Plant Autographs and Their Revelations

pronounced. It used to be thought that the motor machinery of the leaf was simple, permitting only a single up or down movement. The results of my investigations show, however, that this is by no means the case ; for the mechanism is capable of producing very complicated movements, not merely up and down, but also twists to the right and left. The pulvinus itself may be regarded as consisting of four quadrants, left and right, upper and lower, numbered

Fig. 1 18. Showing the course of four nerve-strands from the four sub¬ petioles to the pulvins. {Mimosa pudica.) The lower figure is a diagrammatic section of the pulvinus with its four quadrants. Quadrants i and 4, which give rise to left- and right- handed torsions, are respective!}^ in nervous connection with sub¬ petioles I and 4. The lower efifector 2 is connected with sub-petiole 2, the response being a rapid down-movement. The upper quadrant 3 is in connection with sub-petiole 3, the response being a slow up- movement. The observer is looking towards the parent stem.

I, 4, 3, and 2 respectively (fig. 118). Now when the left quadrant i is locally stimulated by a feeble electric shock or by a beam of light, the leaf answers not by a fall but by a left-handed twist. If the stimulus be transferred from the left to the right quadrant 4, the response is a right- handed torsion. Stimulation of the upper quadrant 3 pro¬ duces a slow up-movement, while that of the lower quadrant 2 causes a more rapid down movement. The leaflets attached to the four sub-petioles are carried like so many flags by these muscle-like reactions of the four quadrants.

If the green leaflets carried by the four sub-petioles are kept covered so as to be sheltered from light, and if sunlight com¬ ing from the east strikes the right quadrant 4, the leaf as a whole twists to the right, carrying all the leaflets on the four sub-petioles with it, as if to face the sun which they cannot see. On the other hand, if the sunlight from the west strikes the left quadrant on the opposite side, the twist is reversed and the leaflets face the west.

So much for the turning movements caused by direct stimulation of the motile organ by light. But under natural conditions the pulvinus is shaded from direct light by the shadow cast by the leaflets. How then does the leaf turn to the light when the motile organ is not directly stimulated? Nothing simpler: by the pulvinus being made aware of the state of affairs through a message from the leaflets. I have shown elsewhere ^ that there are definite nerve- connections between the four sub-petioles and the four quadrants. Thus a moderate electrical stimulus applied to the left sub-petiole initiates a nervous impulse which, reaching the left quadrant i, produces its characteristic response, namely, a left-handed torsion. In the same way, stimulation of the right sub-petiole produces a right-handed torsion. Stimulation of the middle sub-petioles produces up or down movements. These results are also obtained if the sub-petioles carrying the leaflets are stimulated not by an electric shock but by the stimulus of light.

A man rowing a boat, when pulling only one oar, circles round and round without any forward movement. It is when both the oars are pulled with equal vigour that a directive and purposeful movement can be produced. If a number of moths are anchored by means of thin threads ^ ‘The Dia-Heliotropic Attitude of Leaves as Determined by Trans¬ mitted Nervous Excitation,’ Proceedings of Royal Society, B, vol. 93, 1922, p. 153. which do not interfere with the free movement of the wings, they will be found all pointing to the light. This is because both their eyes are then equally stimulated by the light, and the nervous impulse transmitted to the wings makes them beat at the same rate and with equal vigour. Had one moth by chance been aslant to the light, then one eye would have received a stronger stimulus than the

Thus in the plant, a slanting beam of light falling on one of the sub-petioles, say number i on the left, causes a twist, by which the leaf turns round till the fourth sub¬ petiole comes into the line of light; but the effect of stimulation of that sub-petiole will tend to make the leaf twist to the right, putting on a break, as it were, to the former movement. The two opposing reactions balance each other when the two sub-petioles are equally illu¬ minated, this being the case when they are perpendicular to the incident light. The lateral adjustments of the leaf as a whole are thus made by the two sub-petioles i and 4, which are situated externally. The balancing adjustments up or down are made in response to nervous excitation transmitted by the two middle sub-petioles. It is clear that equilibrium is only possible when the leaf-surface, consisting of the leaflets carried by the four sub-petioles, is equally illuminated, and this can only occur when the upper surface of the leaf as a whole is perpendicular to the incident light. Thus the leaf is adjusted in space by the co¬ ordinated effects of four nervous impulses initiated at the perceptive region of the leaflets actuating the four motor quadrants in the pulvinus at a distance.

