The Physiology of the Ascent of Sap

leaves. After this the sap had to ascend i cm. and cross over to the left through a distance of about 4 mm., which was the diameter of the stem ; it then caused erection of the leaf d. After this the flow of sap was reversed in a downward direction, and the successive erections of the leaves took place in a reverse order, D, e, f. The respective time intervals enable us to determine the velocity of movement in an upward, in a transverse, and in a down- ward direction. The specimen, it should be remembered, was under considerable drought and the general rate of the flow of the sap was therefore slow.

Table VIII. — -Giving the Rate of Ascent and the Rate of Reversed Flow downwards The average rate of ascent was thus found to be 19 mm. per minute, and the rate of flow in a downward direction to be 2 "9 mm. The period required for the transverse conduction through 4 mm. was five and a half minutes, or at a rate of 0*7 mm. per minute. Thus, representing the slowest rate of transverse conduction by i, the rate of reverse flow downwards would be 4, and the normal ascent rate 27.

The velocity of ascent may be determined with the highest degree of accuracy by the Duplex Method, in which the latent periods of the absorbing and responding organs are eliminated. plant with roots ; the difference is due to great resistance offered by the fine root-hairs to the entrance of water. The velocity of ascent is modified by the plant's previous history as regards the favourable or unfavourable con- ditions to which it had been subjected.

The effec t of excessive drought is to lower the rate of ascent of sap. Sunlight, acting as a stimulus, retards the rate of ascent : the after-effect of this stimulus is persistent ; anisotropy is thus induced between the sun-exposed and the shaded side of the same stem. The velocity in the shaded side is higher than that in the sun-exposed side. The velocity of flow of sap in the dowTiward direction is about eight times slower than in the normal upward direction. The rate of transverse conduction is about 27 times slower than that of normal ascent.

The Potograph — Effect of physiological agents in modification of ascent — Effect of diminished internal pressure — Effect of stimulus — Modify- ing influence of tonic condition — Effect of variation of temperature on ascent and on growth — The critical thermometric minimum — Drooping of leaves during frost — Phenomenon of accommodation — Effect of anaesthetics — Effect of poison — Method of cxudation^ — Strasburger's experiments — Summary. The various crucial tests discriminating the pulsatory activity of living tissues have been given in Chapter IL These are, the effects of diminished pressure, of the action of stimulus, of the modifying influence of tonic condition on response, of variation of temperature, of the critical thermometric minimum, of small and large doses of anaesthetics and of poisons. In the present chapter will be considered the application of these physiological tests to the ascent of sap, not only in intact specimens with roots, but also in cut stems.

In addition to the two reliable and sensitive methods for the investigation of the velocity of the ascent of sap and its induced variations, namely, those of the response of drooping leaf and of drooping stem, a third method, the method of the Potograph, is now introduced as an indepen- dent and confirmatory test. The experiments were carried out, unless stated to the contrary, with cut stems. The rate of water-movement may be indirectly deduced from the successive readings of the index of a potometcr.

The results obtained with the apparatus in general use are not, however, free from error. The readings again are necessarily discontinuous, and important phases of the induced changes are thereby missed. In order to determine any variation of water-movement, a laborious process of construction of curves from the given data is necessary. It is therefore impossible to obtain any im- mediate indication of the normal rate of suction and its induced variations.

For overcoming these drawbacks I devised the Poto- graph, by which the curve is directly obtained ; the inspection of the curve is sufficient to afford all the in- formation as to the normal rate of water-movement and the direct and the after-effects of external agents on that movement. The apparatus consists of (i) an arrangement by which the immersed part of the specimen may be readily subjected to the action of different excitatory or depressing agents ; (2) a potometric tube by which the normal rate of suction and its variation is observed ; and (3) a contrivance by means of which the excursion of the water- index and its time-relation become recorded. For this last, 1 employ two different methods : the first is automatic, in which the image of the opaque index is thrown upon a photographic plate allowed to fall at an uniform rate by means of a clockwork. The second is a much simpler device, that of following the movement of the index with a recording pen resting on a drum, round which is wound the paper for record ; the drum is kept revolving by a clockwork at a known and adjustable speed. When the excursion of water is followed in the way described, a curve is obtained the ordinate of which represents the quantity of water sucked up, and the abscissa the time. The slope of the curve gives the rate of water-movement ; so long as this is uniform the slope remains constant. If any stimulating agent increases the rate, there is an immediate flexure in the curve, which becomes steeper. A depressing

agent lessens the slope of the curve ; the arrest of water- movement is indicated by a horizontal record. ^ The new type of the Potograph is shown in fig. i6 ; it possesses several advantages over its predecessor, one of which is facility of regulating the temperature of water. The stopcock s allows the introduction of water or other solutions into the vessel. For this purpose the Fig. 1 6. The Potograph The suction of water by the plant is recorded by following the excursion of the water-index in the capillary tube with the recording pen, which traces the curve on the drum d, kept revolving by the clock c. s, stopcock for the introduction of water ; Si for exit ; S2 connects the capillary tube with the plant-vessel.

