The Physiology of the Ascent of Sap

Additional reflexes are caused by other modes of stimulation. The complexity of the life-movements arises from the presence of numerous reflexes sometimes concordant with, sometimes in antagonism to, each other. The various theories that have been proposed in explana- tion of the ascent of sap are admittedly inadequate. The generally accepted view is that the motive power for the ascent is supplied to some extent by the root-pressure, which acts like a force-pump, but chiefly by the backwardly transmitted pull resulting from transpiration by the leaves. It has, however, been conclusively proved that neither root-pressure nor transpiration from the leaves is essential for the ascent of sap, by the experiment in which the root and the leaves were removed, the stem being coated with an impermeable varnish ; on application of water to the cut end of the stem the ascent was found to take place with a velocity of more than i8 metres per hour (p. 36). This high rate of ascent was thus attained in the complete absence of root-pressure and of transpiration. Slow osmotic action, moreover, could not possibly ensure such a rapid ascent. As the movement of sap takes place even in small pieces of cut stem, it follows that the activity underlying the pro- pulsion of sap is not confined to any particular region of the plant, but exists throughout its whole length. Further, the investigations described in the present work prove that the ascent of sap is due to the pulsatory activity of definite layers of cells in all parts of the body of the plant, the exact position of which has been localised. The inves- tigations have included the three regions of the plant — the absorbing root, the conducting stem, and the excreting leaf.

Investigations on the effect of physiological change on the ascent of sap have been carried out by the determination of the normal rate of ascent and its induced variations. Three accurate methods have been devised for this purpose : (i) that of the Erectile Response of the drooping leaf ; (2) that of the Erectile Response of the drooping stem, and (3) that of the Electric Response (pp. 25, 195). The Mechanical and Electric Methods give results which are identical. The velocity of ascent is found to be modified by the physiological condition of the tissue, and mav var}^ from 0*3 mm. to about 1000 mm. per minute.

The rhythmic activity which underlies the propulsion of sap has been demonstrated by the comparative method. Any agent which modifies the pulsations of the leaflet of Desmodium gyrans, or the autonomous movement of growth, has been shown to induce corresponding modifications in the ascent of sap. The characteristic effects of various accelerating or inhibiting agencies not only offer crucial proofs of the essentially physiological nature of the action which maintains the water-transport, but also demonstrate that rhythmic activity is the most important factor in the process.

That the action of external agents induces identical effects in all modes of pulsatory activity, including the ascent of sap, has been demonstrated in a variety of cases, such as (i) diminution of internal pressure; (2) stimulation of tissues in a normal condition ; (3) stimulation of tissues in a sub-tonic condition ; (4) variation of temperature ; (5) the arrest of rhythmic activity at a critical temperature ; I. Effect of Diminution of Internal Pressure. — -Diminished internal pressure, due to drought or to the action of a plasmolytic solution, induces an arrest of movement in Desmodium, the arrest of growth (p. 12) and equally the arrest of the ascent of sap (p. 54) : the cellular pulsations

on which the propulsion of water depends are, under these conditions, brought to a state of standstill (p. 241). Excessive transpiration does not, as is commonly held, increase the rate of the ascent of sap : on the contrary, it has been shown that a condition of drought diminishes the rate of ascent, owing to the reduced activity of the pulsating cells. As pulsatory activity depends on internal hydrostatic pressure, the pulsating cells are relatively more active in the more turgid portion of the plaat. The direction of propulsion of sap thus follows the ' turgor-gradient,' normally, from the root, absorbing water, to the top of the shoot, in which partial drought is produced by trans- piration from the leaves. On supplying water to the leaves and withholding it from the root, the turgor-gradient is reversed ; the flow of sap now takes place downwards. The relative velocity of movement of sap in normal ' up,' reversed ' down,' and in transverse directions, has been found, in typical cases, to be as 27 : 4 : i (p. 49).

2. Effect of Stimulus on Normal Tissues. — Strong stimulus inhibits rhythmic activity ; the pulsations of the Desmodium leaflet, and also the rate of growth, are diminished or arrested by it (p. 14) : similarly it diminishes or arrests the ascent of sap (p. 56). Sunlight acts as a stimulus on herbaceous stems in which there is no thick bark to obstruct the light. The after-effect of long- continued action of light on these plants is to produce a persistent diminution of the rate of ascent, in consequence of which a physiological anisotropy is induced between the sunlit and the shaded sides of the plant. The velocity of ascent on the sunlit side is markedly lower than on the shaded side (p. 47). In trees with thick bark, the incident sunlight causes a rise of temperature and thus enhances the activity of the side exposed to the sun (p. 176).

