The Motor Mechanism of Plants

Very strong indirect stimulation produces a diminution of pressure, an effect similar to that of direct stimulation. The alternate expansion and contraction of the cells active in the propulsion of sap have been recorded by the Magnetic Sphygmograph, which magnifies from 5 to 10 million times. The automatic records clearly exhibit the working of the cellular pumps ; any agent which enhances the pumping activity causes an increase of sap-pressure indicated by an ascending curve, the expansive up-stroke of the constituent pulsation being larger than the contractile down-stroke. Conversely, depressants diminish the pressure, as shown by a descending curve, the down-stroke of the individual beat being larger than the up-stroke.

Ligature produces arrest of cellular pulsation. The arrested pulsation can, however, be renewed by direct or indirect stimulation. 1 propose to study next the effects of various drugs on the propulsion of sap and on the circulation of the blood. The characteristic effect of a drug in enhancing or de¬ pressing the sap -pressure is manifest in the record given by the Optical Sphygmograph. 3 a order to prove that the particular solution produces an identical action on the cardiac tissue,- a parallel experiment was carried out with the animal heart. The stimulating character of a. drug is demonstrated by enhanced frequency or increased ampli¬ tude of the heart-beat recorded by the Resonant Cardio¬ graph, while the effect of a depressant is evidenced by the diminution of the amplitude or by the slowing down of the pulsation. For facility of comparison I will reproduce the characteristic cardiogram of the animal and the spliygmo- gram of the plant under the action of clifferent drugs. These may, according to their action, be conveniently divided into three classes : (i) stimulants, which produce in general an enhancement of activity ; (2) depressants , causing an in¬ hibition of activity; and (3) stimulant-depressants , producing stimulation 111 minute doses ana depression in moderately large doses.

For the detection of changes of activity in the animal heart, the Cardiograph is more direct and sensitive than the Sphygmograph. But the only apparatus which records the pulse-beat of the plant in the propulsion of sap is the Magnetic Sphygmograph. The similarity of effect of drugs on animal and plant may be tabulated as follows : Table XXIII. — Parallel Effects exhibited by Animal- Cardiogram and Plant-Sphygmogram Increased sap-pressure, up-stroke of pulse larger than down-stroke Diminished sap-pressure, j down - stroke larger than up-stroke

The result, both in animal and plant, is modified (i) by tonic condition of the tissue, and (2) by the dose and the duration of application. Experiments were carried out with the heart of a fish (Opbiocephalus) and of the frog, the results obtained being very similar to each other. The effect of drugs on the heart of the fish is described elsewhere.1 The cardiograms given below were obtained with the heart of the frog. The sphygmograms of the plant were taken with shoots of Cosmos, Centaurea, and Antirrhinum.

Experiment 232. Effect on animal heart.— Dilute solu¬ tion of camphor causes an enhancement of the activity i iw. 227. Effect of dilute solution of Camphor on heartbeat of Frog. Normal feeble pulsation (upper record), enhanced by application of camphor (lower record). of the animal heart. The stimulative effect is more easily shown with a specimen which is in a somewhat depressed condition. It is obvious that little further enhancement can be induced in a heart that is already in a condition of

maximum activity. The record (fig. 227) shows that the feeble activitv of the heart was greatly enhanced after the application of dilute solution of camphor. The various solutions may be applied on the heart for absorption, or they may be intiodueeci by hypodermic injection. Experiment 233. Effect on [riant. — The effect of campnor on the plant is shown in fig. 228 ; the original amplitude of pulsation in ’ a state of balance was feeble, so the record appears as a horizontal line Application of dilute camphor produced a sudden increase of pressure, shown by the ascending curve, in which the constituent pulsations show the up-stroke to be larger than the down-stroke.

Fig. 228. Effect of Camphor in en¬ hancing the sap- pressure by in¬ creasing the pun p- ing activity. I obtained similar results with other stimulants such as musk, provided tht^ dose is not excessive (see figs. 230, 231). Note increase of pres¬ sure indicated by EFFECT OF DEPRESSANTS of each pulse larger Morphine may be taken as a typical vAntirrh?nurn)!r°kL depressant which lowers the pulsatory! Experiment 234. Effect on ammo! heart. — lhe normal ! activity undergoes depression when the narcotic is injected into the heart (see fig. 232).

