The Motor Mechanism of Plants

The movement of sap was indicated, as described in the. previous chapter, by the responsive movement of the leaf- indicator. The question arises : Is it at all possible to detect the movement of sap in the stem itself, during the passage of the hydraulic wave ? Taking the somewhat analogous case of changes in the blood-pressure in the artery of an an imal, the pressure is found to be increased by enhanced activity of the heart-pump, and lowered by its depression. In the human subject fhe radial artery is on the surface at the wrist and pressure- variation in it can be recorded by the Sphygmograph, which consists essentially of a series of magnifying levers, at one end of which is a button placed on the artery, and at the other the writing-lever ; all that is necessary for a satisfactory record is the proper adjustment of the pressure of the button on the throbbing artery. Such a record would obviously be impossible if the artery, instead of being near the surface, were buried under other tissues.

In the parallel case of the changing sap-pressure in the stem, the channel for transport of fluid (as will be explained later) is the internal cortex, which functions both as the pumping heart and the artery for transport. The passage of an hydraulic wave would theoretically give rise to an expansion of the stem, attended by an increase of lateral pressure, followed by return to its original size, with restoration of the original pressure. The difficulty in the record of variation of sap pressure in the stem arises from

the fact that the conducting channel, unlike the radial artery, is buried under other tissues ; the area of the channel is, moreover, insignificant compared with the total section of the stem. The alternate dilatation and contraction of the stein produced by the hydraulic wave must therefore be infinitesimal, and may at first sight appear to be beyond means of experimental detection. The problem was solved by arranging for a sufficiently high magnification of the infinitesimal changes of pressure at a point in the stem during the passage of the hydraulic wave. Instruments of dilferent degrees of sensitivity were de v ised for this purpose ; the first, previously described, gives a magnification of 1000 to 5000 times; the second carries it to 25,000 times, and the third to about 10 million times. The highest magnification is necessary for obtaining the record of the individual pulse- wave in the propulsion of sap.

I he principle of the High Magnification Sphygmograph will be understood from the diagram (fig. 199). The stem is placed between two rods, one of which is fixed, and the other movable. Two V-shaped pieces of ivory carried by the two rods make contact c at diametrically opposite points on the stem. The fulcrum-rod F of the movable lever L is supported on jewelled bearings. The movable lever is made of porcupine quill, which combines lightness and rigidity in an unusual degree : the inertia of the lever is practically negligible. The pressure of the lever on the stem is adjusted by the spring S acting at right angles to the fuicrum-rod. The magnification produced by the first lever is 5 times. This is further magnified by the simple or compound magnifying recording-lever.

The application of water at the cut end of a stem in a state of incipient drought gives rise to an hydraulic wave (in reality 10 a succession of wavelets) ; this causes dilata¬ tion of the responding cortical cells at the sphygmographic contact along which the sap is being propelled. The dila¬ tation and increase of pressure produce an expansive movement E of the lever outwards to the right, contraction producing the opposite movement C. In the High Magnifica¬ tion Sphygmograph the movement of the tip of the first lever is further magnified 5000 times by the compound system of levers previously employed in my High Magnification Crescograph; the total magnification is therefore 25,000 times. Under similar magnification the wave-length of

The stem is placed between a fixed rod and a movable lever l. Looking from the left, expansion and increase of pressure cause movement of lever towards right e, contraction and diminution of pressure bring about movement towards left c ; s, diagrammatic view of spring which acts at right angles to the fulcrum-rod f. sodium light would appear to be lengthened to 12 • 5 mm. The record is taken on a plate of smoked glass oscillating once in 10 or 20 seconds (fig. 200). Adequate precautions have to be taken against all mechanical disturbance. 'Hie whole apparatus is placed on a heavy stand supported by brackets screwed on to the wall. The preliminary test is to mount a dead twig in place of the living stem, when the perfectly horizontal record obtained demonstrates freedom

I will briefly explain the principle and working of the Plant-Sphygmograph . The turgor of the cortex at the sphygmographic point of contact is in a balanced condition ^ when the quantity oi sap brought up from below is equal to that removed by the transpiration-current. O11 withdrawal of the supply of water, the accession of sap is arrested while the loss persists. The result is an increasing contraction and diminution of pressure recorded by, 'say, a down-curve ; an up-curve indicates an expansion and increase of pressure.

