The Physiology of the Ascent of Sap

)f Transpiration the record of Chrysantheniuni with root ; tlie upper, of the shoot without root. of Nicotiana latissima gave off about 16 c.cm. of sap in five days, but its shoot absorbed 200 c.cm. A similar disparity was exhibited in other cases also. Further, it is very improbable that the secretory capacity of the root is sufficient of itself to compensate for loss of water due to transpiration.' ^ The apparent anomaly arises from the erroneous supposition that transpiration from leaves is merely a phenomenon of physical evaporation. It has been shown,

however, that it is effected by the activity of living cells, not only of the terminal layers from which excretion actually takes place, but also of a system of cells extending throughout the plant from the absorbing root to the ex- creting leaf. There is thus a co-ordinated physiological mechanism such that each region of the plant controls and is controlled by the rest. Examples of this have already been described, where drought, by depressing the cellular activity, affected not only the absorption by the root, but also the conduction of sap through the stem and the trans- piration from the leaves.

Thus an increased absorption in one region affects the rate of ascent or of excretion in a different region. The root-hairs by their attenuated channels offer great resistance to the inflow of water into the plant. Consequently the removal of the root accounts for the enhancement of the quantity of water absorbed, as in Sachs's experiment, and of the transpiration from the leaves, as seen in Table XVIL ; for the root (on which the supposed root- pressure depends), far from increasing the rate of ascent, actually impedes the flow of sap.

Table XVII. — Showing the Absolute Kate of Transpiration for 24 Hours in Chrysanthemum, with and without Root We shall next attempt to determine the cause of the diurnal periodicity of transpiration. We have here two variables, namely, the daily variation of temperature and the recurrent change of light and darkness. The daily variation of temperature was recorded by a thermograph placed near the transpiring leaf : but there was, unfortu- nately, no apparatus available for the continuous record of the variation of light.

This difficulty was, however, overcome by an apparatus which I have devised for the automatic record of variations in the intensity of light. ^ Selenium exhibits a diminution of resistance under light, and the increasing deflection given by a galvanometer in circuit with a battery of voltaic cells gives an indication of the increasing intensity of light. But the continuous passage of a current gives rise to a counter electromotive force, and the selenium-cell then becomes unreliable. For overcoming this difficulty, the photo-electric cell was placed in the fourth arm of a Wheatstone-bridge, so that there was no deflection in the galvanometer under condition of balance in darkness. A vertical tube projects over the selenium-cell, which is closed with an electro-magnetic shutter. The battery of cells is normally cut off from the circuit, which is closed at definite intervals by a clockwork which actuates three keys in succession. By these means the battery is closed, the shutter of the selenium-cell is opened, and the deflected index of the galvanometer is recorded on a moving piece of paper. The keys are then automatically opened, and the process repeated at definite intervals of time, which may be varied from five minutes to an hour.

I reproduce curves of diurnal variation of temperature and of light taken in March (fig. 39), which will give a general idea of the hourly changes. In summer the light 1 The detailed account of the Radiograph will be given in vol. iii. of the Transactions, Bose Institute. appears about an hour earlier, and disappears an hour later ; in winter the day is shorter by about two hours. The temperature-curve shows a steep rise, attaining its maximum at the thermal noon, about 2 p.m. ; the fall is more gradual, and the minimum temperature is attained between 5 and 6 a.m. The above is true in settled weather

Fig. 39. The Curves of Diurnal Variation of Light and of Temperature conditions ; in unsettled weather there are fluctuations in the diurnal curve. As regards light, there is a very steep rise in intensity during the forenoon, the maximum being attained at noon ; the thermal noon, as already stated, is later by about two hours. There is a steady decline in intensity from noon to 4 P.M., after which the fall is abrupt till the total dis- appearance of light after 6 p.m.

