The Physiology of the Ascent of Sap

10 and II A.M. there was a balance between the sap sent up by the root-stock and the loss arising from the excretion by the leaves : after ii a.m. the loss was the greater, and the condition of turgor fell below the normal ; hence the root-stock began to exhibit a negative exudation by sucking back the exuded sap. The activity of suction attained its maximum about 2 p.m., after which the temperature began to fall and the transpiration decreased, resulting in a transition from negative to positive exudation

Fig. 55. Record showing the Positive and Negative Exudation in Cucurbita with a Side-branch The up-curve represents positive exudation, and the down-curve Fig. 56. Positive and Negative Exudation exhibited by a Tree after 5 p.m. The two transitional points are thus at 11 a.m. and at 5 p.m. ; at the former transpiration ]ust exceeded exudation, and at the latter exudation began to exceed transpiration. The record also gives a rough idea of the relative ex- cretion from the leaves. The average rate of exudation in the plant at night, from 6 p.m. to 6 a.m. in the morning, was 23 c.c. per hour. The loss by transpiration at night is very much less ; we found it to be about 10 per cent, of that in the day-time (p. 119). Hence the average rate of exudation from evening to morning may be taken to be about 25 c.c. per hour. At 10 a.m. exudation and

transpiration were balanced. Hence the rate of transpira- tion at that hour may be regarded as 25 c.c. per hour. From II A.M. to 5 P.M. the plant should have exuded at a rate slightly greater than 25 c.c. per hour, since the rate of exudation would be at its maximum at the thermal noon at 2 P.M. Neglecting this correction, the total exudation for the six hours between 11 a.m. and 5 a.m. should have been at least six times twenty-five, or 150 c.c. Instead of this, the plant sucked in 80 c.c, on account of excessive transpiration during this period. The total loss by trans- piration is thus 150 + 80 or 230 c.c, or an average loss of 38 c.c per hour. The hourly loss by transpiration at midday is thus greater than the 25 c.c. which the plant is able to supply ; hence a condition of drooping of the leaves becomes noticeable at that period.

I obtained similar results with Pithecolobium. The leaves were very nimeroLS, hence the normal exudation was relatively low : nevertheless, the diurnal curve of exudation shows a remarkable similarity to that of the Cucurhita with a side-branch. Here also the exudation continued throughout the night and early morning. It was arrested at about 9 a.m. ; at 10 a.m. there was nega- tive exudation which continued till 4 p.m., after which positive exudation was resumed (fig. 56).

We are now in a position to explain the quantitative relation between absorption and the sum total of excretion from the plant. In the case where the gain is greater than the loss, the turgor is increased, the xylem-reservoirs ^are filled up, and the internal pressure exhibits an increase. A cut made in the plant in this condition is followed by exudation of sap. Under the opposite condition of excessive loss by transpiration, the expenditure is greater than the income ; the turgor falls below par, the xylem-reservoirs become emptied, the intra-vascular pressure changes from

positive to negative, so that water, instead of being exuded, is sucked in at the cut surface. The contrasted effects of pressure and exudation in trees with and without leaves are given in the following table : Table XXa. — Showing the Variation of Pressure and of Exudation in Plants with and without Leaves In plants bearing leaves, the diurnal variation of pressure and of exudation is such that the maximum is attained at thermal dawn, and the minimum at thermal noon.

In plants without leaves, the diurnal variation of both pressure and exudation is exactly the opposite : the maxi- mum is attained at thermal noon, the minimum at thermal dawn. When the internal pressure undergoes rapid diminution, positive exudation becomes converted into negative. Exudation from the Mango-tree— Chemical analysis of the exuded sap — Period of maximum pressure — Absence of exudation from hole drilled into the tree — The existence of a cavity due to disintegration of alburnum — The lateral injection of sap by active cortex — Enhanced' secretion due to local rise of temperature — Summary.

The results of investigation given in the previous chapters show that the various phenomena of pressure and exudation, though apparently anomalous, are yet capable of satis- factory explanation by taking full account of the numerous factors which complicate them. A very curious phenomenon has, however, been recently brought to my notice, which could not be explained by the consideration of the different factors which have already been enumerated. This unexpected occurrence is that of the periodic ' weeping ' of an intact Mango-tree in the suburbs of Calcutta. The mysterious event came to be regarded as of evil omen, and thus roused considerable alarm among the people in the neighbourhood.

