The Physiology of the Ascent of Sap

It is supposed that light enhances transpiration, either directly by widening the opening of the stomata or in some other way at present little understood. That the widening of the opening of the stomata under light plays but a subor- dinate part in enhancing transpiration is made probable by the following considerations : (i) Bonnier and Mangin found that exposure to light enhanced transpiration in Fungi ; the effect of light is thus independent of the presence of stomata.

(2) In ordinary leaves the transpiration is modified by the action of light on the upper surface which bears no stomata. temperature is more pronounced than that induced by exposure to light. The increase may therefore be more or less due to the heat-rays present in the light (4) Light docs not always induce an enhancement of transpiration ; under definite conditions it causes a diminution. Action of Light on the Widening of the Stomata and on Transpiration

The following experiments were undertaken to find the relative variation of transpiration due to the widening of the stomata under light. A leaf of Thunhergia placed in a dark corner of the room at constant temperature at 30° C. exhibited a transpiring activity of 38. It was next brought near the window and subjected to the action of the stronger diffused light of the sky. This should, accord- ing to the generally accepted view, enhance the trans- piration by the widening of the stomata under light. The temperature might have been slightly higher near the window, though a sensitive thermometer did not indicate any difference. But in spite of the greater illumination and the possible rise of temperature, the leaf exhibited a diminished rate of transpiration which was lowered from the normal 38 to 29, a diminution, that is to say, of 23 per cent. The theory that the widening of the stomata en- hances transpiration is thus seen to fail in this particular case, which shows that transpiration is actually diminished under light.

In a second experiment I made continuous observation of the transpiring activity of two different leaves of Nauclea, A and B, from 6 p.m. to 6 am. next morning. The results obtained were very similar (fig. 33) ; it will therefore be sufficient to give a detailed account o^^ the variation of transpiration in b. The fall of temperature from 6 p.m. to midnight was fairly uniform, from 32-8° C. to 29-8° C, or a fall of half a degree per hour. The transpiring activity

declined from 116 to 71 -5. It will be noted that the trans- piration was still considerable, though the leaf had been subjected to continuous darkness for several hours. The subsequent changes were practically determined by variation Fig. 33. Curves exhibiting Variation of Transpiration at Night in two Leaves, a and b Note sudden depression of transpiration by a fall of half a degree in temperature at midnight, and an enhancement of trans- piration by a rise of one-third of a degree after 2.30 a.m. Light early in the morning caused a transient enhancement of transpiration.

of temperature ; there was a sudden fall of temperature of half a degree after midnight to which the leaf responded by a diminution of transpiration from 71-5 to 55, i.e., a depression of 23 per cent. At 2.30 a.m. there was a rise of temperature of one-third of a degree, and the trans- piration exhibited an enhancement of lo per cent. Light began to appear at 5 a.m. ; this induced a transient rise, which subsided after a time, and the curves of temperature and of transpiration subsequently followed a parallel course.

The experiments described above show (i) that the effect of rise of temperature on transpiration is far more pronounced than that of light ; (2) that the heating effect of light may often account for increased transpiration ; and (3) that light sometimes induces a diminution of trans- piration instead of an enhancement. The pulsation of Desmodiiim is arrested by exposure to strong light, which also retards or arrests growth. We have seen, moreover, that while moderate stimulus retards growth, a feeble stimulus enhances it. The growth of a less excitable sub-tonic tissue exhibits acceleration under an intensity of light which in a more excitable tissue would induce retardation (p. 17).

As regards the relative effectiveness of various rays in retarding growth, it is known that while the more refrangible rays of the spectrum, blue and violet, are highly effective, the red rays at the opposite end of the spectrum are less effective or ineffective. Bearing these facts in mind, 1 undertook the following experiments with the object of ascertaining whether or not the process of transpiration responds to the action of light of different wave-length in the same manner as the process of growth.

In order to determine the relative effect of the less refrangible red at one end of the spectrum, and of the more refrangible blue-violet rays at the other, I at first employed a bichromate of potash solution as a light-filter to separate the former, and an ammoniated solution of copper sulphate for the latter. The results I obtained with these filtered lights were often found to be anomalous : in consequence, as I discovered later, of the impure character of the light, due to the overlapping of the spectra. The light transmitted through the bichro- mate solution contains red, orange, yellow and green, while that transmitted through copper sulphate includes green, blue and violet rays. I therefore made special light-filters with coloured glasses which gave red in the region of Fraun- hofer's lines B and C, the wave-lengths being from 680 to 600 /i/i ; and blue-violet light in the region of Fraunhofer's line G, the wave-lengths being from 460 to 380 ^ifi. I thus obtained two spectral lights widely separated from each other.

The source of light was an incandescent electric lamp of 200 candle-power. This was used in preference to the arc-lamp, the light from which cannot be maintained constant, which radiates a very large amount of heat-rays. The incandescent lamp was placed inside the lantern and a slightly divergent beam was allowed to fall on the leaf for a definite length of time by manipulating a shutter. The interposition of the colour-screen and a parallel-sided glass trough filled with alum-solution eliminated the heat- rays. The room was kept in perfect darkness.

