The Motor Mechanism of Plants

1 lie following considerations explain the unexpected result. 1 lie ra^c of flow of the sap depends directly on the pumping activity and inversely on the resistance offered by the channel for the transport of w^ater. In a plant with roots, the attenuated channels of the root-ha:rs offer great resistance to the inflow ; whereas in the case of a cut shoot the resistance offered by the absorbent cut surface of the stem is considerably less. The ascent of the sap has been shown to occur when the plant is deprived of its roots ; nor is the sap-movement entirely dependent on the pull exerted by the transpiring leaves. It can be shown that the movement of sap takes place even in a piece of bare stem, from which both the roots and leaves have been removed. The specimen is taken in the condition of drought, and the bare stem is coated with an impermeable varnish. After making a fresh cut at the lower end, the specimen is mounted suitably on the Potograph, and the rate of suction measured for every successive minute during a period of So minutes. The results show that even in Lie total absence of transpiration the ascent of sap persisted for a considerable length of time.

Experiment 214. — This was carried out with two different species of plants, Centaurea and Impatiens ; the activity of Centaurea is about 10 times greater than that of Impatiens. The results are given in the following table : Table XX. — Giving Rates of Suction in Varnished Bake Stems of Centaurea and Impatiens. The results show that the rate of ascent at first increases, then attains a climax, alter which it undergoes diminution (fig- 217). The question arises: By what mechanism is the sap transported upwards in a bare stem with one end immersed

Curves showing rates of suction in varnished steins urea (dotted cur /e) and Impatiens' (continuous cur\e). experiments, the rate of ascent underwent subsequent diminution. In explanation of the remarkable results described above, it is necessary to bear in mind certain universal characteristics of rhythmic tissues which have been demonstrated in the previous chapters. It has been shown that the arrested pulsation of the heart is revived by increased internal pressure (p. 256), and that the pulsation of the stomach is revived by distension (p. 289). In Desmodium also, the pulsatory activity brought to a standstill under drought was revived by turgor-distension caused by irrigation (p. 255).

In regard to the increase of rate of ascent observed at the beginning of Experiment 214, the pulsatory activity of every section of the stem was in a state of arrest due to drought. The absorption of water at the lower end renewed the activity at that point, and gradually that of others higher up as the liquid pumped up reached them. In process of time a large number of cellular pumps became activated with resulting increase in the rate of ascent. Thus in Centaurea the suctional rate of 18 at the beginning increased to 27 ir the course of 4 minutes ; and in Impatiens likewise, ( the initial rate, which was 2, became enhanced to 3-5 after , 6 minutes.

The question why the rate of ascent after attaining a climax should undergo a continuous diminution has next to be considered. The answer is to be found in the law guiding the direction of propulsion of neristaltic waves , already established (p. 292). it was there shown that the direction of propulsion is from the more to the le*s activated end of the organ. In the present case the lower end of the I stem was activated by turgor-distension caused by irrigation ; |

hence the unidirectioned flow was from the hydrostatically f tense and activated lower end to the less turgid upper end, m still in a state of drought. But the upper end soon becomes | turgid and distended by the accession of water pumped up from below. The differential activity which determines the j rate of flow undergoes, after a while, a diminution, i he resultant flow thus tends to disappear when the two ends become equally turgid and active (see Sg. 217). I

The important role of transpiration in directing the flow of sap upwards in the plant will now be briefly considered. When the shoot and the root of the plant are equally turgid the activities at the two ends balance each other, rendering any ascent of sap impossible. The transpiring leaves do not raise water by exerting a sucking force as such, but trans¬ piration ensures differential activity by causing incipient drought in the transpiring region. The turgor-tension gradient extends from the root to the apex of the stem ; it is the differential physiological activity induced at the two ends that drives the sap upwards against the force of gravity, ihere is a limit to the height of water which can be raised by physical forces ; but there is no such limit in the case of physiological action.

