The Physiology of the Ascent of Sap

Interaction between distant organs — Sachs's experiment of the growth of a branch inside a dark box — HydrauHc convection and nervous conduction — Importance of stimulus in maintenance of life-activity — The Leaf a catchment-basin for reception of stimulus — Stimulation of internal cortex by transmitted excitation of sunlight — Antagonistic action of hydrauhc and nerve reflexes — Dual impulses under stimulus — Opposite effects of direct and indirect stimulus — Explanation of opposite geotropic responses in shoot and in root — The co-ordination of nervous reflexes — Dia-heliotropic attitude of leaves — Summary.

One of the most difficult problems in plant-physiology is that of finding an explanation of the interaction between distant organs. What are the links by which they are connected with each other ? Numerous examples may be cited of the influence of one part of the plant on a distant part ; I may refer to a very interesting instance described by Sachs. A plant kept in darkness becomes abnormal in its growth, and the motility of its sensitive organs dis- appears ; but in Sachs's experiment, a long shoot of Cucur- hita was made to grow inside a dark box, the rest of the plant being exposed to light. The covered part of the plant, in these circumstances, showed normal growth of stem and leaves, and produced normal flowers and a large fruit. The tendrils inside the box, moreover, were found to be fully as sensitive as those outside. Some chemical substance or substances must therefore have been conveyed from the organs outside, causing a regulation of the normal growth of the organs inside the dark chamber.

It is a well-known and important fact in the physiology of animals, that certain chemical substances, termed hor- mones, produced in certain organs, are carried in the circulating blood to other organs or parts of the bod}' in which their regulatory action becomes manifested. There is no doubt that hormones are likewise produced in plants ; and that these chemical substances, produced in one organ, are conveyed to distant organs in the sap distributed by cellular activity. This transfer of matter may be dis- tinguished as the hydraulic convection associated with circulation.

The distant members of the plant-body are also put in communication with each other by nerve-connections ; for I have shown elsewhere that a nervous system exists in plants, by which excitatory or nervous impulses are transmitted with a definite velocity which in different plants varies from 30 mm. to less than i mm. per second. There is a particular aspect of the action of stimulus which is of fundamental importance in the life of the plant. The continuance of its normal functions depends on external stimulus to maintain the tissues in an optimum tonic con- dition ; for deprivation of stimulus reduces the plant to an atonic condition, in which all life-activities are brought to a standstill. Turning our attention to particular instances, we find that growth and movement in plants depend on the turgid condition of the tissue, which is determined by the cellular activity which maintains the ascent of sap.

We have seen that all rhythmic activities are maintained by the action of stimulus. We observe here a regulatory process which is met with in all physiological actions. Beginning with the tissue at the lowest tonic level (due to prolonged deprivation of stimulus), the incidence of stimulus initiates and enhances the activity to a maximum, the tonic condition of the tissue being raised at the same time to an optimum. Continued stimulation above this point causes a partial inhibition, but this need not be regarded as a permanent depression, for the after-effect of such stimulation is often to cause an enhancement of activity. These effects have been illustrated in various modes of rhythmic activity ; in the pulsation of Desmodium, in growth (p. 15), and in the cellular pulsation effecting the ascent of sap (p. 236).

It is thus clear that for the maintenance of the ascent of sap in a tree, the internal cortex should be excited through- out its length either by direct or by transmitted stimula- tion. The root-cells are locally stimulated by mechanical irritation of friction against the soil. As for the great length of the cortex in the trunk of the tree, covered as it is by the thick bark, direct stimulation of the active internal cells by external stimulus is impossible ; it can only be effected by transmitted stimulation. There thus arise two questions : the first relates to the external stimulus which by its transmitted excitation maintains the cellular activity of the internal cortex ; the second relates to the nervous path by which the excitation reaches that active layer.

Among the external stimuli, none is more potent than light. All the conditions favour the transmission of its stimulating effect to a distance by the nervous channel, which is the phloem in the vascular tissue. The expanded lamina of the leaf, in which the vascular bundles are spread out in fine ramifications, is not merely a specialised structure for photo-synthesis, but also a catchment-basin for the stimulus of light, the excitatory effect of which is gathered into larger and larger nerve-trunks for transmission to the interior of the plant. It is very significant that the internal cortex in which pulsatory activity is to be main- tained abuts upon the phloem through which excitation from outside is conducted.

