The Physiology of the Ascent of Sap

Lowering of temperature likewise induces a diminution of the rate of growth till, at a critical temperature, growth becomes arrested. This critical point varies in different species of plants ; but in several tropical plants examined, it was found to be about 22° C. Anaesthetics when given in large doses act as poisons, causing the death of the plant. With regard to the action of poisonous agents in general, the amount of the dose is of importance ; the striking general result which I have ob- tained in this connection is, that while a poisonous solution of moderate strength arrests or abolishes all life-activity, a small dose enhances it. Opposite effects are thus pro- duced below and abovs the critical dose. With strong poisons the range of safety is very narrow ; with less toxic agents, however, the range is wider, and by regulating the dose it is not difficult to produce either a stimulating or a toxic effect.

Ether is less toxic than chloroform, and it is easy to obtain with it the stimulating effect of a small dose. Though the application of chloroform is apt to prove fatal to the plant, yet even here we can obtain the opposite effects of small and large doses without great difficulty. For when a large quantity of chloroform is applied, the plant absorbs it slowly ; the preliminary effect is therefore the same as that of a small dose in the enhancement of activity. Long continued application, however, brings about the toxic effect. While all modes of rhythmic activity are enhanced by the application of small doses of anaesthetics, continued application produces arrest of activity and ultimate death, as is illustrated in the following records of Desmodmm pulsations and of growth.

Desmodium Pulsation. — Beginning with the effcc^ of ether, the leaflet used was in a slightly depressed state, so the introduction of dilute ether-vapour into the plant- chamber induced an enhancement of activity (fig. 6, a). Continued application arrested the pulsation ; but the arrested activity could be revived by substituting fresh air for the ether- vapour. The effect of chloroform is shown in the record (fig. 6, b). The leaflet in this case was in a state of standstill ; the preliminary stimulating effect is seen in the renewal of the arrested pulsation. The continued action of chloroform caused arrest, and the death of the plant as seen in the spasmodic contraction and the resulting down-movement.

Growth. — I have carried out numerous experiments on the effect of anaesthetics on the growth of various organs. (a) Effect of small dose of ether in enhancing the pulsation of (b) Effect of chloroform ; note the preliminary enhancement, followed by depression, arrest, and death as indicated by the spasmodic contractile movement downwards. The results obtained are similar in all cases. The specimens were placed in a closed chamber with an opening for the

Fig. 7. Effect of Anaesthetics on Cirowth (a) Enhancement under small dose of ether ; (b) preliminary enhancement followed by spasmodic death-contraction, under the action of chloroform. passage of the connecting link by which the plant was attached to the High Magnification Crescograph. The magnification employed was about looo times, tlie successive dots in the records being at intervals of fifteen seconds. The anaesthetic vapour was intro- duced into the chamber by means of an inlet pipe. The record was taken on a moving plate, and the first part of the curve indicates by its slope the normal rate of growth. Application of chloroform produced a preliminary enhancement of growth, seen in the sudden erection of the curve. Continued application induced an arrest, as seen at the turning point of the curve. This is the critical point, for further application of the anaesthetic produced a sudden spasmodic contraction giving rise to the reversal of the curve (fig. 7, b). The apex of the curve demarcates life from death. After this reversal, spots of discoloration appeared in the plant ; these spread very rapidly and the specimen became wilted as a consequence of death.

In regard to the action of different poisons, it must be remembered that a certain substance may prove very toxic to one plant and not so much so to another. Plants may also become accommodated to the action of a poison. Growth. — Poisons retard or abolish growth. Thus in a particular experiment, the application of one per cent, solution of copper sulphate depressed the rate of growth from the normal 0-45 //- to 0-13 /^ per second. Prolonged application of the poison killed the plant.

