The Nervous Mechanism of Plants

an animal nerve is arrested by the interposition of an electrotonic block, the conduction being restored on the cessation of the electric current. Similarly, transmission of impulse in the plant can be repeatedly arrested or restored by the alternate application and removal of the electrotonic block (pp. 29, 75). Block of conduction by local cooling.— The animal nerve when excessively cooled, loses its power of conduction.’ A parallel effect is observed in the nervous conduction in the petiole of Mimosa. Local application of ice paralvses the conducting power, which persists for a time even after the return to a normal temperature. The paralysis is

quickly removed by electric stimulation of the benumbed tissue (p. 74). application of poisonous solutions on a narrow zone in the petiole of Mimosa induces a depression of the power of conduction, which culminates in a permanent abolition. The time required for the abohtion depends on the virulence of the poison employed, being quicker under potassium cyanide than under copper sulphate solution (p. 77). These results conclusively prove that the transmission of excitatory impulse in plants is essentially similar to that Of the nervous impulse in animals.

The phloem in the petiole is now admitted by some observers to be the tissue that conducts excitation. On the protoplasmic continuity is essential for , — ^ ® possessing numerous perfora- p^toplasm, were regarded as the special conductors of ex- This supposition IS, however, erroneous for two the sieve-tubes discharge a different function ; and (2) the conducting phloem consists mainly of tubular cells with imperforate septa. Protoplasmic continuity is by no means essential to the conduction of excitation, for in the animal there is no such continuity across a nervejunction where neurone joins neurone, the separating membrane being known as a synapse. Similar synapsoidal membranes are shown to exist between contiguous conducting cells in the plant. °

The anatomical structure of the conducting phloem in the stem is m every way similar to that in the petiole • both contain the tubular cells which are effective ii^ conduction. Main conducting strands in the sfew.-— There are two opposite main phloem-strands in the stem which converge and rneet at the apex (p. 40). This particular distribution explains why, under unilateral stimulation of the stem the ascending impulse crosses over at the apex and becomes reversed at the opposite side (p. 5).

Nerve-connection between stem and leaves.— The. two main phloem-strands in the stem give off lateral branches to the leaves, thereby assuring conducting continuity between the stem and the leaves. The impulse initiated by the stimulation of the stem is propagated in a centrifugal direction to the leaves, while the excitation initiated by the stimulation of the leaf travels centripetally to the stem and is there conducted up and down, causing the fall of other leaves borne on the stem.

Distribution of conducting strands. — It is difficult to trace the course of the nerve-strands in the plant ; but on staining with haematoxylin, the conducting phloem becomes stained deep violet against the surrounding tissue. The phloem in each bundle is not single but double, the xylem being interposed between the two. The bundle is in fact bicollateral (p. 37). The imperforate septa between the tubular conducting cells act as synapsoidal membranes. The effect of the interposition of synapses in the path of nervous impulse in the animal is characterised as follows : (i) Conduction is irreciprocal or unequal in two directions : {2) Under mode-

rate stimulation, the resistance or block to the passage of impulse is diminished, a phenomenon known as ' Bahnung ’ or Facilitation : (3) Under excessive stimulation, an increase in the block occurs at each synapse, producing a fatigue of conduction. Irreciprocal conduction. — At the neuro-muscular junction of the animal, conduction is irreciprocal ; there is no obstacle to prevent the impulse from the nerve passing into the muscle, but the excitation of the muscle does not pass backwards into the nerve. Similarly, at the junction of the conducting and the contractile tissues of Mimosa, conduction is irreciprocal. Excitation of the conducting nerve passes easily into the pulvinus, but the contraction of the pulvinus does not affect the enclosed nerve-endings

Continuity between irreciprocal and preferential conduction.— Under a minimal stimulus, conduction is irreciprocal in various sensitive plants, i.e. it takes place in one direction and not in the opposite. Under increasing intensity of stimulus the S3iTiapsoidal block becomes more and more effectively forced. Conduction is, however, quicker in the preferential direction (p. 48). ‘Bahnung’ or Facilitation occurs as an after-effect of moderate stimulation. A formerly ineffective stimulus now becomes fully effective. Stimulus thus canaHses its own conducting path (p. 51).