The mode of procedure in the following experiments was : (i) the observation of the normal rate in darkness ; (2) observation of the effect of exposure to blue-violet light for five minutes ; (3) observation of recovery which was practically complete in the course of twenty minutes ; minutes ; (6) the effect of exposure to blue-violet light for the second time. A complete cycle of operations was thus carried out with a particular leaf. In other cases the order of pro- cedure was reversed, that is to say, the observation was taken first with red, and afterwards with blue- violet light.

The following is the detailed account of a typical experi- ment. The normal transpiring activity of a leaf of Thun- hergia in the dark was found to be 45. On exposure of the more sensitive lower surface for five minutes to blue- violet light, the rate of transpiration underwent a diminution to 36, or a variation 0/— 20 per cent. On the cessation of exposure, the normal rate of 45 was restored in the course of twenty minutes. Red light was next applied for five minutes, and this caused an enhancement of rate to 59, or a variation of 31 per cent. After an interval of twenty minutes, recovery was nearly complete, the rate being 42. Exposure to blue-violet light was resumed for five minutes ; this caused a depression to 33, i.e., a variation of —21 per cent. ; this is practically the same as the variation of — 20 per cent, obtained at the beginning of the series.

The table on p. 113 gives the results obtained with three other specimens of Thunbergia and one specimen of Nauclea. In all the cases given above, it was invariably found that the blue-violet light, which is effective in inducing a retardation of growth, also caused depression in trans- piration ; red light, which is ineffective in retarding growth, acted like a sub-minimal stimulus, inducing an enhancement of the rate of transpiration. These results show that the average depression under blue-violet light was about — 36 per cent., the mean acceleration under red light being about + 68 per cent.

I also experimented on the effect of stimulation of the less excitable upper surface of the leaf : curiously enough, this gave responses similar to those obtained by mechanical stimulation of the two sides, but reversed. In this experi- merit, the blue-violet light acting on the upper side caused an acceleration of transpiration, while the red induced a depression. The effects produced were, however, feeble compared to those induced by the stimulation of the more excitable lower side : the increase of transpiration caused by blue-violet light was -|-4"5 per cent, of the normal, the depression by red being 9 per cent.

Table XVI. — -Showing the V'ariation of Transpiration under Red and Blue-violet Light for Exposures of Five Minutes We study next the effect of various anaesthetics on transpiration ; of these carbonic acid gas ma\^ be taken as the mildest ; chloroform, on the other hand, as the most intense and toxic after long application. A broad inverted funnel was made to enclose the trans- piring leaf. After measuring the normal rate of transpira- tion of a leaf of Nauclea, a glass jar containing carbonic acid was emptied over the funnel. This increased the

transpiration almost three-fold, from the normal 28 to 70, after the application of the gas. Continued application of the gas, however, caused a slight depression compared with the normal. In another specimen of the Nauclea leaf, the first effect was an enhancement from the normal 106 to 133 ; continued action of the gas induced a depressed activity of 100. I may here describe the remarkable effect of moderately high temperature in modification of the above effect. The result described above was in winter, in January of the present year. The experiment was

Fig. 34. Effect of Carbonic Acid Gas in Enhancing the Rate of Transpiration Fig. 35. Effect of Ether ; Prehminary Enhancement followed by Depression repeated in May, when the room temperature was 32° C. The preliminary effect of enhancement was now practically absent, and the rate of transpiration became depressed from the beginning under the action of the gas. Thus in a given experiment with Nauclea the normal rate of 200 was depressed to 163 in the course of a few minutes.

The enhanced transpiration under carbonic acid is shown in a record (fig. 34) of the balanced Transpirograph, in which the balance is upset in an upward direction. The vapour of ether was applied in a manner similar to the above. The immediate effect was an enhancement of transpiration as seen in the upset of the balance upwards, followed by depression under the continued action of the anaesthetic as seen in the down-curve (fig. 35). Finally, I studied the effect of chloroform-vapour on the transpiring lamina. This is seen to induce a great depression of the rate, as indicated by the upsetting of

Fig. 36. Effect of External and Internal Application of Chloro- form in depressing Transpiration the balance downwards (fig. 36). In another experiment, dilute chloroform was applied to the cut end of the petiole by a side-tube. This internal application also caused a great depression of transpiration, as seen in the right-hand record. The results of applications of the anaesthetic to the lamina and to the petiole are thus identical. Transpiration and its induced variations can be auto- matically recorded by the Micro-Transpirograph.

Plasmolytic agents applied at the cut end of the petiole induce a diminution of transpiration of the loaf. Application of feeble electric stimulus to the lamina causes an enhancement, while stronger stimulus induces a depression or arrest of transpiration ; the after-effect of a moderately strong stimulus is often an enhancement of transpiration above the normal. Mechanical stimulus of moderate intensity applied to the upper surface of the leaf induces an enhancement of the rate of transpiration ; the same stimulus applied to the more sensitive lower surface causes a depression of the rate.