Experiment 235. Effect on plant. — The depressing action | of morphine on the pulse-beat is shown in the record (fig. 429)- The sap-pressure is seen to undergo a rapid depression, the constituent pulsations of the descending curve exhibiting * down-strokes of each pulse larger than the up-stroke. The most remarkable results were obtained in both animal and plant by' the alternate application of a de¬ pressant and a stimulant. For a depressant I us£d a solution of bromide of potassium.

Experiment 236. Antagonistic reactions in animal heart. — After application of solution of bromide of potassium, great depression occurred in the amplitude of pulsation. A solution of musk, one part in a thousand, was next applied ; it not only neutralised the depressing action of th% bromide, but considerably en¬ hanced the activity' (fig. 230). Experiment 237. Parallel reactions in plant. — The effect of bromide was to produce a great depression, the diminished sap-pressure being shown by down-curve of the record, the down-stroke of each pulsation being larger than the up-stroke. Subsequent application of musk caused a great enhancement of pumping activity and re¬ sulting increase of pressure (fig. 231).

Such antagonistic reactions are even more strikingly exhibited under the action of a poison and its antidote. Experiment 238. Effect of morphine and atropine on animal heart. — Morphine produced a depression of pulsa¬ tory activity. Application of atropine caused a revival by exerting a physiological antagonism to die action of morphine (fig. 232). Experiment 230. Effect on plant.— An exactly parallel effect was produced in the plant ; unde r the continued action of morphine the plant was on the point of death, as shown by

the movement of the light-index to the extreme left. Applica¬ tion of itropine, at this stage, renewed the pulsation ; the Fir,. -30. \ntagonistic action of Musk and Bromide of , Potassium on heart-beat of Frog. X, normal : Br, dep less ion induced by KBr solution ; Musk, enhanced activity under the reagent. beam of light now moved to the right, demonstrating the revival of the life- activity . The effect of these agents is greatly modi tied by the strength of the dose. A minute dose, generally speaking, induces stimulation, whereas a large dose causes depression which may culminate in death.

Experiment 240. Effect of strychnine on heart-beat. A dose of strychnine, one part in a thousand, caused a maiked enhancement of cardiac activity (tig. 233) ; a 2 per cent, solution produced, on the other hand, great depression and Experiment 241. Effects of different doses- of strychnine on pinnr. — Parallel effects of stimulation and depression "fere l produced in the plant by minute and large doses of strych¬ nin* A dose of a solution of one part in a thousand acted

Fig. 231. Effect of KBr in depressing and of Musk in enhancing a.s a stimulant ; but a dose of 1 per cent, solution caused marked degression (fig. 234). '1 his acts as a deadly poison to botli plant and animal. Its subcutaneous injection causes death of the anima! in a short time In the plant likewise it causes death, as evidenced bv permanent stoppage of pulsation. N, normal ; Morph, depression under morphine ; Atrop, revived activity under atropine.

lh« feeble normal pulsation (upper record) enhanced by application of dilute strychnine (lower record). A minute dose of the venom is, however, found to pro¬ duce stimulation both in animal and in plant. Left : enhancement of sap-pre?sure under a minute dose. Rigut : diminution of sap-pressure mder a strong dose (Cosmos). Drugs induce, in general, parallel modifications in the pulse- beat of animal and plant. Cardiac stimulants are also found to stimulate die cellular acdv;ty in the propulsion of sap, and to produce an increase of sap-prescure. Cardiac depressants, on the other hand, cause a diminution of sap- pressure. Camphor, caffein, and musk act as stimulants alike for the animal and the plant, while potassium bromide and morphine act as depressants.

The effect of a stimulant can be quickly reversed by that of a depressant. 1 he increase of sap- pressure under camphor, for example, can be transformed into diminished sap- pressure by KBr, and the two opposite reactions can be alternately induced many times in succession. I he effect of a poisonous drug may likewise be counter¬ acted by the action of an appropriate antidote, as shown by revival of activity by atropine after poisoning by morphine.