_ Fatigue of conduction.— Pditx excessive or long con- Sj^e conduction exhibits a depression or _ The excitatory reaction has two concomitant expressions : the mechanical response of motile organs, and the electric response of all tissues, motile or non-motile. The fact that galvanometric negativity is a concomitant of excitatory reaction is proved by simultaneous observation of mechanic^ a.nd electrical response of Mimosa to transmitted excitation, when the two responses are seen to take place practically at the same time (p. 105). The electrical and mechanical responses are independent manifestations of the common excitatory process, for the response by galvanometric negativity takes place even when the leaf is restrained from physical movement. The fact that the excitatory process is independent of the process of contraction is proved by the persistent excitatory electric response of the pulvinus when rendered immotile by absorption of excess of water (p. 107). The transmission of excitation in ordinary plants is proved by the electric response, since it takes place even in non-motile tissues. The invisible nervous impulse is detected by the electric change of galvanometric negativity which accompanies it.

Localisation of the conducting nerve in the petiole of Mimosa by the Electric Probe. — On thrusting in the Probe perpendicularly to the diameter of the petiole by steps of 0-05 mm., an excitatory transmission inducing strong galvanometric negativity was found to occur only when one or other of the two phloems in each bundle, the external or the internal, was reached (p. 113). The two phloems are therefore the conductors of excitation. The existence of the two phloems in each bimdle was also demonstrated by selective staining.

Determination of the velocity of transmission by diphasic electric response.— Txffo electric contacts were made with the conducting nerve in the petiole of Mimosa and record obtained with the Einthoven string-galvanometer. The excitatory impulse arriving at the proximal contact A induced electric negativity of that point ; the later arrival of the impulse at the distal point B produced a reversal of the recorded curve of response. The time-interval between the two phases of response, and the distance between A and B, sufificed for the determination of the velocity, which was the same as that obtained by the mechanical method (p. 109).

Electric response to transmitted excitation in ordinary plants. — 'Examination of the midrib of leaves showed that the phloem contains tubular cells similar to those which conduct excitation in Mimosa. The transmitted excitation along the conducting phloem has been detected by electric response of galvanometric negativity. The various characteristics of conduction evidenced by mechanical and electric response of Mimosa have also been found in the electric response of ordinary plants. The conduction is irreciprocal under feeble, and preferential under stronger, stimulation (p. 102). ' Bahnung’ or Facilitation of conduction is observed, the removal of block by successive stimulations being demonstrated by a staircase-increase of the electric response (p. loi).

N ervous conduction takes place not only in the ‘ sensitive, ’ but in all plants. Two distinct impulses are generated under indirect stimulation, a positive followed by a negative. The velocity of the positive impulse is greater than that of the negative : but the reaction to the positive is relatively slow and feeble, whereas that due to the negative is abrupt and intense. Effects of positive impulse. — ^The positive impulse gives rise to an enhancement of turgor, causing (i) expansion

and erectile response of the motile leaf or leaflet ; (2) acceleration of the rate of growth of a growing organ ; (3) an electric response of galvanometric positivity. Effects of negative impulse. — ^The negative impulse is excitatory, and gives rise to a diminution of turgor, manifested by (i) contraction and responsive fall of the motile leaf ; (2) retardation of the rate of growth ; (3) an electric response of galvanometric negativity. The resultant response is determined (a) by the intensity of the stimulus; (b) by the distance of transmission; (c) by the conductivity of the tissue ; and (d) by the tonic condition. The two responses of opposite sign, a feeble positive followed by a stronger negative, are exhibited by a semi-conducting tissue. On reducing the intervening distance, the predominant negative overtakes and masks the feeble positive (p. 85). When the distance is reduced to zero, stimulation becomes Direct, and the resultant response is one of contraction, of retardation of the rate of growth, and of galvanometric negativity. Under the condition of sub-tonicity, when conductivity is depressed, the excitatory impulse is arrested and the positive impulse is alone transmitted.

