The Physiology of the Ascent of Sap

Each dot represents exudation of 50 c.c. of sap. Note separation The Automatic Tilting Recorder, already described on page 134, was strapped high up in the tree. A short pipe allowed the sap to fall on to the tilting lever. This latter was adjusted so as to be upset after the collection of 50 c.c. Table XXI. — Showing Rate of Exudation from Phoenix sylvestris for every Hour of the Day and Night of sap. Each successive dot in the record thus represents the exudation of 50 c.c. of sap. Inspection of the record given in fig. 61 gives a vivid idea of the various rates at different times of the day and night. The dots are very- wide apart from 11 a.m. to 2 p.m., the minimum rate being attained after i p.m. The dots, on the other hand, are closest about 2 a.m., when the rate of exudation is at its maximum.

The total quantity of exudation from 6 a.m. to 6 p.m. was only 700 c.c, while that for the succeeding twelve hours of night was as much as 2,150 c.c, or three times as great. In seeking an explanation of this characteristic difference in the exudation during the day and the night, it is to be remembered that the total loss of fluid depends upon two factors : (r) the excretion by the leaves, and (2) the exudation from the wounded surface : consequently maximum transpiration should correspond with minimum exudation and vice versa. The maximum transpiration would occur at the period at which the temperature of the leaves is at its highest. This is determined (i) by the highest temperature of the surrounding air which is attained- at about 2 p.m., and (2) by the rise of tempera- ture of the leaves by the direct action of sunlight, the intensity of which is at its maximum at noon. Hence the resulting temperature is at its highest between noon and 2 p.m., i.e., at about i p.m. The maximum transpiration at 1 p.m. thus corresponds to the minimum exudation of 32 c.c. per hour. The temperature fell gradually after 2 p.m., with diminution of transpiration and corresponding increase of exudation, the latter attain- ing its maximum at night about 2 a.m. Sijbaria is situated in an open country and the minimum temperature there is attained earlier than in the town. The curve of exudation of this Palm (fig. 62) thus follows the general course of

diurnal variation of temperature, but in an inverse manner, i.e., maximum exudation at minimum temperature and vice versa. I have been able to obtain independent con- firmation that the exudation is actually diminished by transpiration from the leaves. After completing my ob- servations with one particular Palm, 1 cut off the leaves which still remained at the top of the tree. This was found to enhance the normal exudation by 25 per cent. Fig. 62. The Curve of Diurnal Variation of Exudation Phosnix sylvestris

There are additional causes which produce a slight modification of the loss by transpiration : among these may be mentioned the varying intensity of the wind, its changing direction, and the varying percentage of moisture which it contains. At Sijbaria the land and the sea breezes alternate, and after a lull in the evening the air-current increases in intensity ; the wind also veers round. Neg- lecting these minor variations, the curve of exudation is seen to follow closely the diurnal variation of temperature. This is clear in the record of the Palmyra Palm, in which variation of temperature is also recorded (see p. 186, fig. 63).

I next describe the variation of the rate of exudation in the Palm3Ta Palm from hour to hour. An automatic record of the exudation was taken in the usual manner. The rate during successive hours of the day and night is given in the accompanying table. A thermograph was tied immediately above the spadix, and gave an automatic record of the variation of temperature. Table XXII. — Showing the Rate of Exudation for every Hour of Day and Night {Palmyra Palm)

From the data given in the above table we find that, as the temperature rose from 36-5° at noon to 40° C. at 2 P.M., the exudation became lowered from 45 c.c. to -^y c.c. After the attainment of the highest temperature at 2 P.M., thermal noon, the temperature fell at 3 p.m. to 36-5°, and the exudation was enhanced to 44 c.c. Ignoring for the present the disturbance produced at 4 p.m., the temperature underwent a decline from 6 p.m. to 4 a.m. next morning, the fall being from 32° to 24 -7° C. This was attended by an increase in exudation from 73 c.c. to 118 c.c. All these variations in the exudation in the Palmyra Palm arc characteristically similar to those in the Indian

Date Palm : that is to say, the minimum exudation takes place at thermal noon, and the maximum at thermal dawn. A sudden disturbance in the record occurred, as mentioned above, when the temperature rose abruptly Fig. 63. The Upper Curve shows the Diurnal Variation of Exudation in Palmyra Palm, the Lower Curve the Diurnal Variation of Temperature ; the Down-curve indicates a Rise and Up-curve a Fall Note the general parallelism of the two curves which is disturbed by incidence of sunhght s on the spadix ; this induced local rise of temperature and enhancement of exudation.