In the interior of the plant the distribution of the vascular bundles is such that no mass of living tissue is too remote to be excited by the stimulus conducted by the nervous channels. How reticulated they may often be, even in the trunk, is seen in the photograph of the distribution of the vascular bundles in the main stem of Papaya (fig. 91). This net- work, of which only a small portion is seen in the photo- graph, girdles the stem through- out its whole length, and in this particular case there were as many as twenty such layers, one within the other.

It is contended that all parts of the plant are, by means of nerve-conduction, maintained in the most intimate communi- cation with each other. It can only be in virtue of the existence of a system of nerves that the plant constitutes a single organised whole, each of whose parts is affected by every influence that falls upon any other. Fig. gi. Photogra])h of a Layer of Fibro-vascular Tissue in the Stem of Papaya We have seen that two different impulses are transmitted to a distance, the hydraulic impulse propelling the sap, and the nervous ■ impulse conveying the excitatory dis- turbance. Both these impulses, the hydraulic and the nervous, also produce movements at a distance. Thus irrigation of the root of Mimosa gives rise to the erectile response of the distant leaf. If, instead of irrigation, we apply a strong stimulus to the root, say a prick with a pin or an electric shock from an induction coil, the transmitted nervous impulse induces a fall of the leaf. The hydraulic impulse is thus antagonistic to the nervous impulse. Even in ordinary response and recovery we observe these opposite actions The erectile movement of the leaf is due to the

ascent of sap to the pulvinus along a definite channel. Stimulation of the leaf induces a contraction and expulsion of water from the pulvinus which escapes by the same channel through which the ascent took place, but this time in a reverse direction. The two phases of the normal response, viz. the excitatory down-movement followed by erectile recovery, are thus brought about by the excitatory and hydraulic actions respectively. The fact that the hydraulic expansion opposes, and may even neutralise, the excitatory action is seen in the response of Mimosa. The apparent insensitiveness of the plant early in the morning is partly due to the excessive turgor of the pulvinus at that time of the day. Again, application of water to the pulvinus induces an expansion and inhibition of response which may be restored by the withdrawal of the excess of water by glycerin.^

The phenomena of movement in plants present in- numerable difficulties. Hardly any responsive movement has been observed of which an example directly -to the contrary may not also be found. It has therefore appeared hopeless to unify these very diverse phenomena, and there has been a tendency towards a belief that it is not any definite physiological action, but the individuality of the plant that determines movements which are for its own advantage. The teleological argument thus advanced is, however, no true explanation ; it rather confuses the real issue and diverts attention from the discovery of the efficient cause. The complexity that baffles us arises from the combination of numerous reflexes, sometimes concordant, and at other times in antagonism to each other.

The term ' reflex ' has been defined as the ' reaction in which there follows on an initiating reaction, an end- effect reached through the mediation of a conductor itself incapable of the end-effect.' ^ 2 C. S. Sherrington, The Integrative Action of the Nervous System, p. 6. Now the invisible hydraulic impulse initiated by the irrigation of the root causes an end-effect, namely the erectile response of the leaf at a distance ; we may there- fore regard this particular effect produced at a distance as the hydraulic reflex. There is a different end-effect, due to transmission through the plant-nerve of excitation which causes the fall of leaf ; this is the nervous reflex ; the hydraulic reflex induces, as already stated, an expansive and the nerve-reflex a contractile end-effect. A complexity thus arises in the motile response of growing and of pul- vinated organs due to the two reflexes antagonising each other. The recognition of the existence of these two distinct reflexes makes it possible to offer a full explanation of various effects which have hitherto appeared to be anomalous.

Response in Pulvinaled Organs. — Stimulus applied at one end of the long petiole of Averrhoa Caramhola causes successive fall of the sensitive leaflets. There is, however, a preliminary effect which had not been noticed, but which comes out very clearly in the mechanical record obtained by a magnifying lever. The record given in fig. 92 exhibits response to the stimulus of an electric shock applied at a distance of 50 mm. from the responding leaflet. It is seen that it gives rise to two distinct impulses, one positive and the other negative. The hydraulic positive produced an erectile response of the leaflet shown by the down-curve, while the excitatory negative caused a rapid fall of the leaflet. This negative impulse reached the leaflet forty-four seconds after the application of stimulus ; the velocity of the excitatory impulse is in the present case i • 1 mm. per second, which is slower than the positive impulse.