A shoot is taken in a state of drought ; the cut surface at its lower end is touched with dilute glycerine to prevent Movement of sphygmographic lever highly magnified by com¬ pound levers and recorded on oscillating smoked plate of glass. Water is applied at the cut end of stem by pressure of indiarubber bulb. J rapid drying of the exposed cells. The shoot is suitably mounted on the apparatus in a U-tube partially tilled with water Oj. other solution. .By manipulating the pressure- bulb, the water can be successively applied to or withdrawn from the cut end of the stem tor definite lengths of time.

Under drought, the increasing diametric contraction of the stem is, as explained, shown by a down-curve. Appli¬ cation of water for a definite length of time gives rise to an hydraulic wave, which, on reaching the small group of respond¬ ing cel’s at the contact, causes a brief dilatation and increase of pressure shown by an up-curve ( see fig. 201). The latent period, or delay in responsive dilatation, depends on the intervening distance and on the activity of the ascent of sap. The expansion caused by the passage of the brief hydraulic wave beyond the contact is then followed by contraction. The sphygmogram of a brief hydraulic wave consists of an up- followed by a down-curve.

Experiment 200. — Successive responses of equal ampli¬ tude were produced by application of water for equal lengths of time. Uniform sphygmograms were obtained with a large number of herbaceous plants, such as Impatiens, Duration of application of water represented by thick lines below. Successive dots at interv: Is of 20 seconds. - Cosmos, Centaurea, Chrysanthemum coronarium , and the young stem of the tree Leucosceptrum canum. Two succes¬ sive uniform responses of the tree-stem are given in fig. 2ui. The effect of change of external conditions is manifested, as in the following experiments, by appropriate changes in the amplitude of response.

Experiment 201. — A rise of temperature has been shown to enhance the activity of ascent (Experiment 197). In the Tixe feeble activity at 20° is increased a-- 340 C. and decreased present case three successive hydraulic responses were obtained for applications of equal duration of water at 20°, at 340, and once more at 20°. The amplitude of response at 340 was very much larger than that at 20° (fig. 202). The recovery from enhanced response was incomplete within the time allowed, hence the base-line is displaced upwards.

Owing to pl^siological hysteresis , the response after a cycle of experiments is not exactly the same as at the beginning. I describe the effect of change in the rate of transpiration on the propulsive activity of the stem, as recorded by the Sohygrriograph. Experiment 202. — I took a stem of Centaurea. bearing ten leaves,, in a state of incipient drought ; the distance between the cut end of the stem and the sphygmographic contact was 25 mm. Water was applied for a definite short period at the cut end, and the resulting hydraulic wave reached the active cells at the sphygmographic contact in the course of 10 seconds ; the peristaltic wave not only arrested the increasing contraction under drought, but re¬ versed it into dilatation, which continued for 10 seconds, even after the withdrawal of water. The amplitude of response was 53 mm. and the recovery to the original pressure was completed in the course of 3 • 5 minutes.

Experiment 203. — The loss by transpiration was then reduced by the removal of six out of the ten leaves^ The latent period for the responsive dilatation was now reduced from 10 to 5 seconds, and the amplitude of response increased to 118 mm. T he expansive reaction persisted for no seconds after the removal of water-supply, and the period of recovery to original pressure was prolonged from 3*5 to 9 minutes. The difference due to change in the rate of transpiration is shown in the follow? ng table :

The difference may be explained by comparing the stem with an elastic indiarubber tube into which water pumped from below escapes at the upper end through a vent, the aperture of which can be increased or diminished. The responsive dilatation is slight when the vent is large, and the restoration of the tube to its original form takes place quickly on cessation of pumping. If the vent be reduced, the same, duration of pumping will cause a greater dilatation, and the recovery of the pipe to its original form will be greatly prolonged. In a plant with numerous leaves, the vent for escape of water is large, while it becomes reduced when a number of them arc removed.

I now offer experimental evidence to prove that the passage of the hydraulic wave is brought about by cell-to- c c', spkygmographic contacts ; l l', indicator-leaves giving the phytographic record (Impatiens). cell transfer of sap along the propulsive layer which is con¬ tinuous in the stem and in the leaf. This is demonstrated by the definite sequence of the responses which occur at two points distant from each other. The method of experi¬ ment is explained by the diagram (fig. 203). The hydraulic wave initiated by the application of water for a short time to the cut end of the stem (Impatiens) in a state of drought, first reaches the contact points CC' in the stem, the response being recorded by the Sphygmograph, and then travels on to the leaves L L', the response of one of which is recorded by the Phytograph (cf. fig. 203).-

Experiment 204. — The two records (fig. 204) show that the passage of the same hydraulic wave produced response first of the stem at the sphygmographic contact (lower record), and then of the leaf higher up (upper record). The moment of application of water is indicated by the vertical line/ The hydraulic wave reached the sphygmographic contact 80 seconds after application of water and produced dilatation. After the passage of the wave beyond the con¬ tact, the down-curve indicated contraction and diminution of pressure. The upper record, as already stated, is the phytogram of the indicating leaf, recording the passage of the same hydraulic wave. It will be noticed that the leaf- response occurred 60 seconds later, which is the time required for the ascent of sap through the intervening distance of 80 mm. The velocity of transport was 1 *3 mm. per second, which is the average rate in the stem of Impatiens at that particular season of the year, as found x'rom other experiments.