stopcock Sj, for exit of water, is opened and S2, in con- nection with the capillary tube, closed. After the intro- duction of water or a solution, s and Si are closed and Sg opened. A thermometer inserted into the water-vessel indicates the temperature. The spiral of platinum wire for electric heating is placed at the bottom of the vessel, care being taken that the rootlets do not come in contact with the heating coil. Electric connections are made

1 For greater detail, as also for the more sensitive Method of Balance, cf. Plant-Response and Electro-Physiology. with the coil by means of the electrodes e and e'. The current from a battery of cells is regulated by a rheostat, and the rise of temperature in the water-vessel may thus be adjusted without any difficulty. In order to secure accurate results it is necessary that the temperature of water in the vessel should remain constant. For ordinary experiments in which the effect of variation of temperature is not required, the temperature of the vessel does not in practice vary from the temperature of the room. But when we wish to study the physiological effect of variations of temperature, complications arise from" the gain or loss of heat by the water in the vessel. This is reduced to a minimum by enclosing the plant-vessel in an insulating cover of thick felt, or by placing it inside a box filled with mica-dust. It is also easy to construct a correction-curve for the particular apparatus. The error introduced in neglecting this correction is, however, less than 2 per cent.

Having described the different methods for obtaining the record, we may now enter upon the detailed study of the effects of physiological changes in inducing variation of the normal rate of the ascent of sap. Diminished internal pressure may be produced by the action of drought or by plasmolysis. These were shown to induce a depression or arrest of the pulsation of Desmodium gyrans and of growth (p. 12). A condition of drought diminishes or arrests the ascent of sap. This is not solely due to the absence of water for transmission, but also to the depression of the pulsatory activity of the cells. Thus in a series of experiments carried out with the cut stem of Chrysanthemum, the conducting power of the specimens subjected to excessive drought was found to be depressed to as much as one- thirteenth the normal rate (p. 45).

The application of a plasmolytic solution of KNO3 diminishes or arrests the ascent ; thus a dilute solution of KNO3 applied at the cut end of the stem of Impaticns arrested its erectile response ; a stronger solution applied to a different specimen induced not only an arrest but an actual reversal, that is to say, a drooping movement (Fig. 17, a, b). The effect of plasmolytic solution in diminishing the rate of suction was also determined by the independent method of the Potograph. The normal rate of suction of a cut stem of Croton was 36 c.mm. per

Fig. 17. The Effect of Plasmolytic KXO3 Solution in Arrest of Ascent of Sap Irrigation at vertical line induced normal erectile movement. Application of KNO3 solution at arrow arrested the response. The left figure shows the effect of strong, the right figure minute ; application of dilute KNO3 solution reduced it to 17 c.mm. per minute. It has been shown that in normal specimens the effect of stimulus is to depress or inhibit pulsatory activity, whether in Desmodiiim or in growing organs. In experi- menting on the effect of stimulus on the ascent of sap, a cut stem of Impatiens was taken, into which two pins had been thrust at a distance of two centimetres from each other, these serving as electrodes for the passage of induction-

shocks. After attainment of an uniform erectile response, strong electric shock was applied (at arrow, fig. i8, a). This is seen to have induced an arrest of ascent in the course of fifteen seconds ; the arrest persisted for a considerable length of time. Fig. 1 8. Effect of Stimulus on the Ascent of Sap in Normal and Sub-tonic Specimens (a) Efifect of strong stimulus applied at arrow in arresting ascent. (b) Effect of stimulus of moderate intensity inducing arrest with

(c) Effect of stimulus on sub-tonic specimen in which ascent was at a standstill. Stimulus of moderate intensity initiated ascent for a short time ; stronger stimulus at s' produced persistent ascent. In another experiment, the induction-shock applied was only moderate. This gave rise to a temporary arrest, followed by recovery alter three minutes. Results similar to those obtained with electric stimulus were also obtained with other modes of stimulation, such as that of light. The stimulus of sunlight has already

been shown to induce a persistent diminution of the rate of ascent (p. 47). The effect of stimulus on a sub-tonic specimen has been shown to induce renewal or enhancement of pulsation in Desmodium, or an enhanced rate of growth in growing organs (p. 15). The effect of stimulus on a sub-tonic tissue is thus diametrically opposite to that on the normal. Similarly, the ascent of sap in sub-tonic specimens is enhanced under the action of stimulus. This is seen in the record of a sub-tonic specimen of Impatiens. The sub-tonicity of the specimen is evidenced by its inability to suck up water even after irrigation, the record remaining horizontal. Application of electric stimulus of moderate intensity at s induced a transient renewal of the ascent ; stimulus of stronger intensity applied at s' induced a renewal which persisted for a considerable length of time (fig. 18. c).

