The Motor Mechanism of Plants

The leaves on the shoot were then removed, and the two ends of the bare stem subjected to differential stimulation. The result is given in Experiment 222, the rate of ascent of sap being + 10*9 in the place of +3*6 of the identical plant with root and leaves. In other words, by inducing differential activity of the two ends of the bare stem, the rate of propulsion became three times more active. The leaves and roots in the intact plant evidently do not exploit the full potentiality of the propulsive mechanism.

The various results of separate and combined effects are diagrammatically shown in fig. 219, in which a is the Poto- graph with the bare stem. The responsive movement of ascent is indicated by the dotted, and the reverse down- movement by the continuous arrow. The separate effects are shown in b and c, and the combined effects in d. In all cases the down-movement of sap, due either to stimulation of A o, depression of B, or by simultaneous stimulation of A and depression of B, is indicated by a full arrow. The ascent of sap, on the other hand, is indicated by a dotted arrow, caused by stimulation of B or depression of A, 01 by simultaneous stimulation of B ahd depression of A, the letters in these cases being in dotted outline. To Recapitulate :a b shows the flow of sap down¬ wards (full arrow) due to stimulation of the upper end, and the reversed flow of sap upwards (dotted arrow) by the

stimulation of the lower end ; c shows the flow of sap down¬ wards (full arrow) by the depressed activity of the lower end, and the reversed How upwards (dotted arrow) by the depressed activity of the upper end ; d shows the combined effects of enhanced activit}^ of the upper and depressed activity of the lower end (longer full arrow) ; the combined effect of enhanced activity of the lower and the depressed activity of the upper end is indicated by the strong ascending current (longer dotted throw).

I now describe an additional and very sensitive method for investigating change in the direction of flow of the sap induced by differential activity of the two ends of a bare petiole or stem. The following experiment was carried out with the petiole of H ibiscus mutabilis . A complete hydraulic circuit was made of the petiole and a capillary glass tube, the two being connected by indiarubber tubes, all the tubes filled with water, with the exception of an air-bubble in the middle of the capillary tube, the bubble is adjusted as follows : one end of the tube is immersed in a large vessel of water, and the other end is lowered gradually. This drives out the air and fJls the tube with water ; in order to trap a bubble of air, one end is raised slightly above the water and after¬ wards closed with the linger. The enclosed bubble is then brought to the middle by placing the tube horizontal under water and removing the linger : the bubble is adjusted to any position in the capillary tube by a slight tilt in one direct ion or the other. Ihe tube is connected under water to the ends of the petiole by means of short lengths of india- rubber tube. The preparation is now taken ouf of the water and permanently mounted on the table fo~ experimental observation. The activities of the two ends of the petiole in contact with the water in the indiarubber tube are nearly, though not exactly, the same. There is a residual

flow (say from the basal end B on the right to the apical end A on the left) on account of slight difference in the activity of the two ends. In order to induce differential action, semi-cylindrical hollow containers of cork are fitted to the two ends of tire petiole, these being alternately heated or cooled by means of warm or ice-cold water. The resulting movement of sap per minute is measured by observing the excursion of the bubble by means of an eye-piece provided with a micrometer (fig. 220).

The normal movement of ascent, denoted by a p!us sign, is from the basal end B to the apical end A. In the bare r, plant; 1 1, indiarubber tubes; c, capillary glass tube ; m, micro¬ meter eye-piece tor reading movement of air-bubble enclosed in the capillary tube. petiole the sap-movement was practically zero when water at the same temperature was applied at the two ends. Experiment 224 Effect of enhanced activity of basal end B, and of depressed activity of apical end A (Hibiscus).— The basal end B was stimulated by application of heat, and the apical end A depressed by application of cold.

Responsive flow B to A = 4- 24 divisions per minute (enhanced flow upwards). Experiment 225. Reversal of direction 0] flow; by stimu¬ lation of apical end A and depression of basal end B — The differential activity of the two ends was reversed by application of heat to the apical en<l A, and of cold to the basal end B. Experiments carried out with the more active Antir¬ rhinum and Cosmos gave even more striking results. In all cases the resulting movement of sap was from the more to the less active or depressed end.

Law of Directive Flow of Sap The results given establish the following : Effects essentially similar were observed in the propaga¬ tion of peristaltic waves in the heart and in the stomach of the animal (Chapter XXI). Irrigation of both ends of a bare cut stem produces equal turgor- tension at the two ends ; there is practically no flow of snp in the stem. This state of balance is upset by unilateral stimulation or depression by the application of heat or cold respec¬ tively. The direction of flow of sap, either upward or downward, demonstrates the law that the responsive flow of sap is from the activated to the less active or depressed region.