I obtained similar double responses with Biophytum and other sensitive plants under modes of stimulation as diverse as electrical, chemical and thermal. Stimulation causes local contraction of the excited cells, with expulsion of water which gives rise to a hydraulic wave, the velocity of which is generally greater than the propagation of the excitatory protoplasmic change constituting the nervous impulse. The hydraulic wave gives rise to the preliminary

Down-curve represents erectile response ; and the up-curve exhibits the responsive fall. erectile response ; the subsequent excitatory fall is brought about by the nervous impulse. In the record of the two reflexes given in fig. 92, the excitatory response is more intense than the hydraulic ; hence if the two impulses reach the responding organ at about the same time (which will be the case when the stimulus is applied at or near the responding point), the positive becomes masked by the predominant negative. Application of stimulus at a distance causes the slow-

moving negative impulse to lag sufficiently behind the positive so as not to mask it.^ In semi-conducting tissues, the excitatory impulse undergoes rapid diminution or abolition with the distance ; hence a stimulus of moderate intensity applied at a distance induces only the positive response. It thus happens that while direct application of stimulus causes contraction, application of the same stimulus at a distance induces the opposite effect of expansion.^

Responsive Movements in Growth. — I obtained parallel effects in experiments on growth. Direct stimulation of the growing region was found to induce a retardation in the rate of growth which culminated in actual contraction. Strong stimulus applied at a distance from the growing region gave rise to a diphasic response, an acceleration followed by retardation. When the stimulus is moderate or feeble, the excitatory impulse is unable to reach the distant responding region ; the hydraulic impulse is trans- mitted, the resulting reflex being an expansion, an en- hancement of turgor, and an increase in the rate of growth. The positive tropic curvature in growing organs is due to the contraction of the proximal side by direct stimulation, and the expansion caused by the positive hydraulic impulse transmitted to the distal side.

The opposite responses of the shoot and the root to the stimulus of gravity find their explanation in the fact that in the former stimulation is direct and in the latter indirect. In the shoot the perceptive and the responding region is one and the same ; but in the root, it is the tip of the organ 1 I obtained similar diphasic effects in electric responses, a galvano- metric positivity followed by negativity. This would explain the positive electric response in animal tissues which is often found to precede the normal negative.

which alone is sensitive, while response takes place in a region at some distance from it. It has been shown in the previous paragraph that the movements in response to direct and to indirect stimulation are opposite to each other. From the fact that the geotropic stimulation of the shoot is direct, and of the root indirect, it is inevitable that an identical stimulus should in the two cases induce responses of opposite sign. We have seen how the hydraulic and the nervous re- flexes antagonise each other, and how the resultant move- ment is due to the algebraical summation of the two effects. There are again various nerve-reflexes, which by their co-ordinated action produce what is usually regarded as a specific response for the advantage of the plant. As an example of this we find that the leaves of Mimosa, subjected to one-sided illumination, place themselves at right angles to the direction of the stimulus, apparently for the purpose of absorbing the largest amount of light. This directive action even takes place when the motile pulvinus is kept shaded, whilst the four sub-petioles carrying the numerous leaflets are exposed to light. The result must evidently be due to a nervous impulse transmitted from the four sub-petioles, bearing the leaflets, to the pulvinus at the base of the main petiole.

Detailed analysis shows that the pulvinus itself is a complex organ whose four quadrants act as four distinct effectors with characteristic responsive movements (fig. 93). When the left flank of the pulvinus (i) is alone stimulated by light, the result is a left-handed torsion or movement against the hands of the clock. Stimulation of the lower quadrant (2) by light gives rise to a responsive movement which is downwards. Stimulation of the right flank (4) by light induces a right-handed torsion. The action of light on the upper quadrant (3) causes an up-movement. This

is what takes place when the stimulus is localised at each of the four quadrants : whereas strong and diffuse stimula- tion gives rise to the fall of the leaf due to the predominant response of the lower half of the pulvinus. Moreover, the same effects are induced by the separate stimulation of the four sub-petioles bearing the leaflets. When light is thrown on the left sub-petiole (i) the response is a left-handed torsion : stimulation of sub-petiole (2) induces a down-movement ; that of sub-petiole (3) causes

Fig. 93. Transverse Section of the Pulvinus of Mimosa (lower figure) showing the Quadrants which are in Nervous Com- munication with the Four Sub-petioles bearing the Leaflets (upper figure) . ^an up-movement ; and, finally, stimulation of the right sub-petiole (4) induces a right-handed torsion. I have also found by the Electric Probe that it is the phloem in the vascular strand in the main petiole which transmits excitation initiated in each of the four sub-petioles. There are thus four separate nerve-strands which connect the four sub-petioles with the four effectors in the pulvinus.