Experiment 214a. Propulsion of sap in the midrib in absence of r^ot-pr assure and of transpiration. — Though the fact that the ascent of sap^ can be observed in a bare stem is sufficient by itself to invalidate the assumption that ioot-pressure and suction by the transpiring leaves are essential for it, yet the evidence of the following experiment with a leaflet of Lupin is of confirmatory value. A single leaflet, droooing from drought, had a portion of its lamina removed so as to leave a shor.: piece of the mid- iib projecting at the lower end. . After being coated with vaseline, it was mounted in the cork of a small bottle. Some warm dilute campfior-watcr was poured into the bottle through another hole in the cork, until it just reached the freshly cut end of the midrib in die bottle. The wilted leaflet now rapidly reared itself up, becoming turgid, and was erect in 4 minutes. It was noticed in the course of 15 minutes that drops of liquid exuded, under the coating

of vaseline, along each side of the midrib on the lower surface. It is clear that in this experiment water was absorbed in the absence of transpiration, for the leaflet was rendered impervious by vaseline, and of course of root-pressure. The f transport of absorbed water can only have been effected by the propulsive activity inherent in the midrib, this activity being heightened by the stimulating action of camphor. Moreover, in the absence of any loss of water by transpira- i tion, the turgidity of the midrib became so excessive that war.er was forced out of it under pressure.

It may naturally be asked, Does the peristaltic wave j always move the sap upwards ? Would it be possible to reverse the direction of the peristaltic wave, so as to cause the sap to flow downwards ? The possibility of this has already been demonstrated in Experiment 205. The two cut ends cf a specimen under drought were successively touched with water, and the passage of antiperistaltic and peristaltic waves detected by means of the Sphygmograph. . The effectiveness of the peristaltic wave v/as found to be • far more intense than that of the antiperistaltic.

Experiment 215.— Reversal can be demonstrated by even simpler methods. For instance, when water is withheld from a potted plant, the stem bends over under the drought, | and the drooping leaves hang down. If a glass of water be ^ then raised so that the apical end of the drooping stem is immersed in it, the upper part of the stem will absorb water | and become more turgid than its lower end. The pulsating activity becomes revived at the upper end ; the gradient of turgor-tension is reversed and the sap now flows down¬ wards, that is, against the direction of normal ascent. I he I reversed flow is demonstrated by the sequence of revival I of the drooping leaves, which takes place from the tip of the

Experiment 216. — Two similar bare stems of Impatiens, with all leaves removed, were taken in a condition of drought. The flaccid stem bent over from the point of support. An equal length of bare stem was taken in each case, of which the apical end will be distinguished as A, and the basal end as B. The cut preparations were suitably supported by a clamp, just behind the bend. In a a piece Fig. 218. Curves showing (a) the effect on transport of sap in a ster. under drought when water was applied to the basal end B, and (6) the effect in a similar specimen under identi¬ cal conditions when water was applied to the apical end a (Impatiens).

ol cotton was attach* d to the lower cut end B for the applica¬ tion of water, and in b a similar arrangement was made for application of water at the upper erd A. Application of wat^r at the basal end B caused an erectile response of the apical end attached to the recording-lever, i he distance between I>, the point of application of water, end the respond¬ ing poin: just beyond the support, was 35 mm. The record (fig. 218, a) is essentially similar to that of an intact plant with leave§, though the response was manifested by t.hn movement of the drooping stem in the one case and by the

movement of the drooping leaf in the other. The successive dots are at intervals of 20 seconds, and the erectile response occurred after the second dot. Application of water at the apical end A produced an erectile response of fhe lower end of the stem, due to trans¬ port of sap in a direction opposite to the normal, i.e. from apex to base. The distance of transmission was the same as in the last case, but the response occurred after 8 dots (fig. 218, b), instead o,f 2. The velocity of transport of sap in the reverse direction is thus found to be at least 4 times slower ; this slower rate is also demonstrated by the more inclined slope of the curve. Other experiments showed that the transmission in the reverse or unusual direction is from 4 to 6 times slower than in the normal direction.

The results described tend to show that the membrane of each individual cellular pump acts as a valve which allows easier passage in the normal direction, preventing, to a certain extent, the backward flow. It is interesting to note that the peristaltic contractile wave of the heart, as also that of the stomach, can be re¬ versed ; it has been further shown that the propagation o'i the wave in the reverse direction is considerably slower than in the normal direction ( see p. 289).