Desmodium Pulsation. — A poisonous solution of potas- sium cyanide was applied directly to the pulvinule of the leaflet ; this caused a complete arrest of pulsation in the course of seven minutes (fig. 8, upper record). In another experiment the poison was applied at a distance, namely at the cut end of the petiole which carried the pulsating leaflet. In this case the arrest of pulsation took place much later, i.e. after thirty-eight minutes, the

delay being due to the time taken by the poison to ascend through the intervening distance (fig. 8, lower record). This experiment also demonstrates that a poisonous solution can pass through a killed tissue, owing to the suctional activity of the cells higher up, a fact that bears Fig. 8. Effect of Poison on the Pulsation of Leaflet of Desmodium In the upper record the poison was applied directly on the pulvi- nule. The lower record exhibits the effect of application of poison at the cut end of the petiole, the arrest taking place much later. The gap in the lower record represents an interval of twenty-four minutes.

upon Strasburger's experiments already mentioned. The matter is discussed in Chapter V. The physiological characteristics of pulsatorj/ activity have now been described, as ascertained in fully investigated instances of plant-movement. It now remains to deter- mine, by the application of similar methods, whether or not the ascent of sap responds in an essentiall}^ similar manner. The following have been shown to be the physiological characteristics of pulsating tissues.

i. Pulsatory activity is depressed or arrested under diminished internal pressure ; ii. Normal pulsation is inhibited by the action of strong stimulus, the after-effect of which may be an enhancement of activity ; iv. The response of a sub-tonic tissue is opposite to that of the normal ; that is to say, stimulus revives the arrested, and enhances the enfeebled, activity ; V. Rise of temperature up to an optimum enhances and fall of temperature depresses, rhythmic activity ;

vi. Pulsation is arrested at a critical point which is the temperature minimum ; arrested pulsation is revived when the temperature is raised above this critical point ; vii. A small dose of an anaesthetic induces an enhance- ment of activity ; viii. Pulsation is arrested under the continued action of a large dose of the anaesthetic ; ix. Rhythmic activity is permanently abolished by the action of poisons. Detection and record of ascent of sap — Mechanical Method of Erectile Response — The Automatic Kecorder^ — Erectile response of Mimosa, Chrysanthemum and Impatiens — The Osmotic Theory — Theory of suction and root-pressure — Ascent of sap in absence of root-pressure and transpiration — Depressed rate of ascent under increasing drought — Ascent of sap in cut stems previously exposed to air--r"unction of the xylem — Summary.

In the study of the ascent of sap great difficulty is en- countered in the measurement of the rate of flow and its induced variations. The withering of leaves, as stated before, is a very crude and unreliable test ; some more exact method is essential. Though the direct observation of the movement of sap inside the plant is practically impossible, yet we may detect and measure some of the effects induced by it. In electric measurements we are unable to see the passage of electricity, but are nevertheless able to detect and measure the current by its various effects, such as the production of heat, the directive action on the magnetic needle, the movement of a string across the magnetic field, the chemical effect, and so on. It is thus possible to construct different types of galvanoscopes or galvanometers possessing various degrees of sensibility. Similarly, by taking advantage of the effects produced by the conduction of flow of sap in a plant, we should be able to construct various instruments for the detection or measurement of its ascent. I have, in fact, devised two different methods for this purpose, namely, those of mechanical and of electrical response. In the present chapter 1 describe in detail the principle and construction of the automatic Mechanical Recorder, reserving the descrip- tion of the Electrical Recorder for a subsequent chapter.

The principle of the method will be understood from the following experiments. A cut specimen of Chrysanthemum coronarium is subjected to drought, when the plant doubles over, the leaves shrink and appear crumpled up and dried ; in fact the plant seems to be dead. But irrigation brings about a marvellous transformation through the ascent of sap ; the original turgor is restored, the bent stem straightens up and the withered leaves spread out in their original vigour. This is shown in the photograph reproduced (fig. 9), in which complete recovery took place in a time as short as fifteen minutes. I also reproduce

photographs of a potted Impatiens subjected to drought. The rate of ascent of sap here is much slower than in Chrysanthemum ; a partial recovery occurred in the course of two hours, complete recovery being attained after four hours (fig. lo). In nature the plant experiences great fluctuations The first shows the effect of drought, the second exhibits partial recovery two hours after irrigation, and the third shows full recovery after four hours. in its state of turgor. Thus in Bengal there was no rain for six months from October last. The temperature in April had risen to 40° C. or 104° F., so that the plants were suffer- ing from excessive drought when the rains came down in the middle of April. There was thus great variation as regards the available source of supply of water ; and we shall presently have occasion to discuss the manner in which variable conditions of drought affect the ascent of sap. Potted plants are similarly subjected to periodic variation : on watering the plant, the stem and the

leaves become turgid : after one or two days the loss by transpiration from the leaves will reach a point when it will be greater than the supply of water through the ascent of sap, the result being a slight drooping of the leaves. Confining our attention to a particular leaf, we find that fresh irrigation causes an erection of the leaf to the horizontal outspread position. This erectile movement does not take place immediately after irrigation ; a cer- tain time is required for the ascending sap to reach the leaf-joint so as to increase its turgor and thus cause the responsive erectile movement of the leaf. I designate this time-interval as the latent period.