3. Effect of Stimulus on Sub-tonic r^sswes.— Rhythmic activity is enfeebled or arrested under the condition of sub-tonicity. The application of stimidus to a tissue in that condition restores its activity ; it renews the arrested pulsation of Desmodium (p. i6), the activity of growth (p. 17), and the ascent of sap (p. 57). 4. Effect of Variatio7i of Temperature. — A rise of tem- perature up to an optimum enhances the frequency of pulsation in the Desmodium leaflet, in the rate of growth (p. 17), and in the rate of ascent of sap (p. 58). A rise of temperature of 5° C, from 30° to 35°, doubles the rate of growth and of the ascent of sap. Fall of temperature has the opposite effect of lowering all pulsatory activities.

5. The Critical Minimum Temperature. — Pulsatory activity is arrested at a minimum temperature. In mature organs of several tropical plants this is about 14° C. The pulsation of the Desmodium leaflet and the ascent of sap in cut stems are arrested at this temperature. Growth is arrested at about 22° C, which is also the critical tempera- ture for arrest of ascent of sap through growing roots (p. 68). One and the same tissue thus becomes a conductor or a non-conductor for the ascent of sap in accordance with the alternate rise or fall of temperature above and below the critical point.

6. Effect of AncBsthetics . — A small dose of anaesthetic has a stimulating effect, while a large dose brings about an arrest of activity, culminating in the death of the organism. The preliminary effect of chloroform is an enhancement of the amplitude of pulsation in Desmodium (p. 19), an enhancement of the rate of growth (p. 20), and an enhance- ment of the rate of ascent (p. 69). 7. Effect of Poison. — As the rhythmic tissue which main- tains the ascent is continuous throughout the length of the plant, the death of a particular zone by poisoning or scalding does not arrest the suctional activity of the un- killed portions above. The suction may therefore persist till the whole length is killed (p. 22). The effect of poison is manifested in the arrest of ascent (p. 71) ; in the con- dition of the plant before and after poisoning (p. 74) ; and, in the case of seedlings of Wheat, by the simultaneous

stoppage of growth and of the exudation of water-drops from the tips of the leaves (p. 72). The above experiments afford conclusive proof that the ascent of sap is due to pulsatory activity of hving tissue. As the effects produced in cut stems are the same as those in intact plants with roots, the activity which maintains the ascent is not confined to any one part of the plant but exists throughout its whole length. Localisation of the Active Layer for the Propulsion of the Sap

The generally accepted view is that the conduction of the sap takes place only through the dead xylem, and it is based upon two different experiments. The first is the ' ringing-experiment,' the inconclusiveness of which has been explained (p. 34). The second is the supposed abolition of the ascent of sap in the stem when its cut end is exposed for a short time to the air. Though the injected air blocks the xylem-vessels, yet in spite of this the ascent of sap has been found to persist (p. 37). This shows that the xylem is by no means essential for the conduction of sap. Other experiments already described offer, on the other hand, conclusive proof that the propulsion of sap is a physiological process of a pulsatory character, carried on by a living rhythmic tissue.

Experiments with the Electric Probe have demon- strated (i) that a definite layer of tissue, namely (in dicotyledonous plants) the innermost layer of the cortex, is in a state of active pulsation which consists of alter- nate contraction and expansion : (2) that there is no such pulsatory activity in the dead wood (p. 218) : (3) that the physiological agents which enhance cellular pulsation also increase the rate of ascent of sap ; and, conversely, that agents which depress or inhibit pulsation induce a lowering of the rate, or an arrest of the ascent (p. 241).

