The Physiology of the Ascent of Sap

Returning to the discussion of the relation between pressure and exudation, we may first enquire whether these exhibit any diurnal variation, and in the case of such periodicity relate it to some definite external change. No definite information is, at present, available. According to Baranetzky, Detmer, Brosig, and Wieler, ' a decided daily periodicity cannot be detected in all cases, and it is even doubtful whether the maximum for a given plant always occurs at the same time.' As regards the cause of this periodicity ' Baranetzky found that an alteration in the periods of illumination caused the daily periodicity to change ; Brosig remarked that in a certain plant no such effect was produced ; Baranetzky again has shown that in many cases no daily periodicity at all is exhibited.' ^

I find, however, that both pressure and exudation show a definite periodicity related to the diurnal variation of temperature ; that it is affected to a small extent by the recurrent action of light and darkness, and that the periods of maxima and minima are modified in a definite way, according to the presence or absence of transpiring leaves. We shall first study the diurnal variation of root- pressure in a root-stock cut about lo cm. above ground. Though the root buried underground is but slightly influenced by external changes, the short piece of stem

above ground is affected by them. The root-stock bore no leaves. Success in obtaining accurate results depends greatly on the sensitiveness and reliability of the self-recording apparatus. I have already described the special devices for recording the rate of exudation. For recording pressure and its variations, the movement of a float, making a tracing on a revolving drum, has been used. But this method is subject to numerous errors : the float is apt to turn round and get stuck to one side of the manometer tube ; friction against the recording surface, moreover, restricts the free movement of the float, introducing con- siderable error into the record. These difficulties have been completely removed by attaching the float to one arm of a recording lever with jewel-bearings. The other arm of the lever records on an oscillating plate of smoked glass the movement of the column of liquid in the manometer ; the error arising from friction is thus eliminated. By a system of double levers, magnification may be increased to any extent desirable. I have thus been able to devise an apparatus for certain special investigations, by which a variation of pressure as small as a millionth of an atmo- sphere may be recorded. For our present purpose, the record of pressure to o -i mm. of mercury is quite sufficient. A diagrammatic representation of the method employed is given in fig. 46. A manometer is connected with one arm of a three-way tube fixed to the cut end of the plant. In certain experiments the plant had a side-branch bearing leaves, shown in dotted outline ; in others, the side-branch was cut off. The diagram explains how the closure of the stopcock s and the opening of Si enable us to record the root-pressure, by means of the recording lever attached to the float ; opening of s and closure of Si allow the record of exudation by means of the tilting lever t.

The internal pressure of the plant is usually measured in relation to that of the atmosphere, but this may lead Fig. 46. Diagrammatic Representation of the Methods of obtaining Record of Exudation by the Tilter t, and of Pressure by the Recording Lever attached to a Float, f, in the Manometer The attached branch and leaves are shown in dotted outline. In certain of the experiments this branch was removed. to serious misunderstanding. Atmospheric pressure has little or nothing to do with the ascent of sap and its various manifestations. It would be desirable to record the absolute pressure, taking vacuum as zero. The results can

easily be converted to the atmospheric scale by subtracting 760 mm. from the absolute value. The plant Zea Mays is very suitable for this investiga- tion, since its root-pressure is considerable. The following experiments with a large number of plants were commenced in February and continued for more than ten weeks. It was cold in February, but later it became excessively warm, on account of a heat-wave that passed over Bengal in April. There were periods of stormy weather, which settled down after a time. In spite of these fluctuations in the weather-conditions, the results were very definite, as will be seen from the following.

The first of the series of experiments with Zea Mays was undertaken in February ; the automatic records obtained show^ that the pressure early in the morning was at its minimum, i.e., 916 mm. of mercury, or 156 "5 mm. above the atmospheric pressure. The internal pressure increased and attained a maximum value of 931 mm. shortly after 2 p.m., which is the moment for the attain- ment of the highest temperature. After this, the pressure declined with the fall of temperature, the minimum being reached once more early in the morning.

