Plant Autographs and Their Revelations

The velocity of transmission of excitation is appropri¬ ately modified according to the vital condition of the plant. It is greater in summer than in winter. Another curious fact observed is that while a stout specimen responds in a leisurely manner, a thin specimen attains its acme of excita¬ tion in an incredibly short time. Such a difference is not unknown even in the human species. In a thin leaf -stalk of Mimosa the speed may be as high as 400 mm. {)er second, or 24,000 mm. per minute. While the velocity of impulse in Mimosa is lower than in the higher animals, it is con¬ siderably greater than in the lower animals, such as Anodon. The velocity in the plant may therefore be regarded as half¬ way between the two. The velocity of transmission in both Mimosa and in animal nerve is increased, within limits, by rise of temperature and diminished by a fall. In Mimosa a rise of temperature of about 9° nearly doubles the velocity.

Fig. 106. The various physiological blocks are interposed at B, in the path of conduction. A certain length of the structure through which the impulse has to travel can be so treated as to retard or arrest its passage (fig. 106). Block hy Cold. — When part of the petiole was moderately lowered in temperature by the application of cold water, the transmission-time is shown by the record to have been prolonged. Excessive cooling by ice-cold water abolished the power of conduction (fig. 107). In order to show that the local application of cold abolished conductivity with¬ out affecting the excitability of the pulvinus, a direct

shock was given to the pulvinus which exhibited its normal response. In connection with this subject, I came across the inter¬ esting phenomenon of paralysis of conduction as an after¬ effect of intense cold, this paralysis persisting for over an hour, even after the return of the tissue to normal tempera¬ ture. Electrical shocks are found to be effective in curing- certain forms of paralysis in human subjects, and I dis¬ covered the very suggestive fact that the lost power of con-

Fig. 107. Effect of Cold in inducing retardation and arrest of Transmission. (i) Normal record; (2) Retardation due to slight cooling; (3) Arrest of conduction brought about by intense cold; (4) Record of direct stimulation. duction in the plant can be very cjuickly revived by sub¬ jecting the paralysed portion of the petiole to the action of tetanising electric shocks. Block by Poison. — I will next demonstrate the action of poison in permanently abolishing the conducting power. A strip of cloth half an inch in breadth was wound round the intervening length of the leaf-stalk, and a poisonous solu¬ tion of potassium cyanide was applied. The eff'ect of this was so great that the conducting power was abolished within as short a time as five minutes. Though the intensity

of shock was increased some eight times, yet there was no response. Direct stimulation of the pulvinus showed that its motility had undergone no change (fig. 108). Electrotonic Block. — In the above case the conductivity was permanently abolished ; in animal nerve, as already stated, a temporary block is produced by maintaining a constant electric current in the path of conduction. The block persists so long as the electric current is maintained.

(i) Normal record; (2) Arrest of conduction after application for 5 minutes; (3) Record showing arrest of impulse, even with strong stimulation ; and the conducting power is immediately restored on the stoppage of the blocking current. An exactly similar result was obtained with Mimosa; the block was put on at B and afterwards removed. This was repeated twice, and the record (fig. 109) shows that the transmission of excita¬ tion was invariably arrested whenever the blocking current was applied, and was restored on the cessation of the current.

Polar Excitation" by Electric Cu,rrent I will now adduce positive evidence in proof of the identical character of the transmission of excitation in the animal and in the plant. It is well known that an electric current causes specific exciting effects in the animal nerve. On suddenly sending a current through a nerve, excitation is produced at the point of the nerve which is the kathode, that is, where the current leaves the tissue. On sudden stoppage of the cur¬ rent, excitation is transferred to the anode, where the cur-

Fig. 109. Record of transmitted excitation with the block off and on. Arrest of transmitted excitation under electrotonic block B, B. rent entered the tissue. The excitation does not remain localized, but is conducted to a distance, shown by the contraction of the terminal muscle. A precisely similar result is obtained with Mimosa and other sensitive plants. I give a sketch showing these effects observed in Bio- phytum sensitiviim (fig. no) ; the figure to the left exhibits the excitation initiated at the kathodic point at the starting of the current, which was conducted in both directions. After the recovery of the leaflets, the circuit was broken and excitation was transferred to the point of anode.

plant at kathode-make and at anode-break, and since the impulse in both cases is arrested by intense cold, by the application of poison, and by an electrotonic block, the Fig. no. Illustration to the left shows excitation induced at kathode-make ; that to the right the effect of anode-break ( B iophytum sensitiznivi ) . inevitable conclusion is that transmission is essentially the same physiological process in both; if it be called ‘nervous’ in the case of the animal, there is equal reason for applying to it the same term in the case of the plant.