So we have four brothers, sun-lovers all, pledged to share whatever sunshine there may be, but each with his own pull at head-quarters for the movement of the whole. Imagine them lying with their faces to the sky. The sun ascending in the east throws a ray which touches the right- hand brother ; he immediately sends the message to his quadrant of the pulvinus, which gives a twist to the right, moving not only the scout who sent the message, but the whole squad to face the sun. If the leaf tends to swing too far, scout number 4 rectifies the matter by a pull the other way.

After all, it is Helios, the Sun-god, who is the source of all movement on earth and of all living beings. It is he who pulls us out of our beds each day, who draws water from the equator to the top of the Himalayas, makes our rivers run and causes the winds to blow. In such mighty work, he does not neglect even the smallest leaf, upon which he descends and of which he makes his chariot. The four nerves of the leaf are as so many reins, by the guidance of which the chariot is raised or lowered, or made to swerve either to the right or to the left.

When a child puts its finger on a flame, before it has had time to make up its mind to cry, the arm is automatically withdrawn. This is brought about by a wonderful reflex mechanism. The strong stimulus of the burn gives rise to an ingoing, sensory or afferent impulse, which, reaching a nerve-centre, becomes reflected and transformed into an outgoing motor or efferent impulse ; this impulse travelling along a new path causes the rapid withdrawal of the hand, which is an involuntary and automatic action. During one of my lectures in London, a late-comer, finding all available space occupied, sat on what he took to be a raised seat, and immediately jumped off — it was a hot-water pipe! This was an unrehearsed experiment in reflex action !

A diagrammatic representation of the reflex arc at a nerve-centre is represented in fig. 119. S is an intense stimulus which impinges upon the surface of the skin ; the sensory nerve conducts the afferent impulse A to the nerve-centre N^C; the impulse now becomes reflected as an efferent impulse E and is sent along the motor nerve inducing contraction of the terminal muscle M. The two nerves, afferent and efferent, though distinct, run together and form what is called a mixed nerve.

I have been able to discover a parallel arrangement in the leaf of Mimosa. It has been shown that there are four vascular bundles, by which the four sub-petioles are put into nervous connection with the pulvinus. Investigation with the Electric Probe showed that each bundle contains two nerve-strands, one external, the other internal and therefore relatively more protected. The existence of two nerve-strands in each bundle was further proved by selec¬ tive staining. What can be the object of this dupli¬ cation ?

In investigating this question stimulus was applied on one of the sub-petioles, say the left, and was gradually increased step by step. The simplest and most convenient way of doing this is by increasing the intensity of an electric shock given by a coil. When the stimulus is feeble or moderate, the response, as already explained, is a left-handed torsional movement. But when the intensity of the stimulus is increased a new class of phenomena makes its appearance.

We will follow the course of the im¬ pulse generated by a moderately strong stimulus applied on the left sub-petiole. The inward passage of excitation along the sub-petiole is shown by the succes¬ sive upward closure of the leaflets. The normally outspread leaflets seen from above appear vivid green ; after the pas¬ sage of an impulse the closed leaflets become an inconspicuous line of grey. When the impulse reaches the small pulvinus of the sub-petiole, this is made to move laterally towards its fellow, the next sub-petiole. The impulse then enters the main petiole and its passage gives no sign for the time being. After a while its arrival at the pulvinus is signalled by the fall of the leaf. The time required for the afferent or in¬ going impulse to reach the pulvinus of the leaf is determined by observing the interval between the closure of the inner¬ most pair of leaflets and the fall of the leaf.

The work done by the ingoing impulse does not end here, for this impulse now becomes transformed in the pulvinus into an outgoing impulse which travels along a new path in the opposite direction. The outgoing impulse reaches the periphery, and its reversed direction of propagation is shown by successive closure of the leaflets of the second sub-petiole taking place in an outward direction (fig. 120). Hence it is clear there is a Reflex Arc in the leaf of Mimosa. The afferent nerve may be described as sensory, the efferent nerve as motor.

A, Moderate stimulation of sub-petiole (i) gives rise to an afferent impulse which causes fall of leaf (not shown in the figure). The afferent impulse is reflected from the pulvinus as an efferent impulse which causes re¬ sponse of leaflets on sub-petiole (2). Afferent impulse represented by full arrow and efferent by dotted arrow. of the leaf and the closure of the inner pair of leaflets of the second sub-petiole gives the speed of the motor impulse, which is significantly different from that of the sensory impulse. I have found that these two impulses are conducted by different nerves, the afferent or sensory impulse being conducted by the external, and the efferent or motor impulse by the internal nerve. This is the answer to the inquiry

(p. 203) as to the significance of the presence of two nerves in each vascular bundle. Under a still stronger stimulus applied on the first sub-petiole, the effect transmitted becomes more widely irradiated, and the sub-petioles numbered 2, 3, and 4 exhibit response in serial succession. These characteristic features of the nervous reaction of Mimosa under increasing intensity of stimulus are remarkably similar to those of the nervous reaction in the animal, where increased intensity of stimulus enables the impulse to spread along more neurons and evoke a more widespread response.