In demonstration of the close relation between the diurnal variation of temperature and of pressure, I reproduce the two curves taken on the same plate (fig. 47) ; the vari- ation of temperature was here recorded by a differential metallic strip-thermometer. The close agreement between the two curves proves that the variation of pressure is practically determined by the variation of temperature. 1 have designated as the thermal noon, that of the lowest temperature as the thermal dawn. The maximum temper- ature, under normal conditions, is attained at or about

2 P.M., and the lowest temperature about 6 a.m. These two periods, speaking generally, correspond to the periods of maximum and minimum pressure respectively. In exceptional cases, as during the stormy condition of the weather which occurred in March, there were numerous fluctuations in the temperature-curve, due to the inter- mittent passage of clouds and changes in the direction of the wind. Under these conditions the pressure-curve was found to follow very closely the curve of temperature ; so close was the resemblance between the two, that one could use the physiological plant-manometer as a sensitive thermometer.

Fig. 47. Record of Diurnal Variation of Internal Pressure in The upper record is of the diurnal variation of temperature. The activity of living cells in the maintenance of pres- sure is proved by the fact that it declined and became finally abolished with the growing physiological depression in the decapitated plant. After the subsidence of the stormy weather, the atmo- spheric conditions became more stable, though on account of the periodic change in the direction of the wind there were two thermal maxima instead of one in the course of twenty-four hours (April 13-14), a preliminary small maximum about 1.30 p.m. and a higher maximum at 4 p.m. The pressure-curve also exhibited a double maximum at

the corresponding periods (fig. 48). This record was taken in the second week of April, when the plants were bearing ripened fruits. The records were taken after a brief period of rain. The maximum pressure was now ii75'8 mm., instead of 951*2 mm. before the rain. . 48. The Dinrual Record of Pressure and of Exudation in Root-stock of Zea Mays exhibiting Double Maxima corresponding to the Two Thermal Maxima At the period of the year when a deciduous tree is bearing no leaves, its diurnal record of pressure may be expected to exhibit a certain resemblance to that of a root-stock without leaves. The following is the record

(fig. 49) obtained with a leafless tree [Poinciana regia). The pressure is seen to undergo a continuous in- crease with the rise of temperature, at- taining a n-iaximum at thermal noon at or about 2 P.M., and then to dechne with the fall of tempera- ture. The diurnal curve of the deci- duous tree is thus similar to that of the root-stock without leaves. The results given above lead to the following generalisa- tion : The diurnal variation of root-pressure in a root-stock without leaves, and in a leafless deciduous tree, is determined hy the variation of temperature, the maximum being attained at thermal noon, and the minimum at thermal dawn.

Fig. 49. The Record of Variation of Pres- sure in a leafless Tree, Poinciana regia (lower record) In this investigation I sought to ascertain whether exudation exhibits any diurnal variation, and secondly, whether this variation of exudation bears any relation to the variation of pressure. In order to study the question of exudation and pressure under identical external varia- tions, I prepared two root-stocks of Zea Mays, which were growing side by side ; A was employed for the record of the variation of pressure, and B for the variation of exuda- tion. As the temperature was high, a possible error in the determination of exudation might arise from rapid evaporation of the exuded sap. For the elimination of

this error I employed the following device (fig. 50). From the india-rubber cork closing the glass tube attached to the root-stock, a narrow glass tube was led to a graduated burette, in the cork of which was fixed another glass tube communicating with the air, evaporation being prevented by a drop of oil which acted as valve. The exudation Fig. 50. Arrangement for Measurement of Exuded Sap Loss by evaporation is prevented by the oil-trap o. of each drop from the plant caused an expulsion of an equal volume of air, which bubbled through the oil ; the quantity of sap exuded at definite intervals was measured by the graduations on the burette.

The determinations of the diurnal periodicity of exuda- tion and of pressure were carried out for twenty-four hours from April 13 to 14, when, it should be remembered, two thermal maxima occurred, one at i p.m. and the other at 4 P.M. The record of plant B, fig. 48, shows that the rate of exudation increased with the rise of temperature. attaining its first maximum at i p.m. and the second maximum at 4 p.m. The record of the pressure of the adjoining plant A exhibits a corresponding change. It is thus seen that, under similar external changes, pressure and exudation exhibit parallel variations.

Light exerts two distinct effects, which may be dis- tinguished as thermal and photic. Absorption of light raises the temperature and enhances the physiological activity ; light also acts as a stimulus, inhibiting the rhythmic activity : the actual result represents the differ- ence between these two effects. The retarding effect of light is, however, masked by the predominant thermal action. Certain characteristic features shown in the records given in fig. 48 indicate, however, the retarding action of light in a very interesting manner. In Zea Mays there is no thick bark to shield the active cells from the action of light, which was exceptionally strong in April. It will be noted that after five o'clock, when the light was rapidly fading, there occurred a transient enhancement of pressure and of exudation, though the temperature had been under- going a slow decline. It would appear that this tran- sient rise was due to the removal of the retarding action of light. The effect due to light is, however, negligible compared with the effect of the diurnal variation of tem- perature.