from 36-5°C. at 3 p.m. to 42-5° C. at 4 p.m. Hitherto the rise of temperature had been attended by a fall in exudation, but now there was an apparent anomaly in the sudden enhancement of exudation from 44 c.c. at 36-5° to 88 c.c. at 42 -5° C. however, found in the following observation. I was watching the record of exudation, when a sudden jump in it drew my attention to the intervention of some disturbing factor. On looking up at the tree, I found that sunlight had just fallen on the exuding spadix, which had previously been in the shade of the leaves overhead. Hitherto, the general rise of temperature had induced a relatively greater loss by transpiration, the result being a diminution of exudation with rise of temperature and vice versa ; so that the curve of exudation had run a parallel course with that of temperature in which the down-curve indicates a rise and the up-curve a fall. But when the sunlight fell on the exuding spadix, the two curves diverged from each other (fig. 63).

The explanation of this is found in the fact that exuda- tion is enhanced by a rise of temperature : the spadix had its temperature, and therefore also its exudation, suddenly increased by the sunlight which fell on it at 4 p.m. A parallel instance of enhanced exudation due to the thermal action of sunlight has already been noted in the ' weeping ' Mango-tree (p. 176). In Palms the exudation of sap appears to be quite independent of root-pressure. Molisch has, as stated before, failed to find any indication of it in Arenga sac- chart/era. The following investigation was undertaken to find out whether root-pressure is generally absent in Palms.

It should be remembered that the Indian Date Palm grows in a dry or even arid soil ; hence necessity compels the tree fully to exploit the scanty and precarious supply of water. For this it sends out numerous roots to a con- siderable distance. The number of roots was found to exceed a thousand, each about i cm. in diameter. I dug the ground to a depth of 12 feet (4 metres) without any prospect of reaching the end ; the thickness of the individual roots remained almost unchanged. I followed another root to a lateral distance of 21 feet (7 metres) and yet the end was not in sight. The trunk of the tree is thus slowly charged with water absorbed by the enormously extended root-system. The fact that the tree is not entirely dependent on the immediate supply from the soil, but that it has a supply stored in reserve, is proved from results of two different experiments. I first tried the effect of copious irrigation ; but it had no immediate effect in en- hancing exudation. It is true that the exudation from trees growing near water-courses is relatively more abundant ; but this is due not to any immediate action, but to the previous storage after slow and long-continued absorption of water from the soil.

I next cut down the Date Palm, which was exuding at the rate of 4 litres a day ; the tree was therefore in a condition of vigorous exudation. After the felling of the tree, the cut ends of the stem did not, however, exude a single drop of sap ; portions of the tissue taken from the interior of the trunk were found to be almost dry, and it was only after considerable compression that a small quantity of sap could be squeezed out. The above experiment proves that there is no root-pressure to cause exudation from the injured surface at the top of the tree, and that the sap is held in the trunk with great tenacity.

The result of the following observation is very striking and of much theoretical interest. After cutting down the tree, it was deprived of any supply of water from the soil. In spite of this the sliced stem in the upper portion of the tree continued to secrete sugar-containing sap for thirty- six hours. This shows that the exudation of sap is not immediately dependent on the absorption of water by the root, but that the secreting activity of the terminal wounded surface withdraws sap held in reserve in the trunk.

We next consider the question of the active exudation from the surface of incision in the Palm. It has been shown that the exudation from the root-stock of Cucurbita is not solely due to root-pressure, but that the terminal layer at the cut surface also takes part in the process. The physiological activity of the terminal layer was demonstrated by the local action of dilute chloroform, which enhanced the rate of exudation (p. 141). The cells of the terminal layer also respond, as we have seen, to various agents such as light, temperature, and so on.

In the Palm the factor of root-pressure is absent ; hence it is the layer of cells at or near the surface of in- cision that is specially concerned in the active secretion. This secretion of sugary liquid at the incised surface of Palm-stems resembles the secretion of a similar liquid by the nectaries of flowers and of digestive fluid by the glands of Nepenthes, since in all these cases the secretory energy is developed in the cells of the secreting tissue. But the secreting tissue of the Palm differs from the glandular tissues in that its activity is not spontaneous. The normal inactivity of the wounded surface is shown by the fact that there is no exudation when the vertical slices arc first made in the upper part of the stem of the Date Palm, nor when a terminal section is made in the spadix of the Palmyra Palm. As already stated, exudation is initiated in the former only after slicing the stem repeatedly for nearly a week, and in the latter, after special preliminary treatment of the spadix for several days.

