Plant Autographs and Their Revelations

a similar peristaltic action. As the pulsating mechanism in plants is not so highly differentiated as in higher animals, the propulsive organ is likened to an elongated ‘heart,’ using that term in a broad sense. The propulsive system of plants may be said to corre¬ spond to the heart and arteries of animals. Where then is the ‘heart’? Is it possible to localise exactly the active cells in the inte¬ rior of the tree and catch them, as it were, the very act of

In attempting this, we have to gain access to the smallest unit of life, the individual cell or its throbbing pulsation. electric pulsation at different layers. The pulsating move- Note the abrupt enhancement at a distance microscopic, and its de- of the record has gone out of the plate. to be beyond the range of possibility. The solution of the problem has, however, been made possible by my Electric Probe, previously employed for localising the layer which perceives the stimulus of gravity.

The location of the ‘heart’ was effected by the em¬ ployment of the Electric Probe in circuit with a sensitive galvanometer. When an electric contact is made with a resting muscle, the galvanometer remains quiescent. But if the contact be m.ade with a beating heart, electric pulsa- tions are generated corresponding to the mechanical pul¬ sations. In locating the ‘heart’ I introduced the Probe, step by step, into the organ ; as soon as it came in contact with the pulsating layer, electric signals were sent out, which were automatically recorded in the galvanograph.

Fig. 76. Section of the Petiole of Brassica, and the curve of pulsatory E, epidermis; C, cortex; Ci, the active internal cortical layer; En, endodermis ; B, phloem; X, xylem ; P, pith. Note the sudden enhancement of activity at the layer Ci. The experiment was carried out as follows : one electric contact was made with a resting tissue, such as the epidermis of the leaf ; the other contact was made by the Probe (fig. 74), which was introduced transversely into the stem by successive steps of o.i mm. No pulsation could be

detected at the epidermis ; as the Probe reached a depth of o.i mm. a feeble pulsation was detected; a similar result was obtained at the depth of 0.2 mm. But when the Probe reached 0.3 mm. the pulsations exhibited a sudden increase ; this was so great that a part of the record went off the plate (fig. 75) ; evidently the Probe had come into contact with pulsating cells. As it was thrust still deeper into the stem, the pulsating activity rapidly disappeared. When a trans¬ verse section of the stem was made along the line of the passage of the Probe, it was found that the maximum activity had been detected when the Probe touched the internal layer of the cortex abutting upon the young vascu¬ lar tissue. Contact of the Probe with the wood did not cause any pulsation, proving that the dead wood does not take any active part in the propulsion of sap. A representation of the dif¬ ferent layers of tissue in the petiole of Brassica with their relative pulsating activity is shown in fig. 76. The curve shows that the activity is most intense at Cl, the internal cortex.

The indicating spot of light reflected from the galvanometer reveals by its alternate swings to the right and to the left the invisible pul¬ sations of active cells in the interior of the plant. The record of electric pulsations of the throbbing layer in the Mango- tree is given in fig. 77. What proof is there that these electric pulsations actually indicate heart-like throbbings in the plant? There are sev¬ eral tests by which the specific activity of the heart may be discriminated, of which the following will suffice : ( i ) The heart-beat comes to a stop when the internal blood pressure

is low, the pulsations being renewed after an increase of internal pressure. (2) The beat also comes to a stop when the heart is in a depressed or sub-tonic condition; stimula¬ tion is then found to revive the beating of the heart. (3) Anaesthetics like chloroform stimulate the heart at the begin¬ ning, but under prolonged anaesthetisation it comes to a Fig. 78. Records of electric pulsations under alternate drought and irrigation. Dotted line below represents condition of drought, con¬ tinuous line of fresh irrigation.

stop, followed by the death of the animal. I shall now show that the pulsatory reactions of the plant are exactly parallel to those of the animal. The sap-pressure undergoes a great diminution under drought, in consequence of which the pulsation of the pro- pulsive layer comes to a stop. An increase of sap-pressure after irrigation revives the arrested pulsation. I reproduce the effects of alternate withholding of water and of irriga- Fig. 79. Effect of stimulus of light in enhancement of electric pulsation in sub-tonic specimen.