Having ascertained that there is nervous conduction in the plant, the next thing is to find out where and what the Fig. III. The Electric Probe for localisation of Nervous Tissue in the P, the probe in circuit with the galvanometer, G ; S, the screw-head, by the rotation of which the probe is forced into the petiole ; I, index by which the depth of intrusion may be determined. conducting tissue is. The passage of an impulse along a nerve does not produce any visible change ; we can, in fact, only detect its passage by the negative electric change

that accompanies it. The nerve imbedded in a non-con¬ ducting tissue may be likened to an electric cable sheathed in non-conducting gutta-percha. There may be a single conductor in the cable, or there may be two conductors. We can pick up messages going through the cable if we Fig. 1 12. Transverse and Longitudinal Sections of a Single Vascular Left figure: Transverse section. The dotted vertical line indicates the passage of the Electric Probe. C, cortex ; S, sclerenchyma ; P, external phloem ; X, xylem ; P^ internal phloem; O, pith.

Figure to right : Longitudinal section of the bundle. Note elongated tubular cells, both in the external and in the internal phloem. (The section passed through one side of the bundle and not through the middle.) thrust in a metallic pin suitably connected with a galva¬ nometer. No messages can be picked up until the pin just touches the conductor; the extent of intrusion of the pin tells us the depth at which the conducting strand is situated. ising the nerve imbedded in non-nervous tissue (fig. m) by making use of the Electric Probe already referred to in previous chapters. The distant end of the sub-petiole of Mimosa is periodically excited whilst the Probe is being thrust into the leaf-stalk by steps, say, of 0.05 mm. at a time. At first no electric change, indicative of nervous impulse, can be picked up; we have to probe to a greater depth. Now a message begins to be received, and at a certain depth the electric disturbance is most pronounced. We take note of this depth of intrusion. As the Probe is thrust in still deeper, the electric indication disappears : the Probe has traversed beyond the conducting tissue and entered a region of non-conducting tissue At a point still deeper, messages are again picked up, and beyond this no impulse can be detected. In this way it is possible to localise the conducting tissue within the hundredth part of an inch. These observations show that conduction of excitation is confined to a definite tissue, which may there¬ fore be termed a nerve.

We then cut a section of the leaf-stalk at the line of the passage of the Probe in order to find out at what points the Probe picked up the messages. The epidermis had given none; the cortex again was plainly a non-conducting wrap¬ ping; strong messages had been picked up when the Probe entered the phloem. As it passed to the xylem or wood, the messages ceased, but they recommenced at the next layer. The second conducting tissue thus detected is a second, internal phloem, until now unsuspected by the plant- physiologist. We have thus localised not only one nervous layer, but two (fig. 112). The significance of this double system of nerves, one external and the other internal, will be explained later.

If we can discover a stain that picks out the nerve- strands, already identified as the vehicle for nervous impulse, then the nerve distribution in the plant can be clearly made out. In this way it would be possible to distinguish two neighbouring systems of tissue having different functions, or to establish the similar functions of two tissues which happen to be separated from each other. The application of haema- toxylin and saffranin stained the nervous tissue a deep violet and made it stand out prominently from other tissues. This test confirmed the re¬ sults reached by the indications of the Electric Probe ; the outer and inner phloems were similar¬ ly stained, which indi¬ cates that they are, in fact, two separate nerves. In the petiole there are four such double strands, each pair starting from each sub-petiole and ending in the pulvinus.

In the stem of Mi¬ mosa itself there are two opposite main vascular bundles, each of which contains a double strand of nerves. These give off lateral branches to the leaves, thereby assuring con¬ ducting continuity between stem and leaves. An impulse initiated by stimulation of the stem can thus be sent in an outward direction to the leaves ; an impulse generated in Longitudinal section of stem bearing leaves on each side. The two ascending bundles F, F' give lateral branches to leaves, and meet at apex. The double phloem stained violet stands out against the background. Pulvinus of leaves shaded.