The study of the time-relation of these reflexes shows that there is a definite interval of several seconds between the arrival of the sensory impulse at the pulvinus and the departure from it of the motor impulse. This is the ‘lost time’ in the reflex ; it is the time occupied by the transition of the impulse from the afferent to the efferent nerve in the pulvinus, which indicates that there is a resistance or block to be overcome. Similar conditions are known to exist in the nerve-centre of the animal, where the block is entirely removed by the administration of strychnine. I find that strychnine has a similar effect upon Mimosa, the block being abolished on treatment with a dilute solution of the drug.

In the animal the essential difference between the sensory and the motor impulse is that the one travels towards the centre and the other away from it. It is impossible to tell whether they travel with the same or different velocity. In the plant it is comparatively easy to measure the two speeds, and it is a surprising fact that the outgoing motor impulse is the quicker of the two. The speed of a nervous impulse, usually speaking, decreases with the distance travelled, and one would think that after the arrival of the sensory impulse at the centre the speed of the reflected impulse would be diminished, since it has had to travel a longer distance, i.e. instead of travelling through the single length of the petiole it has to traverse double that length. My investiga-

tions, however, show that the speed of the outgoing motor impulse is at least six times greater than that of the incoming sensory impulse. The change of a sensory into a motor impulse in the reflex arc therefore connotes not merely a reversal in the direction of propagation, but a great dis¬ charge of energy at the centre, by which the motor impulse becomes far more intense than the feeble sensory impulse that provoked it. It would thus appear that the centre has a speciflc executive function, and that a considerable amount of energy is stored there. The sensory impulse arriving at the centre pulls a trigger and the motor impulse is in consequence discharged with almost explosive intensity and rapidity. It will presently be shown that the principal function of the motor impulse is the readjustment of the outlying organs to meet a threatened crisis. There must always be a cease¬ less alertness and an immediate action in response to the general need, for any disharmony means the destruction of the plant commonwealth.

Moderate stimulation, as has been explained before, makes for the well-being of the plant, such stimulation giving a healthy tone to the organism. Intense stimulation, on the other hand, is inimical to life. Let us see how a moderate or an intense stimulus affects the attitude of an animal. When a kitten is gently stroked, it expands into a round ball, purrs with delight, and is attracted towards the person who is caressing it. But when a big stick is substituted, the nature of the response undergoes a radical change. Instead of expansion there is violent contraction; instead of joyous purring, a painful screech; instead of attraction, a quick repulsion and straight jump for the open door. If the door be closed, the creature hides itself under the sofa. The attitude of the Mimosa likewise is characteristically modified by moderate or by excessive stimulation. The stimulus of light being of advantage to the well-being of the plant, the leaf adjusts itself so as to receive the largest amount of it. But when the light is so intense as to be harmful, a sensory message comes to the centre as a danger signal, and

the order sent out to the outlying organs is for immediate withdrawal. Mimosa pudica is a weed which covers large areas of ground in the tropics. The procumbent stems bear numerous leaves, so that the entire tract appears as a mass of vivid green. The danger which threatens the life of the plant is from grazing cattle, the attack of which is followed by the sudden fall of the leaves. It has been suggested that the movement of the leaves serves the purpose of scar¬ ing the cattle, though they are already accustomed to the swaying branches of trees. Moreover, the cow is not sufficiently intelligent to notice the slight movement of the leaves of Mimosa, nor would it be frightened by it. The nervous reflex may, however, subserve the protection of the plant in a different way. When one of the sub-petioles bearing leaflets is trampled on or bitten, the excitatory im¬ pulse is immediately transmitted throughout the length of the plant. In regard to the behaviour of a particular leaf, the afferent or sensory impulse causes its fall so that it presses itself close to the ground. The reflexes at the centre give rise to motor impulses which make the four sub-petioles approach each other laterally and also cause upward closure of all the leaflets. Nothing could be more startling than the rapid change, by which large patches of vivid green thus become transformed into thin lines of dull grey unnoticeable against the dull background of the soil. Like the kitten hiding under the sofa, the plant escapes danger by making itself invisible!