What now is the explanation of these facts ? I have shown elsewhere that a living tissue may be roused from a state of inactivity to rhythmic or multiple activity by the action of an adequate stimulus. A very inactive tissue would naturally require a very strong stimulus, or a succession of stimuli which become effective by their cumulative effects. In moderately excitable tissues, on the other hand, a less intense stimuhis would be sufficient. Thus the dormant activity of the pulvinule of Desmodium may be revived by the application of a moderate stimulus. Again, isolated cardiac tissue comes to a state of stand-still ; when in this state, application of a prick is found to arouse the quiescent tissue to renewed multiple activity.

I now describe the special treatment to which the in- florescence of the Palm has to be subjected in order to induce exudation. For this purpose, two different pro- cesses have been elaborated in different countries, which may aptly be described as ' butting ' and ' milking,' from the not very far-fetched analogy of the action of the calf to make the cow yield her milk. For the exudation of the sap from the inflorescence of the Arenga Palm, very strong and repeated stimulus is essential ; ' the Malayas, during four or five weeks previous to flowering, inflict repeated blows on the base of the bole with a wooden hammer ; then, when the inflorescence is cut ofi, secretion begins at once.' ^

In Bengal the practice with the inflorescence of the Palmyra Palm is a little different. The long spadix is held tightly between the fingers and kneaded from above downwards, the process being similar to the milking of a cow. This potential milking process is repeated day after day for more than a week. Section of the tip of the spadix is then followed by the exudation of sap. The methods employed in inducing exudation from the previously inactive tissue of the Palm are thus seen to be fundamentally similar. They have one object in common, namely, the arousing of the dormant activity by the re- peated application of a mechanical stimulus, which may be repeated cuts, repeated blows, or repeated kneading. As the result of this treatment, the inactive tissue becomes as active as the glandular tissue, and is thus able to main- tain the exudation even though there is no root-pressure to urge it.

With the exception of the stimulus necessary to initiate it, the process of exudation in Palms is in every way similar to that of the root-stock and of the tree with leaves. The diurnal curves of exudation of Cucurhita with side-branch (fig. 52), that of the Rain-tree (fig. 54), and those of the Phcenix and the Palmyra Palm are very much ahke. The exudation, in all these cases, attains its maximum at thermal dawn, and its minimum at thermal noon.

The ascent of sap, the excretion by the leaf, and the exudation from the cut surface of a root-stock, are thus seen to be all brought about by cellular § j; activityr which extends throughout the length of the tree. Under normal con- ditions, the tree as a whole may be regarded as having two poles, as it were, one at each end, the absorption by the root at one end being distinguished by a plus sign, and the excretion by the leaf at the oppo- site end by a minus sign. The plant may thus be compared to a bar-magnet with positive and negative poles, north and south, at the ends, the intermediate region being apparently neutral. According to the molecular theory of magnetism, the smallest particle in a bar-magnet is itself a magnet with two poles, the juxtaposition of two opposite poles producing the ap- parent neutralisation in the middle. The existence of the opposite poles which neutralise each other may easily be demonstrated by breaking the bar-magnet across at the horizontal line, when a positive or north pole will be found below the upper section and the opposite south pole above the lower section. A similar cut in the stem will bring

Fig. 64. The Mole- cular and Cellu- larModel : plant, right ; magnet, left out the opposite functional polarities above and below the line of separation. The lower end of the cut stem a will be found to absorb, and the upper end of the root-stock E to excrete water. In the hmit each cell must exhibit this polarity, its lower end absorbing water and the upper end excreting it (fig. 64) ; without this, the one-directioned propulsion of sap would be an impossibility.

It will be seen that in the ascent of sap it is quite unnecessary to postulate two forces, one pulling from above, and the other forcing from below. For an identical cellular activity would appear as that of a suction-pump, or as that of a force-pump, according to the particular point in view. We thus arrive at the theoretical conception that it is. the pulsation of the individual cell which is ultimately concerned in the maintenance of the ascent of sap. I go on, in the following chapters, to describe new experimental methods by which it has been possible, not only to detect, but also to record, the pulsation of an individual cell.