A root-stock without a side-branch bearing leaves, and a leafless tree, exhibit a diurnal periodicity of pres- sure. This periodicity is determined by the variations of temperature, the maximum being attained at thermal noon, and the minimum at thermal dawn. temperature, the maximum and the minimum exudation corresponding to the periods of the maximum and the minimum temperature. A relation exists between pressure and exudation, such that an increase of pressure is attended with an increase of exudation and vice versa.

Complexity arising from fluctuating factors of absorption and excretion — Hydraulic and electric model — Diurnal variation of pressure in root-stock — Effect of light — Explanation of irregular variation of pressure in trees — Diurnal variation of exudation in root-stock — Positive and negative exudation — -Exudation from Pithecolobiiim — Summary. In the last chapter we studied the changes in the pressure and in the exudation of root-stocks and deciduous trees, and found that the diurnal variation is determined principally by the variation of temperature, the maximum being attained at thermal noon, and the minimum at thermal dawn ; and it is concluded that the variations of pressure and exudation are to be attributed to responsive variations in the activity of the cells concerned in the propulsion of the sap.

We will now consider the case of the plant which bears transpiring leaves. Transpiration, as we have seen, attains a maximum at thermal noon, and a minimum at thermal dawn (p. ii8). The state of turgor at any time of the day (and also the internal pressure and the power of exudation) will thus depend on the relative gain and loss of water at that particular time. The problem is thus the investigation of the resultant effect brought about by the algebraical summation of the two fluctuating factors, one positive and the other negative. The positive is the absorption of water by the root ; and the negative, excretion by the leaves. Our experience of water and electric supply in the house will, however, give us a clearer conception of this resultant effect. In fig. 51, the diagram to the left represents the water-main with two

side-pipes, of which the larger exit-pipe s may be taken to represent the outlet for transpiration, and the smaller, Si, that for exudation, while m is the manometer for the measurement of pressure. The diagram on the right side Fig. 51. Diagram of Water-supply, showing the Diminution of outflow of Sj by opening the Stop-cock at s, which also causes a Diminution of Pressure indicated by the Manometer m The diagram on the right shows the effect of hghting an arc-lamp in diminishing the current through the incandescent lamp p, and in diminishing voltage indicated by the voltameter v (see text).

represents the electric main, with an arc-lamp and an incandescent lamp, which consume the current. In the water-main, if the stop-cock s is opened widely (increased transpiration), the exudation through Si will be diminished. Similarly, the lighting of the arc-lamp, by withdrawing a large amount of current, will dim the incandescent lamp. The excessive loss will also diminish the pressure, as in- dicated by the manometer and the voltameter, respectively. We may now take up the question of the effect of vary- ing rates of transpiration on pressure and on exudation, beginning with pressure.

I prepared a root-stock of Cucurbita, with a side-branch bearing six leaves (see fig. 46) ; the plant was growing under field-condi- tions. The cut end of the root-stock was connected with the Recording Manometer. The specimen was ex- posed to diffuse light from the sky, and shaded from the rays of the sun. The internal pressure was 860 mm. at six in the morning, but it fell rapidly with the rise of temperature; at thermal noon, after 2 p.m., the pressure was at its

Fig. 52. The Diurnal Variation of Pressures minimum of 68o CuctirbitaWith. Leaves ; the Minimum Pressure mm. The tempera- is at Thermal Noon ^^^^^ ^^^^^ ^^ f^U after 3 p.m., and this was attended by a rise of pres- sure (fig. 52). The diurnal record of the variation of pressure in a root-stock with leaves is thus diametrically opposite to that in one without leaves : in the former, the pressure at thermal noon is at its minimum, whereas, in the latter, it is at its maximum. The reason for this difference is that transpiration increases with the tempera- ture, attaining its maximum at thermal noon ; though the ascent of sap is also enhanced by the rising temperature, yet the loss by transpiration is disproportionately greater. Fall of temperature produces a converse resultant effect.

When the diurnal curve of the above Cucurbita in the shade was exhibiting increasing pressure during the fall of temperature after 3 p.m., sun- light was thrown on the leaves by means of an inclined mirror, thus raising their temperature. Transpiration was suddenly in- creased : this was at once re- flected in the pressure-curve by a responsive diminution of pres- sure (fig. 53). After cessation of the exposure to light, the pressure again increased with the falling temperature, attain- ing its maximum early next morning.