The results of the foregoing experiments pi eve con¬ clusively (1) that the movement of sap is not determined solely by the terminal organs, leaf and root ; (2) that there is a pro¬ pulsive activity in the stem, in the petiole, and in the midrib of the leaf ; and (3) that the direction 0) the flow of sap is not 9 necessarily upwards, hit can be inverted downwards by reversing the differential activity at the two ends of the ofgan. The root is not essential for the ascent of sap. i his is M proved by experiments carried out with the Potograph g showing that the rate of 1

removal of the root. This is explained by the fact that in a plant with roots the attenuated channels of the root-hairs offer great resistance to the inflow ; whereas in a cut shoot, the resistance offered by the surface of the cut' end of the stem, is considerably less. The fact that the leaf is not essential is proved by the rise of sap in a leafless stem in a state of drought, the lower end of which is placed in water. The rate of ascent increases at first and then undergoes a diminution.

It is explained that the flow of sap depends on differential activity at the two ends of the organ. The turgid end is more active than the other end still in a state of drought, lhe resultant unidirectional dow tends to disappear as the two ends become equally turgid. Application of stimulating solution of camphor to the lower end of the midrib of a drooping leaflet of Lupin coated with vaseline not only caused a rapid erection of the leaflet, but also exudation of water under pressure.

I he direction of flow of sap is not necessarily ascending ; it can be rev- rsed by reversing the gradient of turgor-tension. I he effectiveness of the peristaltic wave moving upwards is about 4 to 6 times greater than that of the antlperistaltiq wave moving downwards. The membrane of each individual cellular pump seems to act as a valve which allows easier passage in the normal direction, preventing, to a certain extent, the backward flow. The fundamental factor in the propulsion of sap has now- been traced to the pulsating layer in the cortex, which is continuous throughout the plant. By eliminating the complicating factors of the action ef roots and leaves, the functional importance of this propulsive tissue has been demonstrated. The electric pulsation of the propulsive tissue in every section of the stem proves the continuity of the cellular pumps, the co-ordinated activities of which propel the sap in a definite direction. It has also been shown that the propulsive mechanism in the plant is essen¬ tially peristaltic as it is in the animal. The peristaltic wave in the animal has been proved to follow the stimulation- gradient, under the different modes of stimulation employed for this purpose :

The production of peristaltic and antiperistaltic waves in the plant by differential turger-tension at the two ends, has already been demonstrated in the previous chapter. In ji the present chapter I describe in detail the effect of thermal stimulation, reserving the effects induced by chemical and | electric stimulation for subsequent chapters, the demon¬ stration will be rendered more rigorous by increasing elimination of all but the essential lactors. J

stem has been shown to be practically the same when they are equally turgid. In such a case there is no perceptible flow of sap. The cessation of sap-movement here is not due to lack of activity, but to full activities balancing each other. I attempted to upset this balance by inducing an enhance¬ ment or a depression at one end or the other of the stem. The subject was investigated by the Potographic Method m the following order : A piece of stem, after removal of leaves, is mounted with its lower end immersed in the water of the Potograph ; a hollow cork with a hole at one side is fitted on the upper cut end of the stem for the application of water at the upper end without its exerting any pressure [see fig. 219, a). To secure this, the hole drilled on one side of the cork is 3 mm. above the surface of the stem, thus allowing overflow of any excess. The balanced condition of turgor is, after a while, practically, though not absolutely, perfect. A residual flow, usually in the direction of ascent, persists, but it is so feeble as to be practically negligible. For quantitative measurements, a correction has, however, to be applied for this residual current. The following result obtained with Helianthus gives an idea of the residual effect in the bare stem in the balanced condition, as compared with the normal rate of suction of the cut stem before removal of the leaves. The normal rate of suction of the cut stem with leaves was + Q cubic mm. per minute, the plus sign indicating the movement of ascent. After removal of the leaves, and approximate balance by application of water at both the lower and upper ends, the residual current was 4- 0-2 cubic mm. in the direction of ascent.