The maximum secretion of sugar-containing sap by Phoenix sylvestris is 19 litres, and by the Palmyra Palm 20 litres per day. The total quantity of sap exuded by the Palmyra Palm during its lifetime may be as high as 120,000 litres, the quantity of sugar yielded being 12,000 kilograms. The Palm-tree exhibits a diurnal periodicity of exudation, which follows the general law established in regard to leafy trees, that the minimum exudation occurs at thermal noon, and the maximum at thermal dawn.

Local rise of temperature, the result of temporary exposure to sunlight, induced an enhancement of exuda- tion in the Palmyra Palm, the excretory tissue responding in the same manner as the cells which effect the ascent of sap. There is no root-pressure in the Palms ; the water absorbed by the extended system of roots is stored and held tenaciously in the trunk ; after cutting down the tree the surface incision at the top of the trunk continues for a time to secrete sap. A layer of cells at or near the surface of incision is thus the seat of the secretory activity.

The secretory activity of the terminal layer is brought into play by the intense stimulation caused by repeated cuts, by repeated blows, and by repeated kneading. Electric variation with change of turgor — Electrometric determination of velocit}- — Galvanometric determination — Shock- effect of the hydro- static blow — Simultaneous determination of velocity of ascent by mechanical and electrical methods — Determination of velocity by the Diphasic method — Summary.

In the investigation of the ascent of sap, we have hitherto employed only the method of Mechanical Response, in which advantage is taken of the erectile movement caused by the increase of turgor due to the ascent of sap. We turn now to an independent method for the detection and measurement of the rate of ascent of sap, which will be found to extend our scope of investigation and to lead to the discovery of many phenomena which are beyond the scope of the mechanical method.

This new method is electrical. I have shown in my work on Comparative Electro-Physiology that the electric condition of a tissue undergoes a definite variation under changes of turgor ; a diminution of turgor induces an electric change to galvanometric negativity, while an increase of turgor induces, on the other hand, an electric change to galvanometric positivity. The accompanying table shows the mechanical and electrical concomitants of the changes of turgor in the tissue. A sudden diminution of turgor with contraction occurs under excitation, and a slow diminu- tion of turgor is produced under increasing drought. An increase of turgor is, on the other hand, produced by the ascent of sap after irrigation.

Table XXIII.- -Showing thk Effects induced by Variation OF Turgor Effects of diminution of turgor caused by drought or by stimulus A plant under drought lias its turgor diminished through- out its length. If now we make two electric contacts, A with the stem, and b with a distant leaf, the electric conditions of the two points will be more or less similar, and the galvanometric spot of light will remain stationary. Irrigation will, however, cause an ascent of sap, which will reach the lower contact a earlier, and induce an en- hancement of turgor at that point. This will at once be signalled by a sudden electric change to positivity at A, represented in the record as an up-curve. This arrange- ment will be described as the Mono-phasic method. The second, the Di-phasic method, will be described later.

As the electric resistance between the first and the second contacts is very great, it appeared to me that an electrometer might prove to be a suitable instrument for this research, since its indications are independent of the resistance of the circuit. I am not aware that the Quadrant Electrometer has ever yet been used for physiological investigations. The prevailing impression is that it is very difficult to maintain the high insulation of the circuit neces- sary for the maintenance of the electrification of the needle, and that the indication of the instrument is liable to be disturbed by external electric disturbances. I have, how- ever, been able to render the method not only sensitive

but free from external disturbances. No difficulty was experienced in maintaining the charge of the needle constant. The sensitiveness could be raised so as to give a deflection of I mm. for a difference of potential of o-ooi volt. The zero-position of the reflected spot of light was found to remain steady for days in succession. Fig. 65. The Electrometric Record for the Determination of Velocity Irrigation at arrow caused, after an interval of seven minutes, a sudden erection of the base line upwards, indicating in- duced electro-positivity of the first point of electric contact. [The time-marks are at intervals of twenty seconds.]

The advantage of this method is, that not only does it allow visual observation of the induced electric varia- tion, but photographic records may be easily secured. The resistance of the circuit or its variation produces no change in the deflection ; the latter, moreover, gives the absolute value of the induced electromotive force. The lower point of contact, A, is connected with the insu- lated pair of quadrants, the distant indifferent point being put in connection with the second pair, which is earthed.