The response by pressure- variation is extremely sensitive ; I have been able to obtain re- sponse by merely striking one of the leaves. The internal pressure is caused to undergo variation by the action of light, by mechanical stimulus, by the action of the wind, in fact by the action of any agent which induces a variation of transpiration. Fig. 53. Variation of Internal Pressure by the Action of Light on the Leaves The pressure was rising on account of fall of tempera- ture. Incidence of hght, L, induced a diminution, fol- lowed by recovery on the cessation of light. Applica- tion of hght a second time induced similar result.

The pressure indicated by lateral manometers attached to a cylindrical vessel containing water, undergoes a regular diminution from the base upwards. If there is any leak in the vessel, all the manometers will immediately indicate a diminution of pressure, for there is no resistance to delay the readjustment of the pressure throughout the column. The case is, however, different in a tree, where the tissues offer great resistance to the flow of water. Hence a lateral leak, however produced, will take a considerable time to cause readjustment of pressure throughout the tree. In a tree-trunk, moreover, there is not a single leak, but many leaks through side-branches irregularly disposed at various heights ; the leaves in their turn are subjected to diverse influences which modify transpiration. Under these cir- cumstance;3 the irregular variation of pressure at different heights of the tree is by no means anomalous ; it would have been surprising had it proved to be regular.

We now consider the case of the periodic variation of pressure in the intact tree with leaves. For this I experi- mented with a large Rain-tree {Pithecolohium) growing in the grounds of the Institute. It was about 8 metres in height, with numerous outspread branches. The tree was shaded by the neighbouring trees, except at midday, when the sunhght fell on it : otherwise it was subjected to the diurnal variation of temperature, which attained its maximum at about 2 p.m. A recording manometer was attached to the trunk at a height of a metre from the ground. I give the automatic records of the variations of pressure and of temperature (fig. 54), which show that with the rise of temperature the pressure underwent a decline, and that the minimum pressure was attained at thermal noon. In the case of plants without leaves we saw that the curves

of temperature and pressure were parallel to each other (see fig. 47) : in the present case, the one curve appears to be an inverted reflection of the other. Fig. 54. Diurnal Curve of Variation of Pressure in the Rain-tree [Pithecolobiiim) The upper curve shows the variation of temperature, and the lower curve the variation of pressure. Note that the pressure was at its minimum at the thermal noon, 2 P.M. The diurnal variation of pressure in the root-stock with leaves, and in intact leafy trees, is the reverse of that in plants without leaves. The maximum pressure in plants with leaves is attained at thermal dawn and the minimum at thermal noon.

Root-stock of Cucurhita with side-branch. — Observations were made on the change in the rate of exudation at different times of the day. The specimen was exceptionally vigor- ous, and in spite of the loss by transpiration from a side- branch, the exudation was considerable, specially at night- time. The rate of exudation attained its maximum early in the morning, after which it began to dechne rapidly. After 10 A.M. there was a total arrest, which persisted till 5 P.M. The exudation recommenced after that hour, and increased continuously till it attained the maximum at thermal dawn.

The next problem was to find out what happened during the arrest. We have already found that the internal pres- sure of Cucurbita with a side-branch underwent a rapid diminution with the rise of temperature, the minimum pressure being at or about 2 p.m. (p. 162). For obtaining an automatic record of the change in exudation, I employed the following device.. The free end of a bent glass tube attached to the cut end of the root-stock was immersed in a cylinder partially filled with water in which was a float : a continuous record of the variations in the level of the liquid was obtained by connecting the float with the recording lever. Fig. 55 gives a record obtained from Cucurbita for a period of twenty-four hours, from 7 p.m. to 7 P.M. the next day, which shows that exudation con- tinued till 10 A.M., when it stopped for an hour. After II A.M. the sign of exudation was reversed from positive to negative, that is to say, the cut end of the root-stock began to stick in water instead of exuding it. This negative exudation continued till about 5 p.m., after which the positive exudation was resumed.

There are thus two points of inversion in the curve, one after 10 a.m., and the other after 5 p.m. The explana- tion of the transition from the positive to negative and back once more to the positive is as follows : At night the loss by transpiration was slight, hence there was but little diversion of the sap from exudation. In the forenoon, the temperature was rising, and the loss by transpiration was increasing at a disproportionately higher rate ; between