I also studied the effect of stimulus on sub-tonic speci- mens of plants with roots by the potographic method. The plants were mounted on the recording Potograph and afterwards placed in a dark room, till the normal suction was nearly abolished. This occurred in Zea Mays after twenty-four hours ; but in Impatiens the arrest did not take place till after several days. In Zea Mays the rate of suction declined to 0-24 c.cm. per minute. Electric stimulation of a definite intensity and duration was now applied to the lower end of the plant, one electrode of the induction-coil being dipped in the water-vessel, and the other applied to the stem 2 cm. above the root. The effect of the first stimulation was to enhance the rate from o -24 to o-6o c.cm. per minute, that is to say, it more than doubled the rate. The second increased it to 0-85, and the third raised it still higher to l-o c.cm. per minute, which was the climax, for the fourth stimulation induced a decline.

as in normal specimens. Effects similar to the above were also obtained with Impatiens. Table IX. — The Effect of Electric Stimulus on the Ascent OF Sap in Sub-tonic Specimens Rise of temperature has been shown to enhance the autonomous activities of Desmodium pulsation and of growth ; fall of temperature, on the other hand, causes a depression (p. 17). Variation of temperature also induces similar effects in the ascent of sap. The investigation was carried out by two different methods, first by the Erectile Response of drooping stems and second by the method of the Potograph. The specimens were Impatiens with and without roots.

I will first describe the results obtained by the Erectile Method with a rooted specimen. Record was first taken of the actual rate, showing that the plant was exhibiting a continuous drooping, as seen in the down curve (fig. 19, a) ; watering the plant, at the vertical line, with water at normal temperature, arrested the drooping and brought on the erectile response. Cold water was next applied at c ; this caused a flattening of the curve indicating the relative depression of the rate. Warm water was next

applied at ii, with the result of a great enhancement of the rate of erection, and therefore of ascent, as shown by the erect curve and the increased distance between the successive dots. The record given by a magnifying lever labours under the defect that an arc is described, on account of which the flexure caused by the variation of the rate of ascent is not so pronounced towards the end as in the middle of the curve. The defect arising from the curvature in the record may, however, be eliminated ; it is least pronounced in the middle part of the record through a length of about 5 cm. We take the record on a stationary plate, after adjusting the lever slightly below the middle ; the plate is next moved sideways through about i cm., and the next record is commenced at the same level as the first. This is accom- plished by lowering the plant, the stand on which it is placed being provided with a rack and pinion; Successive records of the effect of different temperatures are thus obtained in the middle part of the plate.

In fig. 19, b, are given successive records showing the effect of variation of temperature on the ascent of sap, the temperature rising from 30° to 35° C. and falling once more to normal 30'' C. It will be seen that it took fifteen minutes to cover a distance of 40 mm. at the beginning of the experiment, and sixteen minutes for covering the same distance, at 30° C, after completing the cycle of temperature- variation ; the two determinations for 30° C. are practically the same, the average period for the same length of the record being 15-5 minutes. At 35° C. the same distance was described in eight minutes. Hence the rate of ascent of sap at 35*^ is about 1-9 times that at 30° C.

I also reproduce, for comparison, a record of growth taken on a stationary plate at normal temperature, under cold, and under warmth (lig. 19, c). Method of the Potograph. — We determine first the normal rate of suction of a cut stem of Impafiens at 30** C, which is the temperature of the room in summer. Warm or cold water is then introduced into the plant- vessel, and the record obtained shows the effect of rise or fall of temperature on the rate of suction. In studying the effect of rise of temperature we introduce warm water, say, at 35° C, or the rise is effected by means of the electric

Fig. 19. Effect of Variation of Temperature on the Ascent of Sap and on Growth (a) Record of erectile response on a moving plate. Cold water water at H enhanced it. {b) Effect of cyclic variation of temperature of 30°, 35°, and 30° C. dots ; enhanced rate indicated by wider spacings of the dots. (c) Effect of variation of temperature on the rate of growth taken heating coil. After the attainment of the steady condition, record is taken of the resulting rate of suction.