The active cortex abuts upon the phloem, which is the conductor of the nervous excitation initiated by external stimulation. This is significant, since direct or transmitted stimulation is essential for the maintenance of cellular pulsation (p. 245). The alburnum is within a fraction of a millimetre of the cortex, and the water expelled during active con- traction of the pulsating layer can be readily injected into the wood-vessels. That this is what actually takes place is shown by the ' weeping ' Mango-tree in which a cavity had been formed by the decomposition of the alburnum on the right side of the trunk, the cortex being uninjured. The lateral injection of water by the active cortex filled the cavity and forced out the plug of mucilage which periodically closed the vent. The outflow took place when the cellular activity was at its maximum at thermal noon. The effect exhibited on the opposite side of the trunk, containing the alburnum intact, was quite different. There was no exudation from a hole drilled into that side. The intra-vascular pressure was, as shown by an attached manometer, at its minimum at midday owing to the water injected into the alburnum being rapidly removed by transpiration. These results afford conclusive proof (i) that the pulsatory activity of the cortex forces water not only upwards in the physiological conduction of sap, but also in a lateral direction into the contiguous alburnum, and (2) that the alburnum is the channel for the mechanical transport of water, the force of injection being supplied by the active cortex (p. 175). The bulk of the xylem serves as a reservoir, the water being pumped into or with- drawn from it, according to circumstances.

The fact that the ascent of sap is maintained by co- ordinated cellular activity throughout the length of the plant has been demonstrated by showing that the effects in different regions are correlated with each other. The activity of the ascent of sap has been estimated (i) from the relative rapidity of ascent determined by methods already described ; (2) from the pressure exerted, as measured by the Recording Manometer ; (3) from the rate of excretion from the leaves, as indicated by the Bubbler or by the Micro-Transpirograph ; and (4) from the rate of exudation from the root-stock or from wounded plants, as shown in the automatic records given by the Tilting Electric Recorder.

Physiological continuity is manifested (i) in the effect of drought, natural or artificially produced, which dimin- ishes not only the rate of ascent (p. 45) but also the exudation from the root-stock (p. 137) and the transpira- tion from the leaves (p. 102) ; (2) in the effect of the removal of the root in increasing the rate of ascent of sap (p. 79) and enhancing the rate of transpiration (p. 11 )'. -The increase of transpiration thus produced was more than 70 per cent. ;

(3) in the effect of stimulus, which retards the rate of ascent in the stem (p. 56), the exudation from the root- stock (p. 138), and the transpiration from leaves (p. 102). The retardation of transpiration is induced no-t only by direct stimulation of the lamina but also by stimulation of the mid-rib and of the petiole (p. 93) ; (4) in the effect of warmth in enhancing the rate of ascent (p. 59), in increasing the rate of exudation (p. 142), and in enhancing transpiration (p. 92). It has been shown that the effect of local rise of temperature due to sunlight caused increased exudation from the Mango-stem (p. 176), and also of sugar-containing sap from the spadix of the Palmyra Palm (p. 187) ;

(5) in the effect of ancesthetics , a small dose of which induces an enhancement of activity, while a large dose retards or arrests it ; whether it be the rate of ascent of sap (p. 69), or the exudation from the root-stock (p. 140), or the transpiration from the leaves (p. 114). There is thus a continuity of physiological mechanism in virtue of which each region of the plant controls and is controlled by the rest. It has been explained that there is no strict line of demarcation between the excretion of leaves, ol glands, and of injured surfaces. In all alike, excretion is effected by the pulsatory activity of the terminal layer ; the apparent difference between them is a question of degree and not of kind. The difference is, in fact, the same as that which exists between the so-called autonomous pulsations of the Desmodium leaflet and the multiple pulsations induced in Biophytum and Averrhoa by the application of a strong or repeated stimulation. The pulsatory activity is autonomous in the terminal glandular layer in Nepenthes ; at the opposite extreme is the excretion from the relatively inactive layer of the wounded Palm. In the latter, the cut surface is at first inactive, and there is no root-pressure to cause any ' bleeding.' It is by the strong stimulation of the repeated w^ounds, of the repeated hammering and repeated kneading, that the pulsatory activity of the terminal layer becomes sufficiently aroused to cause active excretion (p. 190).

The excretion from the leaves and that from the cut end of the root-stock may be regarded as intermediate cases between the two above extremes. In the leaves the excretion from the terminal layer of cells is modified by the state of turgor, since the pulsatory activity of the cells depends on internal hydrostatic pressure. The turgor of the terminal cells is increased by the water conveyed and conducted into them respectively by the vessels in the veins of the leaves and by the cortex that surrounds them. When the ascent of sap is retarded by drought or b}' plasmolysis, the pulsatory activity of the terminal layer undergoes a decline, with a resulting diminution of trans- piration (p. 102).