D, feeble pulsation in dark; L, enhanced pulsation under light; and D^, depression under renewed darkness. tion on the electric pulsation of the propulsive layer. The pulsation is seen to be repeatedly depressed under drought and revived after supply of water (fig. 78). When a plant is kept in the dark for twenty-four hours it becomes so sub-tonic as to lose its power of maintaining the ascent of sap. Stimulation by light or by electric shock is now found to revive the activity of ascent. What can be the fundamental cause of this change? The explana¬ tion is to be found in the records of the electric pulsations. These show that, while the pulsations are practically at

Note preliminary enhancement of pulsation, with longer up-stroke; pulsation arrested on continued application. a standstill under prolonged darkness, the throbbing activity is revived on stimulation by electric shock or by light, and is arrested once more on the cessation of the stimulus (fig. It has been explained how, in the first stage of its applica¬ tion, chloroform enhances the activity of the heart, con¬ tinued action producing depression and arrest. I have also found that the rate of sap-ascent under chloroform shows

a preliminary acceleration followed by depression and arrest. The record of the electric pulsations of the propulsive layer under chloroform shows parallel reactions. The pre¬ liminary enhancement is followed by depression and arrest of pulsation (fig. 80). The results described offer conclusive proof that the ascent is brought about by the pulsating activity of the cells of the propulsive layer, which is the inner cortex surrounding the vascular cylinder. It should be borne in mind in this con¬ nection that, under exceptional circumstances, all living cells are capable of being thrown into rhythmic activity; cer¬ tain layers of cells are, however, naturally more active than others, and it is by the pulsatory activity of those of the inner cortex that the normal ascent of sap is main¬ tained.

The propulsive tissue in a Dicotyledonous-tree is a cylin¬ drical tube which stretches throughout its entire length. This cylinder closely surrounds the young vascular tissue. The function of this long cylinder, as already explained, is not unlike that of the elongated heart of lower animals, in which circulation of the blood is brought about by a series of peristaltic waves. The propulsion of sap in plants is found to be essentially a similar process of peristalsis, in which a wave of contraction squeezes the sap forward. A succession of such peristaltic waves maintains the continuous ascent of the sap.

It may next be asked why should the peristaltic wave always move the sap upwards. Would it be possible to reverse the direction of the peristaltic wave, so as to cause the sap to flow downwards ? What are the conditions which determine the direction of flow? The general law which I have established concerning the direction of the sap-movement is that it floziis from the more active to the less active region. It is evident that if the pulsating activity were equal at the two ends of the organ

they would balance each other, and there could then be no resulting directive movement. A difference of activity at the two ends of the organ may be induced in two ways: first by differential turgor, and second by differential stimu¬ lation. I now proceed to explain what is meant by differ¬ ential turgor and differential stimulation. After irrigation the lower end of a tree is rendered tense and turgid by absorption of water, whereas there is an incipient drought at the upper end in consequence of rapid removal of water by the transpiring leaves. Now the rhythmic activity of the propulsive cells has been shown to be enhanced under increased turgor and diminished under drought. The propulsion of sap is, therefore, from the more turgid and active to the less turgid inactive region. The sap-movement thus follows the ‘turgor-gradient,’ tend¬ ing to equalise the degree of turgor in different parts of the plant.

I shall next show that it is possible to change the normal upward direction of the flow of sap into a downward one, by merely reversing the turgor-gradient. Thus when water is withheld from a potted plant, the stem bends over under drought and the drooping leaves hang down. The pulsating activity throughout the plant is now in a state of arrest. If a glass of water be next raised so that the upper part of the drooping stem is immersed in it, then the upper part of the stem will absorb water and thus become more turgid than its lower end. The pulsating activity becomes revived at the upper end; the turgor-gradient is reversed, and the sap now flows downwards against the direction of the normal ascent. This reversed flow is demonstrated by the sequence of revival of the drooping leaves which takes place from the tip of the stem downwards. I have measured the normal rate of ascent and the reversed rate ; the results show that the rate is much slower in the reverse or unaccustomed direction. In the circulation of the blood the forward pro¬ pulsion is helped by the presence of valves which facilitate the flow in one direction rather than its opposite. The par¬ titions of the cellular pumps in the plant act somewhat simi-

larly in predisposing the flow in the normal upward direc¬ tion. Under normal conditions the root-cells are continuously stimulated by friction against the soil, and this probably starts the peristaltic waves. The increased turgor of the lower end under irrigation also directs the propulsion of sap in an upward course from the more to the less active region. In herbaceous plants the distance of the leaves from the soil-water is not too great, but in tall trees it is necessary to have a nearer source of supply — a ‘soil extension,’ as it were, in the shape of conduit-pipes filled with water. These conduit-pipes are the young wood-vessels (alburnum) which serve for the mechanical transference of water during the emergency of active transpiration by the leaves. When transpiration is feeble, the normal ascent along the cortex supplies every portion of the tree with water; the leaves become turgid and the alburnum filled with sap. During active transpiration, however, the physiological propulsion is not sufficient to meet the demand, and water is with¬ drawn from the wood-reservoir. Two factors are thus brought into operation — physiological propulsion by and along the active cortical cells, and physical transference along the wood or xylem.