A minute dose of strychnine produces similar enhancement of pumping activity in the animal and in the plant. A large dose produces depression or arrest in both. The effects of cobra- venom on the pulse- beat of the animal and of the plant are remarkably similar ; in moderate doses the pulsations are quickly abolished; in minute doses, however, it acts as a stimulant enhancing the rhythmic activity. 1 hese results afford additional and independent proof that the mechanism for the propulsion of sap in the plant is physiological, and is essentially similar in principle to that for the Circulation of blood in the animal.

An explanation of the characteristic effects of chemical stimulants and depressants in inducing variations of sap- pressure at the sphygmographic contact described in the previous chapter, which would appear to be satisfactory is : is determined by the algebraical summation of the two factors A — T ; of these A represents the acces¬ sion of water at the contact point C by suction of water from below, and T the removal of water from C by the transpiration of the leaves higher up on the stem.

oi sap upsets the balance in one direction. A being 11QW greater than 1 (A > Tl, there is an accumu¬ lation of sap at C, with resulting expansion and increase of pressure. ascent and thereby reduces, accession of sap at C, the loss by transpiration remaining .he same. The loss being greater than gain (A < T), the effect at C is contraction and diminution of pressure. Blit this explanation is unsatisfactory because it does not take into account all the felevant facts. It assumes

that the movement of the sap is always ascensional, the direction being determined by the action of transpiration above and of root-pressure below. But it has been pointed out in preceding pages that movement of sap can take place in the absence of tran¬ spiration as demonstrated by the Method of the Spliygmograph and that of Erectile Response (Experiments 205, 216) ; and that its direction can be changed by appropriate conditions. Still more serious is the difficulty encountered in finding an explanation of the following results ;

Application of warm water at the cut end of the shoot has been shown to produce an increase of sap-pressure at the sphvgmographic contact above (Experiment 228). This was evidently due to an enhanced rhythmic activity inducing the pumping up ox water at a quicker rate and producing an increase of pressure ; application of cold at the cut end produced a diminution of pres¬ sure by depressing the pumping activity. What would happen if beat or cold were applied above the point of contact ?

Fig. 235. Effects of thermal stinm - lation or depres¬ sion above con¬ tact c. A plasticine cup v allows applica¬ tion of warm or cold water to the stem for induc¬ ing thermal stimulation sT or thermal depres¬ sion Dx. In the first case^ the in¬ duced sap-move¬ ment is down¬ wards, and in the second case up¬ wards (Antirrhi¬ num). Sphygmographic Record of Effect of Heat and Cold Above Contact Experiment 242. Effect of application of heat above. — I took a cut shoot <sf Antirrhinum and mounted it in the usual way for recording the sap-pressure at the sphygmo- g* aphic contact (fig. 235). The following simple device allowed 1 he application of heat or cold above it. A small cup of plasticine was moulded round the stem above the contact-

point ; the object of the cup was to prevent any leakage of the small quantity of warm or cold water applied on the stem for causing local stimulation or depression. As regards the application of heat, one would anticipate that the enhanced rhythmic activity thus induced would increase the rate of the pumping activity, the sap would be carried away at a quicker rate and would cause diminution of pressure below the pom4- of application. 1 he observed result was, however, the opposite : application of boat, instead of inducing a diminution, caused an increase of pressure at the contact-point below.

Experiment 243. Effect of application of cold. — Ihe experiment was repeated with this difference, that instead of stimulation by heat, depression was produced by ap¬ plication of cold above the contact. It might be assumed that the pumping activity in this case would be more or less paralysed, and the transport of sap to the transpiring leaves would be reduced. There should then be an accumulation of sap at the contact, resulting in an increase of pressure. But the effect observed was a diminution and not an increase of pressure.

The following results may be taken as typical. Local application of cold and heat were successively applied on an identical specimen above the sphygmographic contact, and the change of pressure induced after 30 seconds observed by noting the deflection of the spot of light. Responsive Table XXIV. — Variation of Sap-Pressure by Successive Application of Cold and .Hea* above Contact- Point (Antirrhinum). increase of pressure was indicated bv deflection of Tie light to the right (-j- deflection), while diminution of pressure