The physiological mechanism of the active excretion of water-drops from leaves is not essentially different from the above. Here the terminal layer is often supplied with water under pressure by means of the vessels, these being in their turn filled with water by the activity of the sur- rounding cortex. In the root-stock of herbaceous plants (where the wood element is relatively slight), the excretion from the cut end is due, in part, to the activity of the terminal layer. For it has been shown that when the terminal cells are locally stimulated by dilute chloroform the rate of excretion be- comes greatly enhanced (p. 141). Local rise of temperature also causes an enhancement of excretion (p. 142). These facts prove that every portion of the plant takes its share in bringing about the excretion by the terminal vent, whether this be the leaf or the cut surface of the root-stock. It has been supposed that the exudation from the cut end is a" passive process brought about by root-pressure acting from below. But the source of the pressure is not localised ; it is the result of the co-ordinated pumping activity of cells throughout the length of the plant, including the layer of cells at the cut surface.

That transpiration is a physiological process of excretion by the leaf-cells, the excreted water being removed by the physical process of evaporation, has been demonstrated by balancing evaporation, from an equivalent surface of water, against transpiration. The balance is upset during rise of temperature up to an optimum, transpiration being relatively more active than physical evaporation. The converse effect takes place during fall of temperature. Separate experiments with the Bubbler showed that, in the leaf of Thunbergia, the optimum temperature for transpiration is 33° C, which undergoes a decline on further rise of temperature (p. 128). We thus arrive at a

discriminating test by which transpiration is distinguishable from evaporation. Evaporation is continuously increased with rise of temperature ; transpiration exhibits, on the other hand, an optimum above which there is a decline. The physiological activity underlying transpiration is further demonstrated by the following experimental results. Transpiration is depressed by diminution of turgor, is arrested by the action of stimulus, and is en- hanced by a rise of temperature ; under anaesthetics it undergoes an enhancement or depression according to the strength of the dose.

That the excretion of water from the leaf is an active physiological process is further shown by the fact that it continues even after the abolition of evaporation by smearing both the upper and the lower surfaces of the leaf with vaseline. The actively excreted water-drops are then found collected under the film of vaseline (p. 90). Transpiration exhibits a diurnal variation, the maximum being attained at thermal noon, about 2 p.m. ; the minimum transpiration occurs at thermal dawn, early in the morning.

Thermal radiation, by raising the temperature, enhances transpiration. Light-rays of the more refrangible blue- violet region, acting on the more sensitive lower surface of the leaf, cause a diminution of transpiration which amounts to about 36 per cent. The effect of red light is opposite, being an increase of 68 per cent. (p. in). Excessive loss of water by too rapid ascent and trans- piration, which would endanger the life of the plant, is automatically checked by physiological regulation. During excessive drought in summer the cellular activity is de- pressed, which causes a great diminution in the velocity of ascent (p. 45). The stimulus of sunlight retards the conduction in the stem (p. 46). Finally, while evapora- tion is continuously increased with the rise of temperature,

transpiration undergoes a decline as the temperature rises above the optimum (p. 128), and this acts as a physiological check. The ascent of sap has been explained as due to physio- logical conduction along the cortex and to mechanical convection along the xylem. The lateral pumping by the cortex fills the xylem-vessels with water and causes an intravascular pressure. When the amount of water forced in is greater than what is lost in transpiration, the pressure becomes positive ; when the vessels are losing more liquid by transpiration from the leaves than is being pumped into them by the cortex, the intravascular pressure becomes negative. There are thus two different possible cases, typified by (i) the root-stock without leaves, or the leafless deciduous tree ; and by (2) the root-stock with a side-branch bearing leaves, or the tree with leaves.