The following is the Law of Effects of Direct and In- direct Stimulation: (1) The effect of all forms of Direct Stimulation IS A DIMINUTION OF TURGOR, CONTRACTION, Accession and Depletion of Energy under the Action of Stimulus Stimulation does not always cause a depletion or rundown of energy D ; under certain conditions it causes an accession and storage of energy A. The relative intensity of the two reactions is modified in a definite mariner according to the tonic condition of the tissue ; when the tonic level is above par, the D reaction is predominant (D > A) ; but when the tonic condition is below par, the accession of energy is the more pronounced (A >D), the tonic level being thereby raised to the normal. Under repeated stimulation the arrested power of conduction is restored by the gradual removal of the block offered by the synaptic membranes. The fundamental protoplasmic reaction under the action of stimulus shows parallel changes in the three manifestations of irritability. In a condition of sub-tonicity the motor-excitability, conductivity and rhythmicity are all depressed or arrested ; the arrested activities become revived under stimulation. The effect of stimulation on tissues in an optimum condition is the opposite of that on sub-tonic tissues, depression or fatigue being induced in the various manifestations of irritability.

The nerve of the Fern can be isolated without injury ; it is a vascular strand which includes conducting phloem consisting of tubular cells. The characteristics of conduction in the plant-nerve are in every way <;imi1?r to those in animal nerve, both in normal and modified conditions. In the normal nerve, the negative impulse masks the positive, and the resultant response to indirect stimulation is negative. Conductivity is depressed or temporarily abolished under a condition of sub-tonicity ; of the two impulses the excitatory negative is now arrested, and the positive alone is transmitted. This explains the abnormal positive response of animal and plant nerves to indirect

stimulation. The block to the passage of excitatory impulse is gradually removed under successive stimulations, and the response becomes diphasic, positive followed by negative. The conductivity is fully restored after a period of tetanisation or continuous stimulation, and the abnormal positive becomes converted into the normal negative response (p. 128). Since the results are identical in plant and animal nerves, the physiological mechanism must be the same in both.

The identical nature of the physiological mechanism of nervous conduction in plant and animal is further demonstrated by similar modifications of conduction under the directive action of a constant electric current. A current is homodr omous when its direction is the same as that of the propagated nervous impulse ; it is heterodromous when opposed to the direction of propagation. Effect of heterodromous current. — ‘The velocity of transmission is enhanced when against the direction of the current. The intensity of a transmitted excitation is also increased. Ineffectively transmitted excitation becomes effectively transmitted (p. 141).

Effect of homodromous current. — ‘The velocity of transmission is depressed. An effectively transmitted excitation becomes arrested during the passage of the current Immediate after-effect. — K ‘ rebound ’ takes place immediately after the stoppage of the current. The result ‘ is a transient variation of conductivity, whose sign is opposite to that induced during the passage of the current. The transient after-effect of a heterodromous current is a depression of the normal conductivity ; that of a homodromous current, an enhancement of conductivity above the normal (p. 142).

There is a critical intensity of current above which the above effects undergo a reversal. The following are the Laws of Conductivity Variation in plant and animal nerves, under electric current of feeble intensity : The four quadrants of the pulvinus of Mimosa act as four different effectors with characteristic responsive leafmovements. Under direct stimulation, the upper quadrant responds by an up-, and the lower quadrant by a downmovement ; the response of the left quadrant is by an anticlockwise, and of the right quadrant by a clockwise torsion

The surface of the leaf places itself at right angles to the direction of the incident light. In Mimosa, the motor organ is at some distance from the perceptive lamina ; the attitude of the leaf is modified by separate nervous impulses transmitted from the four sub-petioles bearing the leaflets when stimulated by fight. The definite nerve-connection between each of the sub-petioles and the corresponding quadrant of the pulvinus is proved independently (i) by peripheral and (2) by central stimulation.

Characteristic effects of peripheral stimulation. —Sirniuh.- tion of sub-petiole (i), by moderate electric or photic stimulation induces an anti-clockwise torsion, similar to that produced by direct stimulation of quadrant (i). Subpetiole (i) must therefore be in nervous connection with quadrant (i) of the pulvinus. Similarly, sub-petiole (2) is shown to be connected with quadrant (2), sub-petiole (3) with quadrant (3), and sub-petiole (4) with quadrant (4 (p. 168).