In the case just mentioned, the leaf is the responding organ : but the bent portion of the drooping stem itself may be employed as the responder ; for after irrigation, the ascending sap, reaching the bend in the stem, will cause it to straighten. We have thus two means of detecting the ascent of sap, namely, the erectile response of the drooping leaf, and the erectile movement of the drooping stem. These are so slight that the course of erection from its initiation to uniform movement cannot be made out by mere eye- observation. Moreover, there remains the important element of the time-relations of the response. For the fulfilment of our requirements, it is therefore necessary to devise special apparatus giving automatic records.

The responsive movement induced by the ascent of sap is recorded by the apparatus (fig. ii). The indicating leaf is attached by a thread to a magnifying lever made of fine glass fibre ; the lever itself is mounted on jewel bearings. The magnification may thus be raised from five to a hundred times. The bent tip of the long arm of the lever inscribes the erectile response on a smoked glass plate, kept oscillating to and fro by means of clock-work.

This oscillating device offers the double advantage of eliminating any friction of the recording lever against the glass plate, and of securing the accurate time-relations of the curve of response. Adjustment is made so that the Fir,. II. Automatic Recorder for the. Erectile Response of Drooping Leaf Leaves b and c are attached to two recording levers, b to the upper and c to the lower. The sap reaches b before reaching c ; hence the earlier response recorded by the upper lever. The clock-work for the oscillation and lateral movement of the plate is not shown in the hgure.

oscillation of the plate takes place once in fifteen seconds ; the distance between successive dots therefore represents a definite interval of time. The plant with the slightly drooping leaf is suitably clamped and mounted on a stand, the clamping being just sufficient to prevent slipping ; too great a compres- sion would, obviously, retard the ascent. The lower end of the plant is cut and water applied to it. We shall presently find how the curve of response enables us to determine the characteristics of the ascent of sap and its induced variations.

For the accurate determination of the velocity of ascent, two different levers are employed, as seen in the illustra- tion ; the first being attached to the lower, and the second to the upper leaf, one being vertically over the other. The advantage of this Duplex Method will be described later. The record of the drooping stem is obtained by supporting it by means of a clamp a little below the point where it begins to bend, this bent portion being the responder. Water is supplied at the cut end of the stem, and the erectile response recorded in the usual manner. The arrange- ment for taking record of the response of an intact plant with root is shown in fig. 12 ; the pot containing the plant is placed inside a larger vessel, v, into which water is poured for irri- gating the plant . The Oscillating Recorder illustrated here is of a more compact type than the one previously described.

In illustration of the method described above, I will first describe an experiment with a potted specimen of Mimosa pudica. The plant was in a condition of a slight drought, and the responding leaf was exhibiting a slow and a continuous fall, due to diminishing turgor of the pul- vinus. We know that a sudden diminution of turgor takes place under the action of stimulus which causes a quick Fig. 12. Automatic Recorder for Erectile Response of Drooping Stem

c, clockwork ; v, outer vessel ; s, screw adjustment for rais- ing or lowering the plant. fall of the loaf ; in the present case the gradual diminution of turgor due to increasing drought caused a slow movement of fall. This is seen in the first part of the curve (fig. 13). On irrigation at the vertical line, the fall of the leaf became arrested and then reversed to an erectile move- ment. This took place in the course of thirty seconds, which Down-curve shows gradual fall of leaf under drought. Irrigation at the vertical line induced erectile movement. Application of ice-cold water at arrow arrested the movement in the course of fifteen minutes. The gap in the record represents an interval of ten minutes. (Successive dots at intervals of fifteen seconds.)