Comparing the transpiration-curve with those of light and temperature, we find that the effect of light is practi- cally negligible. The maximum light is at noon, but the maximum transpiration occurs two hours later, at the tliermal noon. The similarity of the curves of transpira- tion and of temperature is, moreover, very striking. This proves that the diurnal variation of transpiration is mainly due to the periodic change of temperature. The question next arises as to whether the increase of transpiration with the rise of temperature is simply due to increased evaporation, or whether a physiological element also enters into the problem : for rise of temperature en- hances not only evaporation but physiological activity also. Hence it becomes necessary to distinguish the effect of one factor from that of the other by some discriminating test. Evaporation no doubt helps indirectly in the uni-directioned ascent of sap by maintaining the turgor-gradient. But the independent factor of physiological activity in trans- piration may exhibit a course which is not exactly parallel to that of evaporation.

It is possible to discriminate between evaporation and transpiration by balancing one against the other. This is done by placing the transpiring leaf on one pan of a balance, say the right, an equivalent area of water-surface being placed on the left pan for evaporation. It has been shown (p. 87) that the equivalent area of water is about one-fifth that of the leaf-surface of Thimhergia. But for our present purpose it is necessary to obtain not an approxi- mate but a very exact balance. This is secured by the Surface-Variator which will be presently described. By this adjustment the loss by evaporation from the water- surface in the one pan is made to balance exactly the loss by transpiration from the leaf placed in the other pan.

If transpiration be a simple phenomenon of evapora- tion, then the loss at the two pans of the balance will be equally affected by variations of temperature, and the index of the balance once adjusted to zero will always remain there. But the index is found to be upset in one direction or the opposite according to the variation of Fig. 40. The Differential Balance The Surface Variator, c, seen on the left pan of the balance. A vertical wire from c passes through a guide-hole in the bent piece of metal, p, the upper pan containing shots for trans- ference to c. The right pan contains the transpiring leaf l, mounted in a vessel with a side-tube with an oil-valve, g, plate for recording movement of index r of the Differential Balance. On the right-hand smoked glass is recorded the variation of temperature.

temperature. This proves that transpiration is not the same as evaporation, but that an additional physiological factor is involved in the process. account of the various sizes of the leaves and their different physiological condition, it is not easy to obtain an exact balance between evaporation and transpiration. This difficulty has been overcome by the device of the Surface Variator, which consists of an inverted cone floating in a beaker of water : the area of the evaporating surface may be continuously varied by varying the depth of immer- sion of the inverted cone used as a sinker. The cone is sunk by the addition of small shots. After obtaining a balance of weights on the two sides of the balance, the shots, which are kept on the small upper pan, are trans- ferred to the sinker. The balancing thus remains the same, the change produced' being only in the area of the evaporative surface of water. The exact balancing of transpiration against evaporation at any definite tempera- ture may be secured by this contrivance without any difficulty. The long index records any variation in the balance throughout twenty-four hours. The record is taken on a small glass plate which oscillates to and fro at intervals of half an hour. The record of diurnal variation of temperature is also taken on a second plate ; for this the usual metallic thermometer is employed. For pre- vention of disturbance of the index by air-currents, it is necessary to place a cover over the apparatus. For this a glass cover is not suitable, since it interferes with the free evaporation and transpiration ; fine netting was therefore substituted for glass in the cover. The whole apparatus was placed in a large greenhouse.

The record of the differential result of transpiration balancing evaporation is given in fig. 41. The trans- piration from a leaf of Thunbergia was exactly balanced at 5 P.M. The index did not, however, remain in the zero position, but drifted to the left till next morning, indicating a depression of transpiration compared with evaporation. Comparison with the thermographic record brought out the interesting fact that this relative depression of trans- piration occurred during the fall of temperature. When

the temperature began to rise next morning, the index began to return to zero. As the temperature rose further, the balance became displaced once more, but this time to Fig. 41. Diurnal Record of Transpiration balancing Evaporation Note the drift of the balance index to left (downwards in the Fig.) with falling temperature, indicating relative diminution of transpiration, rise of temperature inducing opposite effect. ' Reversal of curve above optimum-temperature of 33° C.

the right, showing that the transpiration was relatively increased with the rising temperature. This increase was, however, arrested as the temperature rose above 33° C. ; at 34° C. the balance was upset to the left. The curve of relative enhancement of transpiration which had hitherto followed the rise of temperature became reversed above 33° C. This showed that the transpiration exhibited a relative increase up to that temperature, above which there was a decline.