The particular tree is full grown and about 39 feet high (13 metres). The circumference of the tree is 38 inches (i metre), and the outspread branches with their numerous leaves cover an area of about 90 sq. metres. The exuda- tion or weeping commences every day punctually at i p.m., from a point high up in the tree. This weeping, so- called, is very copious at the beginning, the rate of fall of successive drops being once in two seconds ; it gradually slows down ; the intervals between successive drops come to be five seconds at 2 p.m., eight seconds at 3 p.m., fifteen seconds at 4 p.m., and 150 seconds at 5 p.m., after which

the exudation comes to a standstill. The performance is repeated every day punctually at i p.m., and with the same sequence. Exudation takes place through a small aperture marked with an Examination of the tree brought out the fact that there was a small aperture in the bark, which will be designated as the vent, which has been enlarged in the photograph (fig. 57). Analysis of the exuded sap gave the following results : Total solid after evaporation per 100 c.c. . 1-56 grams. Solids after ignition per TOO c.c. . 0-99 KaCOa in solids after ignition . .0-90

The organic matter consisted mostly of gums and tannins. The problem of the weeping of the Mango-tree is essentially the determination of the cause of the abrupt commencement of exudation at i p.m., approximately the hour of the thermal noon : this suggests an analogy with the maximum exudation of the deciduous tree without leaves which, as previously stated, occurs at the same time. But the Mango-tree bears innumerable transpiring leaves, on which account thermal noon should be the period of maximum transpiration and minimum exudation as we found to be the case with the Rain-tree with leaves (p. 168).

As pressure and exudation have been shown to be closely related to each other, the maximum exudation at thermal noon might be attributed to the occurrence of maximum pressure at that hour brought about by some unknown cause. In order to test the correctness of this supposition, 1 attached a recording manometer to the tree, and found that the pressure underwent a continuous decrease with rising temperature, reaching its minimum at or about' 2 p.m. Fig. 58 gives the record of the variation of pressure from 9 a.m. to 9 p.m. ; the lower record gives, by the spacings between successive dots, an indication of the different rates of exudation at i, 2, 3 and 4 p.m. Thus the internal pressure of the Mango-tree exhibits no characteristic distinguishing it in any way from that of other trees with transpiring leaves.

The anomaly presented here is that the maximum exu- dation should occur simultaneously with the minimum interna] pressure. It was thought that possibly, in the particular zone of the plant, tissues had been developed which were unusually active. Investigation was therefore undertaken to find out whether an auger-hole made at the diametrically opposite side of the trunk would give an exudation at thermal noon, as occurred at the natural vent. There was, however, not the slightest indication of any exudation from the drilled hole.

Fig. 58. Record of Exudation and of Pressure of the ' Weeping ' Mango-tree The lower record shows the rates of exudation at i, 2, 3 and 4 p.m. ; the upper record exhibits the variation of pressure from 9 A.M. to 9 P.M. What then can be the difference, in the same zone of the trunk, which would account for the active exudation from the vent on one side, and the total absence of it from the auger-hole on the opposite side ? In pursuing this enquiry I cautiously removed the bark and the under- lying tissue round the vent. This led to the discovery of a large elongated cavity which was irregular in shape, the maximum length being one metre and the breadth 15 cm.

The cavity was formed by the decomposition of the alburnum, which had fallen to the bottom and become decomposed. Its outer boundary was the rind with its Fig. 59. Sections of the Mango Stem Figure to the left is a magnified transverse section of a young stem. E, epidermis, c, extended cortex, g, gland, p, phloem, X, xylem. Figure to the right is a diagrammatic representation of the trunk with the cavity from which exudation takes place. The sap pumped laterally by the cortex is accumulated in the cavity. On the left side the laterally injected sap is rapidly removed by the alburnum which is under negative pressure on account of trans- piration from leaves. The pressure is indicated by the manometer, m. No exudation took place through the drilled hole, E.

layer of phloem and cortex, which were all in a healthy condition ; the inner boundary was formed of duramen. A transverse section of a young Mango-stem is shown in fig. 59 : to the right of the figure is seen a vertical diagrammatic longitudinal section of the trunk of the weeping Mango-tree. In the former e is the epidermis, under which a thick bark is formed in older trees ; c is the cortical tissue in which there are glands which secrete resin. The endodermis, forming the most internal layer of the cortex, abuts on the phloem, which is separated from the young xylem, the alburnum, by the cambium-layer : the rest of the xylem is the duramen.