the leaves can, on the other hand, travel inwards to the stem and be then conducted up and down to the other leaves and may even cause their fall. It will also be noted that the two main strands of conducting phloem converge and meet at the apex of the stem (fig. 113). This explains how it is that under moderately strong stimulus applied on one side of the stem, the ascending impulse crosses over at the top Fig. 1 14. Frond of Fern; the exposed vas¬ cular strands, N, are shown in the en¬ larged figure to the right.

and becomes a descending impulse on the opposite side (see p. 192). The phloem-strand in the vascular bundle of Mimosa is thus shown to be a nerve conducting excitation. It is impossible to pull this out from Mimosa without tearing it to pieces ; I however succeeded in isolating the nerve from the leaf-stalk of a Fern. The hard casing of the leaf-stalk was broken carefully, and on pulling it apart the vascular nerve-strands were isolated ; they are soft, and white in

colour, remarkably similar in appearance to animal nerve (fig. 114). We will now apply the tests generally employed by animal physiologists on the nerve of the frog, to the isolated nerve of the Fern. The experiments on the frog’s nerve are carried out with the help of a galvanometer which re¬ cords the electric change in¬ duced by the nervous im¬ pulse. The electric records of the plant-nerve under varied conditions are found to be in every way similar to those of the animal nerve. The following ex¬ ample demonstrates this in a striking manner.

It is a well-known fact that a nerve, after lying too long idle, becomes more or less inert, and that it can be stirred into activity by being subjected to continu¬ ous stimulation or tetani- sation. The feeble response of the inert nerve becomes greatly enhanced after a period of tetanisation. This is illustrated in fig. 115, in which the first three re¬ sponses are those of the inert frog’s nerve; after tetanisation the responses are far more pronounced than at the beginning. Results exactly similar are obtained with the nerve of the Fern (fig. 1 16).

The Synaptic Membrane Protoplasmic continuity was at one time regarded as Fig. 1 15. Record of enhancement of amplitude of response as after-effect of thermal tetanisa¬ tion, in frog’s nerve. The first three responses are nor¬ mal. Brief thermal tetanisation is then applied, and the responses subsequently obtained under the original stimulation are enhanced. essential to conduction of excitation in plants. But in the animal there is no protoplasmic continuity across the nerve- junction where neurone joins neurone, the separating mem¬ brane being known as a synapse. This synapse acts like a valve, allowing the impulse in the nerve to travel more easily in one direction than in the opposite. Certain other

Fig. 1 16. Photographic record of effect of tetanisation, T, inducing enhancement of normal response in nerve of Fern. characteristics of the nervous impulse arise from the valve¬ like action of the synapse. A detailed examination of the nervous tissue of the plant shows it to consist of elongated tubular cells, the trans¬ verse septa of which act as synapsoidal membranes. The action of a valve will evidently be facilitated in consequence of its frequent opening by repeated stimulation. A hinge, rusty from long disuse, will require a stronger push than one that has been in frequent use which facilitates its working. A similar result is found in the transmission

of nervous impulse, where ‘the effect of passage of an impulse is to diminish the resistance, so that a second appli¬ cation of stimulus evokes the reaction more easily.’ ^ This characteristic reaction is known as ‘Bahnung’ or opening out of the path by frequent traffic. The characteristics of the nerve of the plant are found to be very similar to those of the animal. In the first place, though the impulse can travel in both directions, there is a preferential direction in which it travels more easily and with greater speed in consequence of the valve-action of the synapsoidal membranes. There is thus a greater facility for centrifugal than for centripetal transmission.

Again, Bahnung or facilitation is produced by previous stimulation. Thus in a typical case, the conducting power of the specimen was so low that the impulse due to the test stimulus applied on the leaf-stalk at a distance of 15 mm. from the pulvinus, failed to reach it. On application of a much stronger stimulus the impulse forced its way through the resistance and was transmitted to the pulvinus with considerable rapidity. The path being once made, it was easy for subsequent impulses to be transmitted, the pre¬ viously ineffective stimulus now becoming effective.

The following observations as regards the growth or degeneration of nervous activity through use and disuse are suggestive. A plant carefully protected under glass from the stimulating blows of the elements looks sleek and flourishing, yet in reality it is flabby and decadent. Anatomically, the nervous tissue is present, but from want of use it is func¬ tionally inactive. It is very interesting to watch, in a plant in this condition, the growth of nervous conduction under the influence of stimulating blows. There is at first no transmission ; after a time excitatory impulses begin to be transmitted; con¬ tinued stimulation enhances the conducting power to a maxi¬ mum.