An enhancement or a depression of activity at either the upper or the lower end of the stem was produced by thermal means, the resulting upset of the balance being observed by the responsive movement of the water- index. A rise of temperature, within limits, has been shown to enhance the activity, while cooling causes a depression. The upper end of the stem A may thus be stimulated or depressed by application of warm or of cold water. Similarly , stimulation or depression of the lower end can be produced by projecting a stream of warm or cold water against the lower end B (see fig. 219, a). Observation of the movement of the water-index is made only after the attainment of steady propulsion of sap under the new condition, lhe increased rate of ascent of the sap is indicated by the quicker move¬ ment of the water-index to the right. Reversed movement of sap downwards and expulsion at the lower end is indicated by the movement of the water-index to the left (see fig. 219). u I

Experiment 217. Thermal stimulation of upper end of item of Helianthus. — The results are given in the following order : (1) the residual flow under condition of approximate* balance after application of water to both lower end upper ends ; (2) effect of stimulation of the upper end by warm water. The resultant responsive variation under differen¬ tial stimulation of the two ends is then calculated from (2) and (1). The number given represents the movement! of the index in mm per minute. Plus sign or up-pointing arrow represents ascent of sap in the stem : minus sign or down-pointing arrow indicates the reverse flow from the upper end A towards the lower end B. v

The enhanced activity induced at the upper end by local application of heat produces a responsive movement of sap downwards from the activated A to the indifferent and less active B. In Helianthus the rates of the normal movement of sap and of its induced variations are comparatively feeble. Far more striking results were obtained with the more active plants, Antirrhinum and Cosmos. Experiment 218. Thermal stimulation of upper end of Antirrhinum . — In order to avoid useless repetition I give only the responsive variation induced in tne rate of move¬ ment of sap.

Here also thermal stimulation of the upper end caused a reversal of the normal rise of sap into a reversed down-movement. Cosmos gave very similar results. Experiment 219. — The application of cold to the upper end induced an effect precisely opposite to the stimulatory action of neat, though relatively less intense. Of the results of two experiments given belowT, one was obtained with the less active Helianthus and the other with the more active Cosmos.

Depressed activity of the upper cud thus induced an up- movement of sap from the relatively more active lower end B towards the depressed upper ena A. I next describe the effects of variation of activity induced at the lower end! The temperature at the lower end B can be raised or lowered by projecting a small quantity of warm or cold water against it, on opening the stopcock of the funnel F contain¬ ing water at the required temperature (cf. fig. 216). After allow ing a short time for the attainment of a steady rate of movement of sap, the index is brought to the middle, and the rate of suction measured in the usual manner. The two following experiments were carried out with Helianthus.

The enhanced activity oj the lower end B produced by heat induced an upward movement of sap from the activated B to the less active A, causing an enhancement of the rate of ascent . The effect of the depressed activity due to application of cold to the lower end B was a down-movement from the relatively ■more active upper A to the less active lower end B. After ascertaining the respective effects of induced varia¬ tions of activity at the two ends, I sought to determine their combined effects. The results of experiments showed that the individual effects became summated under depressed activity of the upper and enhanced activity of the lower end and vice versa.

Experiment 222. Rate of ascent induced in a bare stem under differential stimulation (Helianthus). — On application of water at the upper end, balance was produced, the residual flow being practically zero. Differentia’; activity at the two ends, of the stem was now induced by application of cold water above and warm water below, with the following results. In the next experiment warm water was applied above and cold water below, the result was a marked reversal of sap-movement downwards,

The rate of movement in all cases is determined by the differential activity at the two ends, the flow being directed from the more to the less active end. Fig. 219. Diagrammatic representation of the Potograph and of the" effects induced, by stimulation or depression of the two ends of the stem. Direction of flow of s p from the more stimulated to the loss stimulated or depressed region. a, the Potograph. b effects of alternate stimulation of the upper end as and the lower end bs. as induces flow of sap downwards (full ar^ow) ; bs causes flow of sap upwards (dotted arrow), c, effects of alternate depression of the lower end b„ and of the upper end a0. bd induces flow of sap downwards (full arrow) ; ad, flow of sap upwards (dotted arrow), d, additive effect of simultaneous stimulation of one end of the stem, and depression of the other. Enhanced flow downwards (longe^full arrow) undet as bd, and upwards (longer dotted arrow) under Bs ad.

I was next desirous of obtaining some approximate idea of the normal rate of ascent of sap in. the intact plant with root and leaves, and the rate induced in the bare stem by differential stimulation by cold above and heat below. The experiments, it should be noted, were carried out with an identical plant. Experiment 223. Determination of rate of ascent in intact plant . — The rate of ascent of sap in a shoot of Helianthus without roots was found to be +7-2. The presence of roots has been shown (p. 339) to reduce the rate of ascent to nearly half. Hence in the particular specimen the rate of ascent in Helianthus in intact condition may be estimated as + 3*6 approximately.