We may now visualise the important processes connected with the ascent of sap. The absorbing root-cells are con¬ tinuously stimulated by mechanical friction against the soil, giving rise to peristaltic waves of pulsation along the active propulsive layer of the inner cortex. The direction of the sap-flow is determined by differential turgor, from the more turgid lower part of the plant to the upper part which is in a state of incipient drought due to active transpiration by the leaves. The rhythmic contraction of the active cells propels the sap not only upwards but also laterally into the young xylem, which functions as a reservoir for emer¬ gencies, water being withdrawn from it when the transpira¬ tion is most active.

Since the conclusion of my investigations on the causes underlying the supposed devotional exercises of the famous Praying Palm of Faridpore, I have been embarrassed by many requests to interpret happenings which, at first sight, appeared supernatural. The unexpected occurrence, in the present case, was the periodic ‘weeping’ of a Mango-tree in the suburbs of Calcutta. This tree is full grown and about 40 feet in height. The circumference of the trunk is 38 inches and the outspread branches with their numerous leaves cover an area of about 100 square yards (fig. 81). The so-called ‘weeping’ com¬ mences every day without any ostensible provocation punc¬ tually at I p.M. from a point high up in the tree. It is very copious at the beginning, the rate of fall of successive drops being one in every two seconds. The paroxysm gradually subsides, and the interval between successive drops slows down to five seconds at 2 p.m., to eight seconds at 3 p.m., to fifteen seconds at 4 p.m., to 150 seconds at 5 p.m., after which there is a cessation of all ‘weeping’ for the day. The performance, moreover, is repeated every day at I P.M., and with the same sequence. The mysterious event came to be regarded as of evil omen, and thus aroused considerable alarm among the people in the neighbourhood. In their perplexity they asked me to explain what it meant, and if possible to cure the tree of its distressing symptoms.

It appeared, at first sight, that the pressure of sap inside the tree became somehow suddenly increased at i p.m., and that the sap then forced its way out through some vent high up in the tree. Hourly Variation of Sap-Pressure Wlien the internal sap-pressure in the trunk of a tree Exudation takes place through the small aperture marked with an arrow. The pressure-recorder is seen attached to the tree. is very great, sap exudes from a hole drilled into it. The internal pressure may, on the other hand, be lower than the external pressure of the atmosphere. Instead of there

being any exudation, water is then actually sucked into the hole. The change of internal pressure, either increase or decrease, can be continuously recorded by attaching to the tree-trunk a self-recording pressure-gauge. The pressure at any hour depends on the relative gain or loss of water by a tree. Water is absorbed from the soil and is forced up the trunk by the pumping mechanism as already described. The activity of this is increased, within limits, with the rise of temperature. The moment of the attainment of the highest temperature I designate as the thermal noon, that of the lowest temperature as the thermal dawn. The maximum temperature, under normal conditions, is attained in Bengal at about 2 p.m. and the minimum about 6 a.m.

There are many trees which shed their leaves about winter-time. There is then no loss of water by transpira¬ tion from the leaves, the variation of sap-pressure being determined by changes of the rate of suction under changing temperature. The maximum pressure is at thermal noon at 2 P.M., the minimum pressure at thermal dawn at 6 a.m. The sap escapes at noon with great rapidity from a hole drilled into the trunk. The conditions in trees with a large number of tran¬ spiring leaves are somewhat complicated. It is true that water is absorbed with greatest vigour at thermal noon, but loss from transpiration by the leaves is also greatest at that time; in fact, the loss is even greater than the gain. Hence the diiirnal variation of pressure in leafy trees is the reverse of that in trees without leaves. The maximum pressure in plants with leaves is attained at thermal dawn, and the minimum at thermal noon.

In fig. 82 the upper curve shows the variation of tem¬ perature from 6 A.M. to 6 p.m. The lower curve shows the variation of pressure in a leafy Rain-tree (Pithecolobiiim) . The minimum pressure is seen to occur at the thermal noon about 2 p.M. The pressure-curve appears to be more or less an inverted reflection of the curve of temperature. Ihe highest temperature at noon corresponds to the minimum internal pressure. Hence at noon water is sucked in and not exuded at a drilled hole.