As regards the fundamental mechanism of the movement of sap, the root or the leaves are by no means essential, nor do they fully exploit the potentiality of the physiological mechanism for propulsion. This was demonstrated in a striking manner by an experiment in which, by means of differential stimulation, the rate of ascent in a bare stem was enhanced three times beyond that in the intact plant. An identical law governs the passage of the contractile peristaltic wave in both the animal and the plant.

Results obtained with the Electric Probe, described in a previous chapter, demonstrated the occurrence of electric pulsation indicating the pulsation of the active cells in the propulsive layer. I next attempted to obtain the direct mechanical record of die infinitesimal contractions and dilatations associated with the peristaltic transmission of sap by this layer. It :s possible to induce changes in the blood-pressure by various means which modify the pumping activity of the animal heart. Is it possible to induce parallel changes in the sap-pressure by similar means ?

For such an investigation, the sensitiveness of the High Magnification Sphygmdgraph, already described, proved to be quite inadequate. It was therefore necessary to devise an apparatus which should possess the following advantages : The movement of dilatation and contraction of the channel conveying the sap by peristalsis, should be greatly magnified so as to be rendered visible to a large audience. i he period of experiment should be reduced so as to exhioit all important reactions in the course of a few minutes, during which the external condi¬ tions could be maintained constant

The Optical Sphygmograpti perfectly fulfils all these < conditions. The movement of the primary lever L of the l Sphygmograph indicating dilatation or contraction of the I cortical cells, which has already been described \cf. tig. 199), | is further magnified by reflection from a small concu\ e mirror J carried by a vertical rod 0*5 mm. in diameter, supported j both above and below on jewel bearings. A thin silk thread, tied to the tip of sphygmographic lever L, makes one turn round the vertical rod, the further end being attached to a fine spiral spring by which the pressure of contact can be

Dilatation at the contact-point on the stem by the passage of an hydraulic wave causes a movement of the tip of the lever outwards, producing clock-wise rotation of the mirror and a positive movement of the reflected line of light, siiy to the right. Contraction at the contact-point gives rise, on the other hand, to a movement of the light to the left. The mirror-rod, as already stated, is supported above and below by two jewel bearings, and its rotation can only be pro¬ duced by cellular expansion and contraction causing a puii on or release of the thread. The grip of the thread on the vertical rod, and its support both above and below, render the indications of the Optical Sphygmograph singularly free from all external disturbance. The incandescent filament

of a small pea-lamp can be so focussed by a lens carried by the lamp-holder that, after reflection from the mirror, a bright line of light is thrown on a distant scale or on a moving photographic plate. The complete apparatus is very com¬ pact and portable, as seen in the accompanying photograph reduced to one-half the natural size (fig. 222). The mag¬ nification produced is a million times when the scale is at a distance of 4 metres. The apparatus is also provided with an Applicator which carries three small cups containing respectively a stimulating, a depressing, and a poisonous solution. One or other of these can be readily applied to the cut end of the stem.

A satisfactory sphygmogram in man can be obtained only by means of suitable pressure on the radial artery. \\ ith too feeble a pressure there is no record, while excessive pressure arrests the pulse. Experiment 226. — The sphygmogram of the plant also exhibits similar characteristics. The following table gives the responsive rates indicated by the movement of the line of light, as the pressure at the sphygmographic contact was increased from 0 to 7 units of an arbitrary scale.

I obtained similar results under increasing pressure measured in gramme. It was thus found that when the pressure was slight there was no response by the plant. Under a pressure of 0-7 grm. the rate was 60 cm. per 10 seconds, increasing to 100 cm. under 1 -4 grm. This was the optimum pressure, for when it was increased to 3 grm. the rate was diminished to 32 cm. , at 6 grm. it was further reduced to 12 cm. ; an arrest of response occurred when the pressure was increased to 12 grm.