The activity of transpiration is depressed when the leaf is placed in a field of rapidly alternating electric force. Electric waves also induce depression. Statical electric induction, both positive and negative, enhances the rate of transpiration. Rise of temperature of the leaf caused by thermal radiation enhances the rate. The effect of light is complex on account of the presence of two antagonistic factors : of heat-rays which enhance transpiration, and of Hght-rays which, acting as a stimulus, retard it.

The rays at the two extreme ends of the spectrum affect transpiration in opposite ways. The action of blue- violet light on the under side of the leaf causes diminution ; that of the red rays causes enhancement. The action of light on the upper surface of the leaf induces an effect opposite to that of its action on the lower surface. Carbonic acid induces an enhancement of transpiration ; long-continued action induces a depression. Ether induces a preliminary enhancement followed by depression.

Chloroform depresses the transpiring activity of the lamina. The same effect is induced by application of dilute chloroform to the cut end of the petiole. The above experiments offer independent proof that transpiration is, essentially, not a physical but an active physiological process. Diurnal variation of transpiration in plants with roots — Diurnal varia- tion after removal of the root — The Radiograph — Diurnal variation of temperature and of light — Balancing evaporation against trans- piration— The Differential Balance — The optimum-temperature for transpiration — Summary.

Having studied the effect of physiological variations on transpiration, we may attempt to ascertain if the rate of transpiration undergoes a daily variation. Should this prove to be the case, it will be necessary to determine the external changes to which this variation is due. For the purposes of this investigation it is necessary to obtain a continuous record of the transpiration for twenty-four hours. The self-recording Transpirograph, already described, may be used for this purpose, taking the precaution of reducing its sensitiveness to a considerable extent, for the record would otherwise go off the plate. The sensitiveness may be diminished to any extent desired by increasing the diameter of the tube at the opposite ends of which the plant and the recording float are adjusted. The apparatus was fixed in a place free from vibration, the leaves being exposed to the light of the sky, but not to direct sunlight. Two separate records with two specimens of Chrysanthemum were taken simultaneously on the same plate, which was allowed to fall down vertically at an uniform rate by rneans of a clockwork. It was not necessary to move the plate laterally for obtaining the time-record, since this was secured by the dots in the record produced by the oscillation of the recording plate at definite intervals of half an hour. The distance between

the successive dots then affords a striking indication of the relative rapidity of transpiration at different periods of the day. The general similarity of the two records affords strong evidence that the result is not accidental, but is due to similar physiological ( hanges in both. The record for twenty-four hours obtained with Chrysanthemum with root is given in fig. 37. The vertical The Record of Diurnal Variation of Transpiration [Chrysanthemum)

record is reproduced as horizontal for convenience of in- spection. The successive dots, as already stated, are at intervals of half an hour, and the enhanced rate of trans- piration is seen in the widening of the spacings. The upper record represents the transpiration from 10 a.m. to 4 P.M., i.e., for six hours, and the lower record from 4 p.m. to 10 A.M. next morning, that is to say, for eighteen hours. The distance covered is the same in the two records, hence the average transpiration between 10 a.m. and 4 p.m. was three times quicker than that between 4 p.m. and 10 a.m. We also find that the maximum transpiration occurred at 2 P.M., which was also the thermal noon, or the period of temperature maximum ; the minimum transpiration coincided with the temperature minimum between 4 and

5 A.M. The rate was 1-7 c.c. per hour at 6 A.M. ; it increased with the rise of temperature; at midday it was 9-8 c.c. ; the maximum temperature at or about 2 p.m. was also the period for maximum transpiration, which was 12-9 c.c. After this the temperature fell and the rate of transpiration also declined; at 4 p.m. it had fallen to 9-8 c.c; the minimum transpiration of i c.c. was attained at or about 5 A.M. The maximum transpiration at thermal noon was thus about thirteen times that at thermal dawn.

The record was continued for the next twenty-four hours, but now after the removal of the root. We observe a diurnal variation similar to that in the last experiment, but with a general enhancement of the rate. Thus the ratio of the maximum transpiration of the plant with the root and without it was 12 -9: 22 -3. The ratio at the minimum temperature at 5 a.m. was 1:1-5. The trans- piration of the shoot was thus increased to about i -7 times after the removal of the root.

The table on p. 121 gives the rates of transpiration for twenty-four hours of the plant with and without root. The curves in fig. 38 also show the relative rise and fall of the rates in the two cases ; the lower curve with the root, and the upper curve after the removal of it. The rate of transpiration at every hour of the day and night was relatively greater after the removal of the root. What, now, is the cause of this difference in transpiratory activity ? The answer to this question will also solve the difficulty concerning the disparity between the amount of water absorbed by the root and the power of the stem to conduct and the leaves to transpire it. That this is regarded as anomalous, is clear from the following quotation :

' We may imagine that the water was forced up by root-pressure. . . . But the question at once rises as to whether the amount of water siippHed by the root is approxi- mately enough to replace what is lost by transpiration. Some experiments carried out by Sachs (1873) are worthy of consideration on this latter point. He compared the amount of sap given off in a definite time from the root of a herbaceous plant, with the amount sucked up by the shoot, whose cut end had been submerged. A root-stock