petiole without causing the fall of the leaf. All the leaflets of the four attached sub-petioles, however, underwent excitatory closure in serial succession. The fact that the conductivity of the nerve is independent of the moto-excitability of the pul vinus is also demonstrated by experiment with a stem carrying very old leaves below and young leaves above. The mobility of the pulvinus of very old leaves near the base of the stem is permanently abolished on account of old age. But when a strong stimulus is applied to the petiole of one of these leaves, the excitation is

nevertheless transmitted across its own immobile pul'vinus and causes the fall of the younger leaves higher up the stem. In a Mimosa plant there are leaves of different ages. Of these the youngest are at the top ; lower down come the fully grown young leaves, and further down, leaves which are older. I find that the fully grown young leaves are relatively the most excitable. The following experiment deals with the relation of the age of the leaf to the conduct¬ ing power of the petiole.

Experiment 44. Comparison of conducting power in different leaves. — The electric connexions are so made that the same tetanising shock passes through short lengths of very young, fully grown, and older petioles on a shoot of Mimosa. As the secondary coil was gradually pushed in, the fully grown young leaf was the first to respond to the induction shock ; the secondary coil had to be pushed nearer to the primary by 6 cm. before excitation was effectively transmitted through the very young petiole. For the old leaf a still stronger stimulation was necessary ; the secondary had to be pushed through an additional distance of 4. cm. for effective transmission of excitation. I also determined the relative values of the minimal intensity of stimulus effective in the three cases. Adopting, as before, the intensity of electric stimulus which causes bare per¬ ception in a human being as the unit, I found that the effective stimulus for a fully grown young petiole was 0-3 unit, while the very young one required 2*5 units, and the moderately old petiole 5 units. Hence it may be said that the conducting powers of fully grown, of very young, and of moderately old petioles are as 16 : 2 : 1. In very young specimens the conducting power is but feeble, becoming fully developed later. In old specimens the diminished conducting power is due to permanent physio¬ logical decline.

I Experiment 45. — By the employment of constant current in excitation (see Chapter IX) I was able to determine the different excitabilities of very young, fully grown, and older leaves. The very young leaf at the top of the stem required a current of about 6 micro-amperes for effective excitation. The fully grown young leaf lower down was found to be more sensitive, a feebler current of 4 micro-amperes being effective in excitation. The older leaves below required a far stronger current for inducing excitation. The most sensitive leaf wras thus the second or the third from the top A piece of stem bearing such a fully grown young leaf is found most suitable for purposes of experiment.

1 now return to the temporary abolition of excitability after cutting the piece of shoot. It is to be borne in mind that after feach excitation the plant becomes temporarily irresponsive, and that the excitability becomes fully restored- only after the completion of protoplasmic recovery. A cut or a section acts as a very intense stimulus from the effect of which the recovery is comparatively slow. If the stem be cut very near the leaf, the excitation of the pulvinus is intense, and the consequent loss of excitability becomes more or less persistent. But if the stem be cut at a greater distance, the transmitted excitation is less intense, and the cut specimen recovers its excitability within a reasonable time. I have also succeeded in reducing the depression due to the intense wound-stimulus by previously benumbing the tissue by application of cold.

Quantitative Determination of the Rate of Decay of Excitability after Section Experiment 4b Variation of excitability after section . — In order to follow the history of the changes of excitability which are the immediate and after effects of cutting off the piece of stem, I took an intact plant and fixed the upper half of the stem in a clamp and connected the indicating leaf with the Resonant Recorder. The response of the leaf to direct stimulation by an induction shock of intensity o • i unit was now taken ; the specimen was vigorous and the response obtained was found to be at its maximum, ihe stem

bearing the leaf was then cut below the clamp at the moment marked in the record with a cross, and water was applied at the cut end. The effect of section was to cause the First response normal, x shows contractile fall of the leaf after wound. The subsequent responses exhibit gradual re¬ covery in staircase manner (Mimosa). maximum fall of the leaf, with subsequent recovery. Suc¬ cessive responses to test- stimulations taken afterwards at intervals of 15 minutes showed that depression of excitability had been induced owing to the shock caused by section, and that the excita¬ bility became restored in a staircase manner in the course of an hour and a half (fig. 48). This was the case with a vigorous specimen, but with less vigorous ones a longer period of about 3 hours was