A single reflex caused by the stimulation of one of the sub-petioles gives rise to a purposeless movement in a direction which carries the plane of the leaflets away from the position perpendicular to the incident light. But when the two sul3-petioles (i) and (4) are simultaneously exposed to hght of the same intensity, the two result- ing torsions balance each other. Hence the lateral adjustments of the leaf as a whole are made by the two sub-petioles (i) and (4) which are situated outside. The balancing adjustments, up or down, are made, in response to the excitations transmitted, by the two middle sub- petioles (2) and (3). It is thus seen that equilibrium is only possible when the entire leaf-surface (consisting of the rows of leaflets carried by the four sub-petioles) is equally illuminated ; and this can only occur when the leaf-surface as a whole is perpendicular to the incident light. The leaf is adjusted in space by the co-ordinated action of the four reflexes. The dia-heliotropic attitude of the leaves is thus brought about by distinct nervous impulses, initiated at the perceptive region actuating the different effectors at a distance.^

There are additional reflexes caused by other modes of stimulation, such as that of gravity. The geotropic response is modified by thermal variation ; rise of tempera- ture diminishes the geotropic response, while fall of tempera- ture, within limits, enhances it.^ Thus even in such an apparently simple case as the adjustment of the leaf of Mimosa, there are the following variable factors : (i) the hydraulic reflex ; (2) the nervous reflex caused by the stimulus of light, in which there are four variables depending on the relative intensities of excitation transmitted by the four different receptors of stimulus; (3) the geotropic action; and (4) the effect of thermal variation in modifying the geotropic action.

It will be understood how, by the permutation and combination of these factors, numerous variations will be produced in the resulting response. This accounts for the complexity of life-movements, which are by no means capricious, but are capable of rational explanation on the investigation and the discovery of the numerous factors underlying their manifestations. ^ ' The Dia-Heliotropic Attitude of Leaves as determined by Trans- mitted Nervous Excitation,' Proc . Roy. Soc, B. vol. 93, 1922. 2 Life Movements of Plants, voL ii. p. 513.

The plant is an organised whole, there being interaction between distant organs. There are two modes of inter-communication : (i) the hydraulic convection of fluids by cellular activity ; and (2) the nervous conduction of excitatory protoplasmic change. Regulation of growth may take place by hydraulic convection of chemical substances (hormones) from one part of the plant to another. Direct or transmitted stimulation is necessary for the maintenance of all modes of rhythmic activity, including the cellular pulsation in the ascent of sap.

Excitatory impulse reaches the interior of the plant along the phloem which functions as the nerve of the plant. The active cortex abuts upon the phloem, and is thus stimulated by the transmitted excitation from outside. 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 and larger nerve-trunks and transmitted to the interior of the plant.

The hydraulic and nerve reflexes are antagonistic to each other. The former induces an end-effect of expansion, increase of turgor, galvanometric positivity, and erectile movement of the leaf. The nerve-reflex gives rise to the opposite effect of contraction, diminution of turgor, gal- vanometric negativity, and the fall of the leaf. Stimulus applied at a distance gives rise to dual impulses, hydraulic and nervous. The hydraulic travels the faster, and induces the preliminary erectile response ; the nervous impulse which follows gives rise to the subsequent excitatory fall of the leaf. Of the two effects, the excitatory is the more intense.

In a semi-conducting tissue, the excitatory impulse undergoes diminution with distance, and may thus become extinguished. The result is that while direct stimulus causes contraction, fall of the leaf and retardation of the growth, a stimulus applied at a distance induces expansion, erectile movement of the leaf and enhancement of the rate of growth. The effects of direct and indirect stimulus are thus of opposite sign. This explains the opposite effects of the stimulus . of gravity in the shoot and in the root. In the shoot the stimulus is direct ; in the root it is indirect, since the sensitive root-tip for perception of stimulus is separated from the region of response.

The dia-heliotropic attitude of the leaves of Mimosa and other plants is the result of the co-ordination of nervous reflexes. The pulvinus of Mimosa has four distinct effectors : the left and right quadrants respond by left- handed and right-handed torsions ; the upper quadrant responds by an up-movement, and the lower quadrant by a down-movement. These four quadrants are in nervous connection with the four sub-petioles bearing the leaflets, which are the receptors for the stimulus of light. The leaf is adjusted in space by the co-ordinated action of four reflexes, equilibrium being only possible when the leaf- surface as a whole is perpendicular to the incident light. The dia-heliotropic attitude of leaves is thus brought about by distinct nervous impulses initiated at the perceptive region actuating the different effectors.