Effects of central stimulation. — Stimulation of the central nerve-end in the left quadrant (i) generates an excitatory impulse, which travelhng outwards causes response only in the leaflets of sub-petiole (i). Local excitation of the nerve-end in quadrant (2) causes response in sub-petiole (2). Stimulation of nerve-ends in quadrants (3) and (4) elicits corresponding response in sub-petiole (3) and in sub-petiole Co-ordinated reflex in leaf-adjustment. — When the leaflets of the right sub-petiole (4) are acted on by vertical light, the distant pulvinus undergoes a right-handed or clockwise torsion, by which the leaflets are carried away from a position at right angles to the incident light. But when the two sub-petioles (i) and (4) are simultaneously exposed to light of the same intensity, the two resultant torsions balance each other. The lateral adjustment of the leaf as a whole is thus made by the two sub-petioles (i) and (4) situated externally. The balancing adjustment up or down, is effected in response to excitations transmitted by the two middle sub-petioles. Equilibrium is attained when the leaf-surface as a whole is perpendicular to the incident light. ' The dia-heliotropic attitude of the leaf is thus brought about by distinct nervous impulses, initiated at the perceptive region actuating different effectors at a distance (p. 171).

^ A minimal stimulus applied to one of the sub-petioles gives rise to an in-going impulse causing the characteristic response of the corresponding quadrant of the pulvinus • the excitation remains localised without irradiation to the other nerve-ends. Under a slight increase of the intensity of stimulus, a different class of phenomenon makes IS appearance ; the in-going or afferent impulse, reaching the centre, now becomes reflected along a new path as

an efferent impulse. Thus a reflex arc is formed at the centre (p 175). Relatim between the afferent and the efferent impulses.— Stimulation of sub-petiole (i) or sub-petiole (4) gives rise to a single reflected impulse ; stimulation of sub-petiole (2) or (3) gives rise in each case to two reflected impulses These characteristic effects are explained by the mutual relation of the contiguous nerve-ends at the centre (p 18^) External stimulus applied at the periphery generates an afferent or sensory impulse, which, after reflection at the centre, is transformed into an efferent or motor impulse. There is a marked difference in the velocity of the two i^ulses, by which they may be discriminated from each other, the motor impulse being about seven times as quick as the sensory (p. 182). The transformation of the sensory into the motor in the reflex arc connotes not

Separate conducting nerves in the samebundle.— Div&ct evidence is adduced in demonstration of the existence of two separate nerves for the motor and the sensory impulses he characteristic of the motor nerve being that conduction at a much quicker rate than in the sensory, that there are two conducting phloems, one and the other internal, in the same bundle has 1st c of the sensory impulse ; stimulation of the internal phloem gives rise, on the other hand, to an impulse wSh

li^i^the d ^ The real difference he ife T- Under natural conditions, the centre by the external phloem; the excitation is then tr^sferred to the inner phloem of the contiguous nerve and conducted outwards as the efferent impulse (p. 191). Successive Reflexes under Stronger Stimulus AND THE Lost Time ' in Reflex A more widespread response ensues on the application of a stronger stimulus. Stimulation of the left subpetiole (i) then causes successive reflexes at the centre the sub-petioles (2), (3) and (4) responding one after another in the sequence 2-3-4. Stimulation of the right sub-

petiole (4) causes successive reflexes in the reverse order of 3-2-1. The ‘ lost time ’ in reflex.— time lost in the second reflex is normally about 4 seconds ; on account of decrement. it is longer at the third reflex. The total lost time in the second and third reflex is from lo to 19 seconds. Abolition of ‘block’ under strychnine. — The block, causing ‘ lost time ’ in reflex in the animal nerve, is entirely removed after administration of strychnine. A similar effect is also observed in the reflex of Mimosa ; the resistance to the passage of the impulse at the centre becomes completely removed by the absorption of the proper dose of strychnine. The ‘ lost time ’ is then reduced to zero, as is shown by the simultaneous response in all the sub-petioles.

In the light of the results summarised in this chapter, it can no longer be doubted that plants, at any rate vascular plants, possess a well-defined nervous system. It has been demonstrated that excitation is conducted by the phloem of the vascular bundle, and that conduction in this tissue can be modified experimentally by the same means as is that in animal nerve. The conducted excitation may, therefore, be justly spoken of as nervous impulse and the conducting tissue as nerve.

It has been further shown that, as in the animal, it is possible to distinguish sensory or afferent and motor or efferent impulses, and to trace the transformation of the one into the other in a reflex arc. The observations involve the conception of some kind of nerve-centre. No structure corresponding to the nerve-ganglion of an animal has, indeed, been discovered in the pulvinus of Mimosa pudica, but it is not impossible that the physiological facts may one day receive histological verification.