Correction for thermometric effect. — In carrying out experiments on variation of temperature we have to apply a correction for the thermometric effect. It is to be re- membered that the vessel of the potometer acts as the bulb of a thermometer. When the water in the vessel is at a higher temperature than that of the room, there is a loss of heat through conduction and radiation. The loss can be greatly reduced by a non-conducting cover. Again, other things being equal, the rate of loss of heat and of fall of temperature will be greater the greater the difference between the temperature of water in the vessel and that of the surrounding temperature. With a difference of one or two degrees, the rate of loss will be very slight. During the fall of temperature the water in the vessel will contract, and the index will show this by a movement which is in the same direction as that of the suction by the plant. For obtaining the absolute rate of suction we have, therefore, to apply a correction, which is to be subtracted from the observed rate. When the water in the vessel is at a lower temperature than that of the room there is a gain of heat and a consequent expulsive movement of the water-index, which is in a direction opposite to that of suction. The actual suction will be greater than what is observed, and we have to add a correction for the true rate. It is therefore necessary to obtain a correction-curve for different tempera- tures applicable for the particular apparatus.

Experimental method of obtaining the correction-curve. — For determining this correction, a glass stopper closes the aperture through which the lower part of the plant is inserted into the vessel. The water of the vessel is raised 5 degrees above the temperature of the room, this being the maximum rise generally employed in the experiments. Observations are commenced after the attainment of a steady condition. The thermometer inside the vessel shows the rate of fall of temperature ; and the movement of the water-index the rate of contraction of the water due to the fall of tempera- ture. It was found that, under the conditions of the experi- ment, the temperature fell from 35-5° C. to 34-5° C. in the course of forty minutes, and the total contraction of the index was 98 mm. ; the average rate of contraction is therefore 2 -4 mm. per minute for the mean temperature of 35° C. The average rate of suction of Impatiens at 35° is, on the other hand, about 135 mm. per minute. The correction for the apparatus at a temperature 5° C. above that of the

room is thus i-8 per cent. For temperature five degrees below that of the room the correction is of the same order but of positive sign. For smaller differences of temperature the correction is negligible. In studying the effects of change of temperature on the ascent of sap, a rising temperature can be kept under better control by the electric heating of the platinum coil immersed in the vessel than by pouring in hot water. Lowering of temperature is effected by the introduction of cold water. The surrounding temperature in Calcutta varies from about 22° C. in winter to nearly 40° C. in summer.

Ejfect of variation of temperature. — The experimental plant employed was Impatiens ; the temperature of the water' at the cut end of the stem was first lowered and the record taken at 25° C. ; it was next raised to 30° C, and afterwards to 35° C. The record obtained gives the move- ment of the index in mm. per minute. The absolute quantit}^ of water in cubic mm. sucked up by the plant is found by multiplying the rate of movement of the index by a constant, which for the capillary tube used was 0-24. The following table gives the rate of suction at different temperatures.

Effect of cyclic variation of temperature.- — An investiga- tion was next carried out on the effect of cyclic variation of temperature, that is to say, of the determination of the rate of suction at different temperatures for both thermal ascent and descent. Observations were made as the temperature rose successively from 25° to 30° and then to 35° C. ; the temperature was next varied in a reverse direction from 35° to 30° and afterwards to 25° C. The two sets of results did not at first exhibit any close agreement. Further investigation showed that this was due to the fact that sufficient time had not been allowed for physiological adjustment to the changed conditions. In studying the effect of a given temperature, the specimen should be subjected to it for at least twenty minutes before taking the record. With this precaution the record of a cyclic change is extraordinarily consistent. The following table gives the results of the observations.

Table XI. — The Effect of Cyclic Variation of Temperature. (Capillary Constant 0-24) It will be seen that the suctional activity is enhanced during rise and depressed during fall of temperature, and that the rate of suction for a given temperature during the ascent and descent is practically identical. Comparison of the rate of Ascent and of Growth at different temperatures. — It will be instructive to compare the effect of variation of temperature on the two autonomous activities of the ascent of sap and of growth, at about the medium temperature between 30° and 35° C. By the Potograph we found the rates of ascent at the two temperatures to be in the ratio of 81 : 150 or as i : 1-85. By the method of Erectile Response also the ratio of the rates of ascent for the same difference of temperature was seen to be 1:1-9. The rates of growth (Table IV, p. 18) at the two temperatures were found to be 0-32 /u, and o -84 /a, the ratio