is the time taken by the ascending sap to reach the pulvinus from the absorbing root. On the attainment of a uniform rate of erectile movement, cold water was applied to the root, at the point in the record marked with an arrow. This brought about an arrest of the erectile movement in the course of about fifteen minutes. We have already seen that the application of cold induces a diminu- tion of pulsatory activity ; the arrest of the erectile response in this experiment is thus attributable to a retardation

of the ascent of sap induced by physiological depression of rhythmic cells. The method of experiment with Mimosa described above, though of much theoretical interest, labours under certain disadvantages. First, it may be supposed that the results are peculiar to ' sensitive ' plants ; secondly, the very great sensitiveness of the pulvinus demands special precaution against accidental disturbance. I therefore prefer to employ the leaves of ordinary plants as indicators of +lie ascent of sap. Most of the investigations described below have been carried out with Chrysanthemum and Impatiens. Chrysanthemum may be grown in Calcutta from December to March, Impatiens is available during the rest of the year. In the records of different cut specimens given in fig. 14, the distance Intervening between the cut end of the stem and the responding organ is the same in all, namely 15 cm.

The response of the leaf of Chrysanthemum in the following experiments is shown in fig. 14, a, that of the drooping stem in 14, h. They are seen to be very similar, the cause of this resemblance being that in the two experiments the leaf and the stem function merely as indicators of the ascent of sap. In the record of the erectile response of the stem (fig. 14, li), it is seen to take place after the third dot, that. is to say, forty-five seconds after the application of water to the cut end of the stem ; as the intervening distance for the transport of sap was 15 cm., the velocity of ascent was 200 mm. per minute. The record (fig. 14, c) was obtained with a drooping stem of Impatiens ; the erectile response occurred 2*5 minutes after irrigation, the velocity of ascent being 66 mm. per minute, or less than a third of the velocity in Chrysanthe- mum. The curve attained an uniform slope in the course of six minutes after the application of water, and this uniformity was maintained for a considerable length of time, in fact so long as the bent portion of the stem did

not become too erect. For securing an uniform curve, the drooping stem should make an angle of about 5° below the horizon. Uniform slope of curve indicates uniform rate of the ascent of sap ; enhancement of the rate under stimu- lating agents is demonstrated by a sudden erection of the curve, also by wider spacings between the successive dots. Induced depression, on the other hand, is indicated by the Fig. 14. Records of Erectile Response of Drooping Leaf and Stem after Irrigation

(a) Erectile response of ' varnished ' specimen of ChrysanthemiDn (d) Erectile response of Chrysanthemum stem the cut end of which flattening of the curve, and by the closeness of the succes- sive dots. Having secured accurate methods for the determination of the rate of ascent of the sap, I defer to a sub.sequent chapter the study in detail of the effect of physiological agents upon it. For the present I will describe certain important experiments which will show definitely that the generally accepted theory of the ascent of sap is quite untenable in its essential details.

The first stage in the process, the passage of water from the soil into the plant, is described as follows by a well-known author ^ : ' The cells of the root-epidermis absorb water osmotically from the soil. The water ab- sorbed by the epidermis is transferred to the centre of the root since the cell-sap is in a state of greater concentration there than it is in the epidermis, and it will continue to be so transferred until a similar osmotic pressure prevails throughout all the cells of the transverse section. Water in the same way will pass osmotically into segments of young vessels while these are still in an embryonic state and possessed of normal cell-contents. When, however, a segment fuses with the next older segment, an immediate dilution of its osmotically active cell-sap must take place since it is essentially water that is found in adult vessels. The question then comes to be, how can water be abstracted from the cell-sap of a parenchymatous cell and transferred to the lumen of a vessel ; one would expect the precisely converse process to take place.'

There is thus a barrier between the parenchymatous cell and the xylem which cannot be crossed by osmotic action. A different explanation has to be found for the transfer of water to and from the xylem according to different circum- stances. This is afforded by the theory of cellular pulsa- tion according to which the liquid is injected by the living cells into the wood-vascular tissue. Pulsatory activity is dependent, as we have seen, on the internal hydrostatic pressure and the resulting state of turgor. The difference of hydrostatic pressure between two points will be one of the factors in determining the direction of the propulsion of sap from a place of higher to a place of lower potential, from the more to the less active region. The sap-movement

will thus follow the ' turgor-gradient,' tending to equalise the difference of turgor in different parts of the plant. Passing now to the consideration of the further move- ment of water in the stem, some idea of the prevalent view will be obtained from the following quotation : ' It is certain that the water is not merely driven upwards from the root, or base of the stem by the root-pressure acting like a force-pump, but that the removal of water from the conducting channels exerts a force transmitted backwards as far as the absorbing organs, causing in these a corresponding entry of water.' ^