Fig. 82. Diurnal curve of variation of pressure in the Rain-tree {Pithecolobiiim ) . The upper curve shows the variation of temperature, and the lower the variation of pressure. Note that the pressure was at its minimum at thermal noon, 2 p.M. As there were numerous transpiring leaves on the Mango- tree, the internal pressure should have been at its minimum at noon. Nevertheless its ‘weeping’ or exudation of sap was then at its maximum. Closer examination of the tree disclosed the fact that there was a small aperture or vent in the bark at a point high up in the tree. This vent was usually closed by a plug of mucilage till forced out by strong

internal pressure exerted by the sap at i p.m. It was an anomaly that the maximum exudation should take place at about noon, when the internal pressure in leafy trees is at its lowest. This led me to undertake investigations on the pressure- variation in the Mango-tree; a hole was drilled for the attachment of a recording manometer at a point diametri¬ cally opposite to the vent. The pressure-variation was found, as expected, to be at its lowest at noon; there was, moreover, not the slightest exudation from the drilled hole. What then could be the difference in the same zone of the trunk, which would account for active exudation from the vent on one side, and the total absence of it from the drilled hole on the opposite side?

I continued the inquiry by cautiously removing the bark and the tissue round the vent. This led to the dis¬ covery of a large elongated cavity of irregular shape, formed by the decomposition of the young wood (alburnum) which had fallen to the bottom. The outer boundary of the cavity was a rind consisting of phloem and cortex in a healthy condition. The inner boundary was formed of duramen or hard wood, which is a non-conductor of sap (fig. 83). The total absence of exudation from the drilled hole on the left side of the trunk, and the copious overflow from the cavity on the right side, can only be due to the structural difference between the two sides — the presence of alburnum on the left and its absence on the right. The active cortex bordered the cavity, and the ex¬ cretion of the fluid that fills it can only be due to a lateral pumping action of the cortex, the activity of which is gradually increased by the rise of temperature which attains its maximum at 2 p.m.

We have next to explain the reason of the commence¬ ment of exudation punctually at i p.m. Further inspection of the tree showed that for the greater part of the day the leaves cast a shadow on the trunk. There was an opening, however, among the branches, such that during the course of the sun from the east to west, sunlight fell directly upon the exuding portion of the trunk exactly at I P.M., causing a local rise of temperature. Consecpiently, the cortex underneath had its activity greatly increased,

Figure to the left is a magnified transverse section of a young stem. E, epidermis ; C, broad cortex ; G, gland ; P, phloem ; X, xylem. Figure to the right is a diagrammatic represen¬ tation of the trunk with the cavity from which exudation takes place. The sap pumped laterally by the cortex is accu¬ mulated in the cavity. On the left side the laterally injected sap is rapidly absorbed by the alburnum which is under nega¬ tive pressure on account of transpiration from the leaves. The pressure was indicated by the manometer, M. No exudation took place through the drilled hole, E.

and the resulting enhanced exudation caused a rapid rise of the level of the sap collected in the cavity. The great increase of pressure expelled the closing plug, with a result¬ ing sudden outflow of sap. Late in the day the sun became hidden by the leaves and the temperature underwent a rapid fall. The weeping of the tree consequently declined and was arrested in the evening. On the left side of the trunk the young wood or alburnum was uninterrupted, and could readily absorb whatever water might be laterally injected into it by the propulsive cells of the cortex. There was, therefore, no accumulation of sap and no exudation from the hole drilled on the left side.

The ‘ weeping ’ was thus due to the accumulation of laterally injected sap in the cavity and its periodic overflow from the vent. The tree wept no more after the destruction of the cavity and the coating of the exposed surface with coal tar. The results given above furnish conclusive proof that the pulsatory activity of the cortex propels the sap not only upwards but also in a lateral direction, so as to inject it into the contiguous young wood-vessels which serve as a reservoir. It also proves that the young wood is a channel for the mechanical transport of water, the force of injection being supplied by the active cortex.

Large quantities of sugar are obtained in India from the sap of Palm-trees. The Indian Date Palm {Phoenix syl- vestris) grows to a height of 30 to 40 feet. The sap is drawn in a special way by wounding the upper end of the trunk (fig. 84). The daily yield of sugar-containing sap from this tree is often as high as 19 litres per day, a litre being equal to about two pints. The sugary sap is drunk fresh, or is used for the manufacture of sugar; it is also fermented for making intoxicating liquor.