We return to the question of the total absence of exuda- tion from the drilled hole on the left side of the trunk and the copious outflow from the cavity on the right side ; this 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 borders the cavity, and the excretion of the fluid that fills it can only be due to a lateral pumping action of the cortex. The cellular activity, as we shall presently find, is greatly enhanced by the local rise of temperature which takes place at i p.m. The cavity, which has been slowly filling up with the excreted sap, now becomes overcharged, and the plug of mucilage which closes the vent is suddenly forced out ; the removal of the obstacle is immediately followed by the rapid out- flow which characterises the commencement of exudation.

On the left side of the trunk the alburnum is un- interrupted ; the active cortex injects water into the alburnum, in which it is rapidly conveyed to the transpiring leaves. There is therefore no exudation from the hole drilled on the left side. The results given above furnish conclusive proof — (i) that the pulsatory activity of the cortex propels the sap not only upwards hut also in a lateral direction so as to inject it into the contiguous alburnum ; and (2) that the alburnum is a channel for the mechanical transport of water, the force of injection being supplied by the active cortex.

We have still 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 east to west, sunHght fell directly upon the exuding portion of the trunk at i p.m., causing a local rise of temperature. Consequently, the cortex underneath had its activity greatly increased, and the resulting enhanced exudation caused a rapid rise of the level of the sap collected in the cavity. This expelled the closing plug, with a resulting sudden outflow of sap. Later 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 became arrested in the evening.

In the ' weeping ' Mango-tree, exudation from a vent on the right side of the trunk took place daily, beginning at I p.m. and continuing till 5 p.m. The vent opened into a cavity formed by the decomposition of the alburnum, the outer wall of the cavity being formed by the uninjured cortex. The internal exudation, effected by active lateral pumping by the cortex, caused sap to accumulate in the cavity. The maximum internal exudation by lateral pumping took place at i p.m., when sunlight fell on the side of the trunk containing the cavity. The local rise of temperature caused a great enhancement of exudation by the cortex at i p.m., such as to set up a pressure sufficiently great to force out the plug of mucilage with which the vent was periodically closed.

There was no exudation from a hole drilled on the opposite side of the trunk. A manometer attached to that side exhibited a maximum negative pressure at i p.m., when the exudation from the vent was at its maximum ; the negative pressure on the left side was due to maximum transpiration at i p.m., which caused rapid conduction of sap by the alburnum. the presence of the alburnum on the left side and its absence on the right side. The above results afford conclusive proof that the alburnum is the channel for the mechanical transport of sap, and that the driving force for the lateral injection of sap into the alburnum is supplied by the activity of the cortex.

The Indian Date Palm — The Palmyra Palm {Borassus flabellijer) — The maximum quantity of exudation in a season — The total yield of sugar — Diurnal variation of exudation in Phoenix sylvestris — Explanation of greater exudation at night — Diurnal variation of exudation in Palmyra Palm — The action of sunlight — Absence of root-pressure — Stimulus for initiation of exudation — The magnetic analogue of polar action of cells in absorption and excretion — Summary.

Many trees before the unfolding of leaves in early spring are filled with sap under considerable pressure, on account of which the sap exudes as soon as a hole is drilled into the tree. The exudation of sap by Palms appears to belong to a different class of phenomena, for which it has hitherto been impossible to offer any explanation. Molisch has shown that in the Palm Arenga saccharifera there is no root-pressure, yet the quantity of sap exuded from an incision may be as much as 4 litres a day. Exudation is even more copious in other Palms. In Phoenix dactylifera it is, according to Semler, as much as 10 litres a day, a value which has been regarded as exceptionally high, since Molisch did not obtain such large quantities.