It is one of the greatest of all mysteries how we are brought into contact wuth the external world; how blows from without are felt within. Our sense-organs are like so many antennae, radiating in various directions and picking up messages of many kinds. All of these, when analysed, are found to consist of shock-effects transmitted along different nervous channels. These shocks are caused by mechanical strokes or by impacts of aerial or aethereal waves, and we perceive them as touch, sound, or light. These messages bear, moreover, a certain potentiality to induce in us a sensation which may either be agreeable or disagreeable. The quality of the sensation is often affected by the intensity of the impinging stimulus. It is well known that while a gentle touch or a moderate stimulus of light, heat, or sound may produce a sensation which may be described as pleasant, an intense stimulus of the same nature causes a sensation which is extremely unpleasant or even painful.

The intensity of the impulse that reaches the perceptive organ depends on two factors — the strength of the external stimulus and the condition of the vehicle that conducts the impulse. Under normal conditions extremely weak stimu¬ lation gives rise to an impulse which is so feeble that it remains below the threshold of perception. Moderate stimulation gives rise to responsive sensation not un¬ pleasant; very strong stimulation, on the other hand, causes an intense reaction of a painful character.

nervous excitation that reaches the central organ. We are subject to human limitations, through the imperfection of our senses on the one hand, and our over-sensibility on the other. There are happenings which elude us because the stimulus is too feeble to waken our senses ; the external shock may, on the other hand, be so intense as to fill our life with pain. Since we have little power to alter the external world, is it possible to control the nervous impulse, so that it shall be exalted in one case and inhibited or obliterated in the other? Does science hold out the hope of any such possibility? This question is plainly fraught with high significance.

The solution of the problem of the modification of nervous impulse and the resulting sensation depends on the discovery of some means of affecting the impulse during transit, so that it can be either intensified or inhibited. How can this be effected? The nerve-circuit may be likened to an electric circuit with transmitter and receiver connected by a conducting wire. The invisible electric impulse sent along the con¬ ductor is detected by the twitch of the needle of the receiving galvanometer. Application of extremely feeble current in¬ duces no answering twitch ; the response of the galvanometer becomes perceptible and then increased under increasing intensity of the impinging electric force. When this force is very strong the answering movemient of the needle becomes violent.

In the nerve-circuit parallel effects are observed as demonstrated in a more concrete way by substituting for the galvanometer needle a contractile muscle, which is also a substitute for the perceiving brain. An extremely feeble stimulus applied at the end of the nerve gives rise to an impulse which is below the threshold of response. Stimulus of moderate intensity gives rise to an impulse which causes a moderate twitch. A very strong shock gives rise to a violent contraction of the muscle; the magnitude of the

response measures the intensity of the transmitted nervous impulse. There is much resemblance between the conduction of electric impulse by a metallic wire and that of excitatory im¬ pulse by a nerve. In the metal the power of conduction is constant, and the intensity of the electric impulse simply depends on the intensity of the electric force that is applied. If the conducting power of the nerve were constant, then the intensity of the nervous impulse and the resulting sensation would depend entirely on the intensity of the impinging stimulus. In that case, modification of sensation would be an impossibility. But there may be a possibility that the power of conduction possessed by a nerve is not constant but is capable of change, so that resistance to the passage of an impulse can either be decreased or increased. Should this surmise prove correct, then we would arrive at the momentous conclusion that sensation itself is modifiable, whatever be the external stimulus.

The modification of nervous impulse can be attempted in either of two ways. In the one case, we may render the nervous path super-conducting so that the impulse due to a sublimal stimulus may be brought into sensory prominence. In the other case, we may block out the pain- causing impulse of intense shock by rendering the nerve a non-conductor. Under a narcotic the nerve becomes a non-conductor and we may thus save ourselves from pain. But such heroic measures are only resorted to in extreme cases, as when we are under the surgeon’s knife. In actual life, we are confronted with unpleasantness without previous notice. A telephone subscriber has the evident advantage that he can switch off the connection when the message begins to be unpleasant. Statesmen and politicians have been known to cultivate convenient deafness; but that is mere pretence, for unpleasant remarks continue to rankle. Few have the courage of Mr. Herbert Spencer, who

openly resorted to his ear-plugs when his visitor became tedious. The thrill produced by stimulation of the receptive outer end of the nerve is sent inward from point to point of the conducting thread as a molecular disturbance. Imagine a row of billiard balls touching one another. A blow given to the first ball is communicated to the next and next, till the last ball starts ofif from the rest in acknowledgment of the blow. None of the other balls appears to have been moved from its place. In the same way each particle of the nerve, though keeping its own place, passes on the impulse.