This upsetting of the balance at about 34° C. at first appeared inexplicable ; further experiments showed that transpiration reaches its maximum at an optimum temperature. For an independent demonstration of this I took a leaf of Thunbergia, mounted on the Bubbler, and placed it in the conservatory, where the previous record had been obtained with the Differential Balance. The light in the conservatory was practically uniform in the forenoon, but the temperature exhibited a gradual increase from 31 • 5° at 10 a.m. to 33 • 5° at 11.30 a.m. The transpiring activity of the leaf was taken for each half degree rise of temperature. The following table gives the result of the experiment.

Table XVIII. Showing the Optimum Degree of Temperatuke FOR Transpiration (Thunbergia) It is thus seen that the transpiration of the leaf was in- creased continuously from 45 to 71, as the temperature rose from 31 '5° to 33°. A sudden decline from 71 to 52 occurred as the temperature rose from 33° to 33 "5°. The temperature of 33° may therefore be regarded as the optimum temperature for transpiration in the leaf of Thimbergia. We thus demonstrate a phenomenon which discriminates transpiration from evaporation. Evaporation is continuously increased with rise of temperature, hut transpiration is en- hanced up to an optimum point beyond which there is a decline.

Since the autonomous activity of growth has a definite temperature-optimum beyond which it undergoes a decUne, it is very interesting to find that transpiration also exhibits an optimum, the existence of which is a remarkable demon- stration that pulsatory activity underlies transpiration. In conclusion, we have now evidence of the existence of a physiological mechanism by which the absorption, the conduction, and the transpiration of water are auto- matically regulated, and the life of the plant maintained. During excessive drought caused by summer heat and intense sunlight, the following regulating devices are brought into play. The excessive loss of water is prevented, first, by the enfeebled rate of ascent of sap that is induced by drought (p. 45) ; secondly, by the stimulus of strong sunlight which retards conduction along the stem (p. 47) ; and finally, by the rise of temperature, which, though it continuously increases evaporation, interposes a physio- logical check on transpiration when it exceeds the optimum. It is indeed remarkable that the physiological activity which maintains the ascent of sap and the process of tran- spiration should possess automatic powers of regulation by which the plant is enabled to survive periods of severe drought.

Transpiration exhibits a diurnal variation which is principally determined by the daily variation of tempera- ture. Transpiration is at its maximum at thermal noon and at its minimum at thermal dawn. The effect of varia- tion of light is comparatively slight. There is a continuity of physiological action throughout the plant, in consequence of which each part of the plant controls and is controlled by the rest. Transpiration is thus increased with the enhancement of the ascent of sap resulting from the removal of the root.

The relative effect of physical evaporation and physio- logical transpiration is determined by balancing evaporation against transpiration by means of the Differential Balance. It is thus found that, with rising temperature, transpiration is relatively greater than evaporation. Transpiration exhibits an optimum temperature which in Thunbergia is 33° C. Above the optimum degree the activity of transpiration, like that of growth, undergoes a rapid decline.

The foregoing characteristic discriminates transpiration from evaporation. Evaporation is continuously increased by rise of temperature, but transpiration is enhanced only up to an optimum, above which there is a depression. The Recorder of Exudation — The Tilter and the Electromagnetic writer — Composition of exuded sap — Continuous record of exudation- — Effect of drought — Effect of mechanical and electrical stimulus- — Effect of poison — Effect of anaesthetics — Continuity of action in root and in shoot — Activity of terminal layer at the cut end — Expulsion of sap by living cells — Summary.

Having studied the relation to the ascent of sap of the activity of the excretory organ, the leaf, at the upper end of the plant, we pass on to consider that of the absorbent organ, the root, at the opposite extremity. It commonly happens, when a plant is cut across at the level of the ground, that liquid is exuded, sooner or later, at the cut surface of the root-stock. This exudation of sap is attributed to a force generally termed ' root- pressure ' ; it is this that has been investigated with results now to be given.