The fact that the phytogram is practically a replica of the sphygmogram proves conclusively that the propulsive mechanism is identical in the stem and in the leaf , the charac¬ teristic responses of expansion and contraction being due in each case to absorption or expulsion of sap by an active tissue. In the records given, the movement at the leaf-joint was magnified 300 times, that at the stem-contact 25,000 times. The amplitude of the two responses is nearly the sajne ; hence the anisotropic leaf-joint, with its larger mass of cortex, gives a response about 100 times greater than that given by the stem with its few layers of cortex at the contact-point.

The fact that there is continuity of contractile tissue in the leaf- joint and the stem has already been demon¬ strated by records of the contraction of the cortical cells in Fig. 204. Simultaneous sphygmogram (lower record) and phytogram (upper record) (Impatiens). Vertical line represents moment of application of water. Phyto¬ graphic response initiated 60 seconds laW than the sphyg- mographic, this being the time of transmission through the intervening distance. Successive dots are 20 seconds apart.

different regions cl Jh.e plant in response to electric stimu¬ lation (Experiments 72, 74). I have in a previous chapter described the peristaltic propulsion of the contents of the stomach by waves of con¬ traction It was further shown that the velocity of the wave is about 4 times quicker in the normal peristaltic than in the opposite artiperistaltic direction. It will be shown later that the propulsion of the sap is likewise a case of peristaltic action. Does the sap always how in one direction or can it be made to undergo a reversal ? If so, is there any difference in the rate of flow in the two directions ?

By the sphygmographic method, clear demonstration has been obtained that the propulsion of sap not only takes place in the normal upward direction but can be reversed into the downward direction. Experiment 205. — The specimen was a stem of Chrys¬ anthemum coronarium in a state of drought, from which both the leaves and roots had been removed. The normal velocity of ascent is considerably higher in Chrysanthemum than in Impatiens. The bare stem, 8 cm. in length, was coated, except at the upper and lower ends, with impermeable varnish. The contact-point of the Sphygmograph was mid¬ way, i.e. 4 cm., from either end. Water was applied alternately above and below by touching the ends with a piece of moist cloth. The movement of sap was down¬ wards in the first case, and upwards in the second.

The first response was obtained when water was applied at the upper end for 3 minutes. The latent period for transmission through 4 cin. was 80 seconds, the velocity in the downward direction being 0-5 mm. per second, and the amplitude of response was 8 mm. The second response was to the application of water at the lower end for 1 minute ; the period of application had to be reduced, as otherwise the record s would have gone off the plate. The latent period was 12 seconds, the velocity in the normal upward direction being 3 cm. per second, this gives the ratio of the velocities in down and up directions as 1 .* 6. fhe mortj

effective upward transmission is also demonstrated by the more erect form of the up-curve of the second response ; the amplitude of response in this case to the application for 1 minute was 23 mm. (fig. 205) : for 3 minutes’ application, it would have been over 50 mm. or so, the ratio of the two amplitudes being of the order 1:6; that is, the trans- Fig. 205. Sphygmograrns of descending and ascending hydraulic waves in a bare varnished stem, caused by successive applica¬ tion of water above and below.

Thick horizontal line below first response represents duration of application of water at upper end of stem. The line below second response denotes the duration of application of water at lower end of the stem (Chrysanthemum). mitted impulse in the normal direction was about 6 times more intense than that in the reverse or unusual direction. A. different method (Experiment 216) gave results which are practically the same as the above. The important fact established is that, the sap-movement can take place in a stem from which leaves and roots have been removed ; the transport of sap can occur, moreover, not only in the normal upward but also in the reverse