Here we have displayed before us the modification of the organism by its environment, the creation of the organ by the cumulative effect of stimulation. The nerve unstimu¬ lated lies passive and inert; but stimulation energises it, and its excitability and conductivity become highly exalted. There is a particular aspect of the action of stimulus which is of utmost importance for the maintenance of the life of the plant. For the continuance of its normal activities the internal tissues have to be maintained in an optimum tonic condition by stimulation, which can only come from outside. Among the external stimuli naturally accessible to the plant none is more potent than light. All the con¬ ditions favor the transmission of its stimulating effect to the interior along the nervous channel — -the phloem in the vascular tissue. The pulsation maintaining the ascent of sap has been shown to come to a standstill when the plant is deprived of light; but after exposure to the stimulus of light the pulse-throb becomes renewed, the sap is again pumped up, and life riots through the channels which serve as arteries. The outspread leaf, in which the vascular bundles are distributed in fine ramifications, is not merely a special structure for the fixation of carbon from the carbonic acid gas in the air, but also a catchment-basin for the stimulus of light, the excitatory effect of which is gathered into larger and larger nerve-trunks for transmission to the interior of the plant. The distribution of vascular bundles in the interior is such that no mass of living tissue is too remote to be stimulated by the excitation conducted by these nervous channels.

Thus all parts of the plant are maintained by means of nerve-connection in the most intimate and rapid com¬ munication with each other. It can only be by virtue of the existence of a system of nerves that the plant constitutes a single organised whole, each part of which is effected by every influence that falls upon any other. In the animal, rapid means of communication between its different organs is often a matter of life and death, for when it becomes aware of any threatened danger by sight or sound, an urgent message is sent along the connecting- nerve to the organ of locomotion, which is immediately set in action to enable it to escape. The attitude of the creature is profoundly modified by the action of the stimulus ; if this be favourable to its well-being, it turns towards it, if unfavourable, away from it.

In the plant also two opposite reactions are induced under the action of moderate and of intense stimulation. The latter endangers the life of the plant, and I shall in a later chapter describe the characteristic movements by which the Mimosa plant evades any source of intense stimulation. I will here speak of the attitude assumed by the plant for the absorption of the stimuli which are for its well-being. It has been already pointed out that light is essential to the assimilation of carbonic acid by the plant; the greatest absorption of light take place when the upper surfaces of its leaves are perpendicular to the incident rays. This perpendicular adjustment is technically described as the dia- heliotropic attitude of the leaves.

Photographs of certain plants grown in my garden (fig. 1 1 7) show this quest for light. The middle figure is a Sunflower growing near a wall, the plant being exposed to light from the western sky. The leaves numbered i and 3 have undergone a twist — right-handed or left-handed — such that the upper surfaces of the leaf-blades are placed at right angles to the incident light. The figure to the extreme right shows that curvature and adjustment of a different species of Sunflower which was grown in the open. In the morning, it bent over to the east and all the leaves showed appropriate movements and torsions to face the light. In the afternoon the plant bent over to the west, all the previous adjustments and torsions being completely reversed. The plant continued to exhibit these alternate swings day after day, till the movements ceased with age.

The leaf-adjustment is shown in a still more striking manner by Mimosa, a photograph of which is reproduced on the left of the illustration. The plant grown in a pot had been exposed to the northern sky. It will be seen that the leaves which directly faced the light have been raised . and so placed that the sub-petioles with their leaflets are at right angles to the strongest illumination. The lateral leaves, on the other hand, have undergone appropriate twists, the plane of the leaflets being adjusted perpendicularly to the light; it will be noted that the petioles to the right and to the left have undergone opposite torsions. After the assumption of this position, the pot containing the plant was moved round i8o°. This brought about a new adjust¬ ment in the course of twenty minutes, the plane of all the leaflets being once more at right angles to the light. The new adjustment necessitated a complete reversal of the former movements and torsions.

How now are these movements produced? The leaves turn as if to bask in the sun. What happens when we turn our out-stretched palms towards the sun? In order to effect this, the very complex muscular machinery in the arm has to be set in action, producing a right-handed or left-handed twist, or a movement up or down. In the case of the leaf, some such complex movement in its mus¬ cular organ, the pulvinus, must be induced by the action of light. For the necessary observations on the plant, I will take Mimosa, in which the motility of the pulvinus is so very