The extreme sensitiveness of the Optical Sphygmograph, and its perfect reliability in physiological investigation, will be realised from the following account of an experiment in which the effects of diverse physiological variations were demonstrated in the course of time as short as 30 minutes. Experiment 227. Effect of physiological variation. — A shoot of Centaurea in a state of drought was suitably mounted and a fresh cut made at the lower end. Water at different temperatures, and different chemical solutions, were applied one after another, each for 4 minutes, by which time the steady rate due to the physiological change was attained. The scale divided into centimetres was placed at a distance of 4 metres, the magnification being a million times. A small electric metronome struck a bell once in 1 second, and the excursion of the light index was observed for 10 seconds. After application of water at 21 : C., the positive movement of dilatation was -f 60 c.rn. per 10 seconds. Substitution of water at 6° lowered the rate to + 16 ; application of water at 340 raised the rate to + 150, or 2*5 times the rate at 210 C. Application of KNG3 solution reversed the response from 4- 150 to — 28, showing that the withdrawal of sap from the responding cells was greater than the absorp¬ tion. Reapplication of water at 210 C. restored the rate to + 56, which was practically the same rate as at the beginning. Poisonous solution of KCN caused a quick reversal of response tu — 50. 1 he power of response was

found to be completely abolished after a time, for there was no recovery after fresh application of water. The results are summarised in the following table : Table XXII.- Response to Physiolog cal Variation, by Optical Sphygmograpji (Centaurea). I now pass on to consider the record of the actual pulse-wave in the propulsion of sap. The pulsations began to be perceptible under a magnification of a million times produced by focussing the light on the scale at a distance of 4 metres, the spot of light appearing io move not continuously but by spurts, each spurt followed by a pause. The focussed spot of light was, however, too broad to give a sharp photographic record. The necessary condition for a satis¬ factory record is a sharpl}' focussed spot of light at a distance of 1 metre, giving a magnification of from 5 to 10 million times.

This high magnification was secured by the method of amplification previously employed in my Magnetic Cresco- graph and in my Magnetic Radiometer.1 I used for this purpose a single magnetised lever in the place of the . com¬ pound system of levers in the High Magnification Sphygmo¬ graph (if. fig. 200). I11 front of the N-pole of the distant end of the magnetic lever is suspended a small magnetic t needle with an attached mirror. As the N-eiid of the lever is raised by the pull of the sphygmogr aph: c lever caused by tissue-expansion, a very large deflection of the suspended needle is produced to the right, which is magnified by the reflexion of the lint of light by the attached mirror. The sensitiveness of the apparatus is very greatly increased by the employment of a perfect system of astatic needles ; by reducing the distance between the N-end 01 the magnetic lever and the suspended needle, the magnification can oe increased from 5 to 10 million times.

In a balanced condition of turgor the average s„ap- pressu re remains constant, as exhibited by thenipproximately horizontal part of the sphygmogram. This balance is, however, not static but dynamic, the absorption of sap by the cells at the sphygmographic contact being equal' to the expulsion. The alternating phases of rhythmic activity are exhibited by pulsatory variation of pressure, dilatation by an up-curve, and contraction by a down-curve. The ampli¬ tude of pulsation in the balanced condition is relatively slight, for any further dilatation of a cell already distended is a matter of some difficulty. The electric record of pulsation in the condition of balance ( see fig. 213) offers no such drawback, for the electric response persists even under conditions which restrain any mechanical movement. The mechanical record of the feeble pulsation in the con¬ dition of balance is only rendered possible by the excep¬ tionally high magnification of about 10 million times. The rhythmic expansion and contraction is exhibited in the pulse-records, expansion as an up-curve, contraction as a down-curve.

The mechanical pulsation, however, becomes very marked even under moderate magnification during variation of pressure, whether increase or diminution. It is then actually possible to follow the action of the cellular pump, and to note the unequal strokes delivered by it during enhancement or depression of the rate of ascent. The frequency of pulsation is found to be increased by physio¬ logical vigour of the tissue, by enhanced turgor, and by optimum temperature. Under favourable conditions the period of a single pulse may be as short as 5 seconds.

1 he simplest and most certain way of inducing variation of sap-pressure is that of the alternate withdrawal and supply of water at the cut end of the shoot. The pumping activity, with its up- ana down-strokes, is now exhibited in a very clear manner. In the first part of the record (fig. 223) the pressure was falling on account of brief with- drawal of water, the diminution being indicated by a fall of the curve. the individual pulses during fall of pressure show that the down-stroke corresponding to the expulsion of sap from the active cells at the point of contact is larger than the up-stroke representing absorption. When water was applied at the cut end, the hydraulic wave reached the active cells at the contact-point in a short time. It is inter¬ esting to note the change in the character of the individual

supply of water (continuous lint below) on vhe pulsation of the active cells. The descending curve indicates a dimi¬ nution, and the ascending curve an increase of pressure. Note down-stroke of pulsation is the larger during diminishing pressure ; du: ing increasing pressure the up-stroke is the larger. Up-stroke represents tissue-expansion ; down-stroke represents contraction (Cosmos). pulsations by which the hitherto diminishing pressure was converted into one of increasing pressure. After a short period of hesitation the up-stroke became pre- dominant ; .a series of such constituent pulsations produced the resultant, increase of pressure indicated by an up- movement of the base-line of the record.