downward direction. The results described completely demolish the hypothesis generally held that the movement of sap depends upon the activity of the terminal organs, the suction exerted by leaves from above, and the pressure exerted by the roots from below. The significance of these results will be fully appreciated in a subsequent chapter where I deal with the subject of peristaltic and antiperistaltic waves in plants. The Sphygmograph also makes possible the determina¬ tion of the periodic variation of sap-pressure in the stem of the plant. This is by no means a simple but is a highly complex phenomenon, for the pressure exerted by the sap is modified by the varying factors of accession of water from the soil and its loss by transpiration from the leaves, is to be remembered that every section of a tree possesses rhythmic activity which, within limits, is enhanced by a rise of temperature. There are the two extreme cases, (i) of a tree with a large number of leaves, and (2). of a tree without leaves. In the first case, though rise of temperature causes enhanced absorption by the root and quicker ascent along the stem, the loss of water by transpiration from the numerous leaves is even greater. Hence the sap-pressure in leafy trees is minimum at the hour of the day when the temperature is at its highest, this period being conveniently described as the thermal noon, which is attained between 12 noon and 3 p.m. according to the season. The loss of water by transpiration is minimum at thermal dawn at about 6 a.m., when the temperature is at its lowest point. The sap-pressure is therefore at its maximum at or about 6 a.m. In leafless trees, on the other hand, the rise of sap increasing with rise of temperature (there being no loss by transpiration) causes maximum sap-pressure at thermal noon and minimum pressure at thermal dawn, lhe diurnal variation of sap-pressure in leafless trees is therefore the,

reverse of that of leafy trees. These facts were established by records of diurnal variation of pressure inscribed by the Self-recording Manometer.1 I now proceed to determine the diurnal variation of sap- pressure by the independent method of the Sphygmograph. An increase of sao-pressure in the stem at any particular period of the day will involve diametric expansion of the cortex, and a diminution will produce contraction ; these periodic changes will recur day after day. Any permanent expansion in diameter by growth in the course of 24 hours would be too slight to affect the record with moderate magnification.

Experiment 206. — The specimen employed was a young Mango tree about 60 cm. in height. The stem bore a rosette of six leaves at the top ; the sphygmographic con¬ tact was made a short distance below the leaves. T magnification employed was at first 2000 times. This gave records of diurnal variation of pressure which were very definite and of so great an amplitude that it would have been impossible to reproduce them within the page. I therefore reduced the magnification to 1000 times. The experiment was commenced at 6 p.m. The upper record is that jj the thermal variation, fall of temperature being recorded as an up-curve. In the lower record, which is that of the diurnal variation of pressure, diametric expansion with rise of pressure is indicated by the up-curve and con¬ traction with diminution of pressure by the dcwn-curve (fig, 206).

temperature (19*5° €.) was attained at .6 a.m., while the highest temperature (26° C.) was reached at 12 noon. The turning-point of temperature from fall to rise is therefore at 6 A.M., and that of rise to fall is at 12 noon. Comparison of the curves of temperature and pressure shows a very striking correspondence. The fall of tempera¬ ture between 6 p.m. and 6 a.m. was attended by an increase The upper is the thermographic record, up-curve representing fall of temperature. The lower is the record of valuation of pressure, the up-curve representing expan don and increase of pressure (Mango).

of sap-pressure. The thermal reversal took place at 6 a.m. and the reversal of sap-pressure occurred at about the same time, 6.45 A.M., the delay being due to physiological inertia. The second reversal occurred at noon and, corresponding to this change from rise to fall of temperature, there was a variation from diminished to increased pressure shortly after noon (£g. 206). The results of sphygmographic investigation are thus fully concordant with those obtaineu by the Manometric Method.

Experiment 207. — In order to investigate the effect of a reduction of transpiration, the experiment was continued with the same specimen (Mango) after removal of all the leaves. It was very interesting to note that the old rhythm persisted, more or less, for several days according to the Note the reversed record compared with that of the leafy tree. Maximum sap-pressure at thermal noon, and minimum pressure at thermal dawn (Mango). habit that had been impressed on the organism. On the fourth day, however, the diurnal variation exhibited a com¬ plete reversal, the maximum pressure and expansion being attained at thermal noon and the minimum at thermal dawn (fig. 2C/). This agrees with the variation of sap- pressure in leafless trees as recorded by the Manometer.

The effect of any slight variation of sap-pressure can be recorded by the Plant-Sphygmograph with a magnification of 25,000 times. Accession of sap at any point in the cortex of the stem is attended by expansion and increase of pressure. Removal of sap is, on the other hand, followed by contrac¬ tion and diminution of pressure. The amplitude of the response recorded by the Sphygmo- graph increases, within limits, with rise of temperature. Change in the rate of loss of water by transpiration appropriately modifies the sphygmographic record.