My investigations have been carried out with two different species of Palm growing in Bengal, from which large quantities of sugar-containing sap are gathered every year, forming no inconsiderable portion of the sugar-supply in the province. The Indian Date Palm [Phcenix sylvestris) grows to a height of 30 to 40 feet. The sap is drawn from the upper end of the trunk. All the leaves situated below are cut off ; vertical thin slices are then cut at the top of the stem, and an inclined V-groovc is made, in which is inserted a small drainage-pipe made of a piece of Palmyra Palm : this is led to an earthen pot suspended from above

Fig. 60. Phcsnix sylvestris The stem is sliced for collection of sap in the pot. (fig. 60). According to the prevailing custom, the surface of the tree to be wounded is one which faces the sun, so that the exuding surface is stimulated by sunlight either in the forenoon or in the afternoon. of November, December, January and February. The sap is drawn for four days at a time, with two following days for rest ; and it is collected on about sixty days in the season. The yield of sap is very considerable ; a tree growing in the dry lands of the Sijbaria Research Station gave an average of about 4 litres per day : another tree growing near a water-course gave as much as 19 litres per day ; and the yield of sap during the whole season is more than a thousand litres. The sugary sap is drunk fresh, or used for the manufacture of sugar ; it is also fermented for making intoxicating liquor.

The sap of the Indian Date Palm, as already stated, is drawn from November to February ; but it should not be inferred from this that the exudation takes place only in the winter months. I have been able to obtain it even in summer ; but it is not worth while doing so out of season. The reason for collecting the sap during the winter months is twofold : first, the amount of exudation is not so seriously affected by loss from transpiration as in summer ; and secondly, in hot weather the sap is spoilt by fermentation. Very special precautions are taken to prevent this even in winter by careful cleaning of the collecting pots ; in spite of this the sap is sometimes spoilt and becomes sour on particular days, when it becomes warm in consequence of change in the direction of the wind.

The Palmyra Palm {Borassus flahellifer) is a tree of very slow growth, and is said to live for more than a century. The tree attains a height of 100 feet (30 metres). The sap is drawn from the cut end of the spadix bearing the flowers ; about nine such spadices are borne at the top of the tree, some of which bear only male flowers, in March. In others the inflorescence bearing both male and female flowers appears about the middle of April. tree. An incision into the spadix is, by itself, ineffective in inducing exudation : a preliminary process is necessary, which consists in bruising the axis of the inflorescence and crushing the young flowers. The axis is also kneaded from above downwards. After going through this preliminary process for several days, a very thin slice is cut off from the tip of the inflorescence. The exudation then takes place with great rapidity. The wound, which becomes blocked by bacterial growth, has to be re-cut every day, the thinnest slice being sufflcient for this purpose.

There is no choice of season for the collection of sap from the Palmyra Palm, since the inflorescence appears only in early summer. Fermentation is very pronounced at high temperatures, and very special precautions have to be taken to obtain the sap unfermented. Ordinary cleaning of the pot is found insufficient for the purpose ; the prevailing custom is to smear the vessel with quick- lime and wash it afterwards. This antiseptic treatment is often successful in securing the fresh sap as a drink. But on hot days the sap ferments, when it is employed for the preparation of the intoxicating toddy.

The tree employed in my experiments was 30 feet in height. The exudation from a single spadix was 2100 c.c, and the total daily exudation from all the spadices was II htres. Trees may yield nearly twice this quantity. The sap is usually collected for four or five months in the year. The tree begins to yield sap at the age of fifteen years, and continues it for a further period of fifty years. The operation is discontinued one year in three. The total quantity of sap exuded by a single tree during its life may be as much as 120,000 litres.

The sugary sap contains about 0'25 gram of mineral solids per 100 c.c. It is very rich in sugar, the content being as high as 10 per cent. Since a Palmyra Palm gives out, as already stated, about 120,000 litres of sap, the yield of sugar from a single tree during its life may thus reach the enormous total of 12,000 kilograms. In order to determine the characteristics of exudation, a continuous record was taken from which the rate of exudation for every hour of the day and night was obtained.