Other results will be described wnich establish the im¬ portant generalisation that any agent which enhances the rate of ascent gives rise to an increase of sap-pressure, the up-stroke of the constituent pulses being larger than the down-stroke. A depressing agent produces, on the other hand, a diminution of pressure, the down-stroke of the constituent pulses being now larger than the up-stroke. Conversely the stimulating or the inhibitory nature of an agent can at once be discovered by its effect on the pulse- record. The sensitiveness of this method of detection is extraordinarily great. Taking for example the action of a stimulating agent, the immediate effect is a moderately large up-stroke followed by a feeble down-stroke ; the effect increases rapidly, s u that the amplitude of the up-stroke becomes so great as to carry the record off the plate. The frequency of pulsation also increases to such an extent that, the individual pulsations tend to merge one into the other

In studying the effect of stimulants on pulsation, I first applied agents which had been previously found to enhance the pumping activity of the ascent of s^p. The high magnification employed introduced the difficulty that long-continued records cannot be taken within the limited size of a photographic plate. Hence the experiment had to be commenced with a balanced condition of turgor giving an initial record which is approximately horizontal. Enhanced rate of ascent of sap upsets the balance in an upward direction indicative of increased pressure. The method of observation, under balanced condition is, as previously explained, less sensitive than that under the condition of incipient drought ; this drawback is, however, compensated by the exceptionally high magnification.

I now proceed to describe the effects of various agents on propulsive pulsation and sap-pressure. Experiment 228. — Balanced turgor was obtained after application of water at 18' ; warm water at 340 was then substituted at H ; the balance is seen to have been upset in an upward direction indicative of increased pressure (fig. 224). The up-stroke of the constituent pulses became larger and larger, and the record soon went off the plate. The mitial horizontal record had so far been obtained by inducing a condition of balanced turgor, and the characteristic effect of any external agent was manifested by the upsetting of the balance ir either an upward or a downward direction. 1 next attempted to employ another method of obtain¬ ing balance, that of bringing the pulsation to a state of standstill. It occurred to me that as the pulsation of the heart can be arrested by a ligature, the same method might be effective in arresting the pulsation of the propulsive tissue in the plant.

Experiment 229. — Ligature below 1 the sphvgmographic contact was effected, without disturbing the record, by means of a screw-clamp. The first part of the record (fig. 225) shows the pulsation during active ascent of sap ; the application of the ligature at e arrested the pulsation, and made the record horizontal. The pulsation of the heart arrested by ligature can be revived b} the application of external stimulation (cf. fg. 160). Similarly revival of arrested pulsation in the pro¬ pulsive tissue of the shoot occurred under external stimula¬ tion. The results of moderately strong direct and of feeble indirect stimulation on the arrested activity of the plant

Fig. 224. Effect of application of warm water 11 ir increas¬ ing sap-pressure in a shoot in the condition of balanced turgor. Effect of Direct Stimulation on Pulsation Arrested by Ligature Experiment 230. — The investigation was continued with the specimen in which pulsation had been arrested by ligature. A tetanising induction-shock was now sent along Fig. 225. Pulsation a became arrested by application of ligature at L Direct stimulation revived pulsation, and caused diminution of pressure, the down stroke being larger than the up-stroke ; after cessation of stimulation, recovery of original pressure followed, the up-stroke being now larger than the down- stroke (Impatiens).

the length of the stem for 2 seconds. This gave rise, after a latent period of 5 seconds, to a diminution of s .p-pressure, the pulse-record exhibiting a down-stroke larger than the up-stroke. After cessation of stimulation there was a recovery, the up-stroke during the process being larger than the down-stroke, till the origina1 pressure was restored (second part of record, fig. 225). For this purpose I removed the lamina from a lateral leaf of Impatiens, leaving only the midrib connected with the stem for the purpose of indirect stimulation. The object of the removal of the lamina was to eliminate the possible complication arising from varying rates of transpiration

Fig. 226. Effect of feeble indirect stimulation in enhancing the sap-pressure (pulsatory up-curve) (Impatiens). from its surface, The sphygmographic contact was 1 cm. above the lateral leaf, and the clamp was 1 cm. below it. The horizontal record at the commencement shows the effect of the ligature. Experiment 231. Effect of feeble electric stimulation on the midrib. — Application of moderately feeble tetanising induction-shock at S caused a renewal of pulsatory activity resulting in an increase of pressure, the up-stroke being larger than the down- stroke (tig. 226).