was shown by deflection to the left (•— deflection). The scale was placed at a distance of a metre. These results were not what might have been expected, and evidently called for further investigation. I Experiment 244. Response of leaf to application of heat or cold above. — Precisely parallel results are obtained when the leaf is utilised as an indicator or the movement of sap. When heat is applied to the stem above the leaf, it exhibits an erectile response evidently due to the forcing in of sap downwards. Application of cold, on the other hand, causes a responsive, fall of the leaf, due to the flowing away of sap. ] These definite effects, recorded by the two different methods of the Sphygrhograph and the Phytograph, cannot be explained on the hypothesis that the upward movement of sap is the ' fundamental reaction, all other results being merely due to modification of the rate of ascent, either increase or dimi- nation. The unfounded character of this hypothesis is demonstrated by the experiments just described ; very definite results have been obtained, which indicate that under certain speciflc conditions the direction of movement of the sap undergoes a complete reversal from up to down. It is. * evident that the old theory must be replaced by a universal law based on the facts of the directive movement of the sap.

The experiments that have been described in previous chapters may be divided into two classes, as follows : absorbed at the lower end and excreted at the upoer end byT the transpiring leaves. stems in which the transpiration-current was com¬ pletely eliminated. In spite of this, application of heat at the lowei end caused an up-movement of sap, while its application at the upper end caused a down-movement (Experiments 217-220). The results prove that transpiration is by no means essential to the propulsion of sap. \\ hat would happen if both absorption and transpiration were completely elimi¬ nated ? Would the responsive movement of sap, and varia¬ tion of pressure induced by it, undergo abolition ? In order to answer the question thus raised, 1 made experi¬ ments with bare stems which were completely sealed, so that there could be no accession of water by absorption nor any loss by transpiration. The detection of responsive movement of sap under these conditions might, it was hoped, lead to the discovery of the basic activity in the propulsion of the sap.

For sealing the stems T employ a mixture of coconur oil and solid paraffin, which melts under a slight rise Df tem¬ perature and solidifies on return to the temperature of the room. The most sensitive specimens for the investigation are young stems of Cosmos and Antirrhinum ; failing these the petiole of Tomato may be employed, though it is con¬ siderably less sensitive. A piece of bare stem or a petiole, about 70 mm. in length, is taken and completely sealed by means of the impermeable varnish.

How can a sap-movement induced in a sealed stem be detected ? I nave shown that the passage of the peri¬ staltic wave carrying the sap can be detected by means of the Spiiygmograph ; that the flowing in of sap causes expansion and increase of pressure at the contact , whereas withdrawal or movement of sap away from the contact is indicated by contrac¬ tion and diminution of pressure (Experiment 204). In order to carry out experiments on peristalsis in the plant parallel to those on the animal stomach, I devised a modified form of the Optical Spiiygmograph. The experimental stem is held horizontal between t ato vertical rods, one fixed and the other movable (cf. fig. 176). The movement of the primary movable lever is further magnified by a reflected beam of

light, as in the Optical Sphyginograph (cf. hg. 221). Obser¬ vations were commenced a short time after the preliminary adjustment, so as to allow disappearance of the irritation caused by handling. After subsidence of irritation the pressure at the contact was found to remain constant, indicating that the sap was in a state of standstill. The sphygmographic contact is made at the middle of the stem. The sap, as already stated, is m a state of stand¬ still at the beginning, as indicated by the sap-pressure at the contact remaining constant. The initiation of sap-move¬ ment under external change can be detected, as previously explained, by the observed variation of pressure. An increase of pressure, shown by the positive deflection of the spot of light to the right (up-curve in record) , indicates a responsive movement of sap tovvaids the contact. A diminution of pressure, shown by the negative deflection (down-curve in record), indicates, on the other hand, a movement of sap away from the contact.

Reference to the generation of peristaltic waves by external stimulation described in a previous chapter (pp. 288, 289), will show that the experiment on the bare and sealed stem is analogous to that on the isolated stomach, the activity of both being at standstill. I t was shown that local stimulation applied at the more excitable upper end of the stomach gave rise to a peristaltic wave in the normal direction of propagation ; stimulation at the less excitable pyloric end gave r se, on the other hand, to an antiperistaltic wave in the reverse direction, the effectiveness of which was 3 to 4 times less than chat of the peristaltic wave in the normal direction. Among the modes of stimulation found effective in initiating peristalsis of the stomach were : (1) electric stimulation, ( 2 } chemical stimulation, and (3) thermal stimulation.