We approach the subject with the conviction that the conclusions already reached concerning the movement of the sap in the stem are equally applicable to the root. The absorbed water is pumped from cell to cell ; and when the rate of absorption is too rapid for this physiological conduction, the excess water is pumped into the wood- vascular tissue in which it is driven upwards by an increasing ' intra-vascular ' pressure, to escape when the root-stock is cut across. The value of the intra- vascular pressure depends upon the relation between absorption and transpiration : when the former is the more active, the pressure is positive ; when the latter, it is negative.

The varying activity of the root-cells may be gauged by the measurement either of the rate of exudation ^ from the root-stock, or of the pressure exerted by the ascending sap ; the latter method of measurement will be described in the next chapter. It is of essential importance to be able to obtain a continuous record of the rate of exudation, so that any induced charge in it can be referred to some definite variation in the environmental conditions.

I have been able to perfect an apparatus by which the normal exudation and its induced variations may be recorded. It consists of a Tilter, an Electromagnetic Writer, and a Revolving Drum round which is wrapped a sheet of smoked paper for the inscription of the record (fig. 42). The cut end of the root-stock is enclosed water- tight in an india-rubber cork, and a glass tube led from it allows the drops of exuded sap to fall on to a contrivance by which they are counted electrically.

The Tilter. — ^This consists of a delicately poised lever, one end of which is spoon-shaped for catching the exuded drops of water. The balanced lever, when upset by the falling drops, empties the liquid into the vessel v, and at the same time completes an electrical circuit, in consequence of which a dot is marked on the revolving drum by the Electromagnetic Writer. The tilting lever is balanced by a sliding weight, which allows adjustment to be made within wide limits. The sensitiveness can be so exalted that it is possible to obtain a record of the fall of a single grain of sand.

1 I have, in the absence of better phraseology, used the terms ' exuda- tion,' ' exudation-pressure,' and ' root-pressure.' Root-pressure is, how- ever, not due to any specific action of the root, but to the co ordinated activity of pulsating cells common to both root and shoot ; ' cell-pressure ' would be a better term. Again, ' exudation ' rather suggests a passive process ; but the expulsion of liquid from the living cells of the plant is an active process. ' Excretion ' is a better term ; but it has become associated with the expulsion of undesirable waste-products, a distinction that is purely gratuitous.

The Eledromagneiic Writer. — This consists of a horse- shoe electro-magnet with a polarised armature to which is attached the vertical marker. Only a feeble current is required for actuating the Writer; a couple of dry cells is found sufficient for the purpose. The duration of electric Exuded drops fall on the tilting lever t from the tube attached to the root-stock r by means of an india-rubber cork. The upsetting of the Tilter completes an electric circuit, causing the Electromagnetic Writer to inscribe a dot on the smoked paper wrapped round the revolving drum D. v, vessel for reception of the exuded sap from the tilter.

contact is short, hence the current drawn from the cells is very small. The Recording Drum. — The drum has three speeds of rotation : one revolution in an hour, or in twelve hours, or in twenty-four hours, A smoked paper is wrapped round the drum, and the strokes of the writing-point inscribe the successive dots. The Writer, if desired, may be made to carry a soft pencil for the inscription of marks on white paper. The Recording Drum may be placed inside the laboratory at a considerable distance from the plant ^rown under field-conditions : the electric connection

with the Tilter is then made by means of sufficient lengths of line-wire. It is easy to obtain simultaneous records of four different root-stocks on the same drum : the necessary adjustment is the employment of four electro-magnetic writers placed one over the other Portable Apparatus. — It was sometimes necessary to have a self-contained and complete set of apparatus for obtaining records of exudation of sap by Palms in a place out of the way. Such a compact apparatus is contained in a cubical box, each side of which is only 15 cm. The box can be locked, and strapped to the tree, out of reach. The apparatus requires little attention, and the automatic records are taken out at intervals of 24 hours.

Each dot in the record represents the exudation of a definite quantity of sap, and the distance between the successive dots represents the time required for the exudation of this quantity. Hence the rate of exuda- tion at any moment can easily be determined. The record itself gives a vivid picture of the normal rate and its induced variations. Enhancement of the rate reduces the distance between successive dots, while de- pression induces the opposite change of widening the intervening distance.