The maximum cellular activity in the ascent of sap occurs when the temperature is highest at thermal noon ; but this gain is counterbalanced in trees with leaves by the loss by transpiration, which is relatively greater. In the root-stock without leaves, and in the leafless tree, the intravascular pressure is at its maximum at thermal noon. Exudation from the cut end of the root-stock, or from a hole drilled in the tree, is also at its maximum at this period. The hole, reaching the wood, drains all the water that is being actively pumped by the cortex into the whole length of the conducting tissue below the hole. The ' bleeding ' produced is therefore considerable. But in the root-stock with a side-branch bearing leaves, and in leafy trees, the loss by transpiration at thermal noon is, as already stated, relatively greater ; a maximum negative pressure thus occurs at thermal noon, and the side-tube sucks in water that is supplied to it. The diurnal variation of pressure and exudation in the root-stock without leaves.

or in tJie leafless deciduous tree, is determined by the daily variation of temperature, the maximum being attained at thermal noon, and the minimum at thermal dawn. The converse takes place in the root-stock with leaves and in the leafy tree (p. 156). Discovery has been made of the pulsations of individual cells ; they have been automatically recorded by electric means, which show that the active cells concerned in the ascent of sap execute alternate expansion and contraction, the period of a single pulsation varying, under different circumstances, from fourteen seconds to several minutes. Deprivation of the stimulating action of the environment brings the pulsation to a state . of standstill ; but applica- tion of stimulus is now found to renew the pulsating activity. Rise of temperature, which enhances the rate of ascent, increases the amplitude or the frequency of pulsation ; lowering the temperature arrests the pulsation and the ascent of sap. The preliminary effect of dilute chloroform is to enhance the amplitude of pulsation, so that the up- stroke in the pulsation, indicative of suction, is relatively greater than the down-stroke ; the rate of ascent of sap also undergoes a corresponding increase. Continued action of the anaesthetic arrests the pulsation, with corresponding arrest of ascent. Diminished internal pressure, induced by the action of a plasmolytic solution of KNO3, gives rise to a characteristic change in the pulsation ; the down- stroke, which represents contraction, becomes greatly increased, while the up-stroke, which represents suction, is reduced ; the base-line dechnes downwards, indicating a persistent diminution of turgor. The final result is an arrest of pulsation and a throttling of the channel for propulsion, with the consequent arrest of the ascent of the sap (p. 241).

sequence of pulsation from cell to cell. This has been demonstrated by the occurrence of definite electric maxima and minima in the path of conduction, the distance between the maximum and minimum being half the wave-length (p. 225). The sap expelled during the contraction of any one cell is absorbed by a cell higher up during its phase of expansion (pp. 142, i'9i). There is thus a propagation of a wave of contraction preceded by one of expansion, in consequence of which the sap is, as it were, squeezed forward. A succession of such waves maintains the continuous ascent of sap.

The plant is a multicellular organism, and hence necessity arises for intercommunication and interaction between more or less distant organs. This is accomplished in two different ways : by transfer of matter, and by trans- mission of motion. The first is exemplified by the hydraulic convection of liquids carrying chemical substances in solu- tion, such as occurs in the circulation of sap ; the second, in the conduction of excitatory change along nerves. The tissue for the cell-to-cell propagation of hydraulic impulse in the propulsion of sap is the internal cortex which abuts on the phloem ; this latter is the tissue for the conduction of nervous. impulses. The two different conducting channels are thus in close proximity to each other.

For the continued maintenance of cellular pulsation in the ascent of sap, direct or transmitted stimulation is essential. When the plant is cut off from the stimulus of the environment, the cellular pulsation is stopped and the ascent of sap becomes arrested. Fresh stimulation renews the pulsation and the ascent. The roots are stimulated by friction against the soil. In the body of the plant, the distribution of the vascular bundles (containing the nervous phloem) is such that no mass of living tissue is too remote

to be excited by stimulus transmitted from the receptive region. The leaf is a catchment-basin for the reception of the stimulus of light. The excitatory effect produced in the nervous elements present in the veins is gathered into larger nerve-trunks and transmitted to the interior of the plant (p. 246). Stimulus gives rise to dual impulses (p. 249). The con- traction of the excited cells is attended with expulsion of water : the hydrauHc impulse thus produced gives rise to an erectile response of the distant leaflet. The excitatory impulse, which travels at a relatively slower rate, causes the opposite movement of a fall. The hydraulic and the nervous reflexes thus act antagonistically to each other (p. 247). The existence of these dual impulses, and the sup- pression of the nervous impulse when transmitted through a considerable length of semi-conducting channel, explain the various tropic curvatures, and the opposite geotropic responses given by the root and by the shoot (p. 251).