The Physiology of the Ascent of Sap

being as i ; 2-6. The induced variations in the activity of ascent and of growth may, therefore, be taken to be of the same order. I have explained (p. i8) that the autonomous pulsation of Desmodium leaflet becomes arrested at a temperature below the critical. This is shown in the record (fig. 20) , in Fir,. 20. Record showing the Critical Temperature for the Arrest which the arrest in a summer specimen took place at 17° C, the pulsation being revived at 18° C. The plant becomes accustomed to a lower temperature in winter, when the critical temperature is 11° C. The mean critical point for the leaflet of Desmodium may therefore be taken as 14° C. When the temperature is lowered, the arrest of pulsation persists so long as the temperature remains at or below the critical point ; rise of temperature above this point revives the pulsation. The same living tissue may thus exist in two different conditions, namely, an inactive and an active state, which can be made to alternate by merely lowering

or raising the temperature below or above the critical point. No proof of physiological activity could be more direct or convincing than the alternate arrest and revival induced by this definite physiological variation. As regards growth, the minimum temperature for arrest in Scirpus Kysoor I find to be 22° C. ; the arrested growth becomes feebly revived when the temperature is raised by 1° C, the rate of growth being now 0 "02 yx per second. The critical temperature for arrest of growth in S. Kysoor is thus found to be about 8 degrees higher than the mean critical point for the Desmodium leaflet. This shows that the growing cells are more sensitive to the adverse influence of low temperature than the fully grown cells in the pulvinule of Dcsniodium.

It is thus seen that lowering of temperature below the critical point arrests the rhythmic activity of the Desmodium leaflet and of growth, a rise of temperature above that point causing a revival. Hence, a crucial proof in demonstration that the maintenance of the ascent of sap is effected by the rhythmic activity of living cells would he afforded by the alternate arrest and renewal of the ascent by temperature- variations beloiv and above the critical point.

In endeavouring to ascertain whether or not the ascent of sap is arrested at a critical temperature, I used the method of Erectile Response for the determination in specimens with roots. The Potographic method was employed, more especially for ascertaining whether or not the critical tem- perature was the same or different for the stem and the root of the same plant. Method of Erectile Response. — I obtained the erectile response of Impatiens after irrigation with water at the normal temperature of 30° C. ; on the attainment of the steady rate, water at 10° C. was applied to the root ; this was found to cause an arrest of the ascent, as indicated by the stoppage of the erectile movement. The temperature was next allowed to rise slowly, and when the thermometer buried in the soil indicated 23° C, there was a slow

up-movement indicating resumption of the ascent of sap (fig. 21, a). The critical temperature for arrest was there- fore below 23° C, or at 22° C. Fig. 21. Record of Erectile Response showing Critical Tempera- ture for Arrest of Ascent of Sap (a) First portion of the curve shows normal rate of erection under irrigation with water at 30° C. Application of water at 10° C. caused arrest in the course of fourteen minutes, after which the plant exhibited drooping. As the temperature rose to 23° C. there was a resumption of ascent, as seen in the erectile movement.

(b) The plant was placed with roots in cold water, which caused an arrest of suction and consequent drooping. When the temperature of water rose to 23° C. the ascent of sap and the erectile movement were resumed. (Impatiens.) of temperature. It would be better, were it possible, to place the root in water and adjust the temperature. But a difficulty would arise in the washing of the soil from the roots, and the fixing of the plant in a vessel of water. By the time this had been accomplished the root would have

absorbed enough water to cause a more or less complete erection of the stem, after which it would be impossible to obtain any further record of any erectile movement. It now occurred to me that the difficulty could be overcome by washing the roots with water at a low temperature, say, at 10° C, and placing them in a vessel of water at 15° C. As this temperature is below the critical point, the rhythmic activity would be arrested, with the resulting arrest of absorption. This surmise proved to be amply justified, and nothing could be more surprising than the fact that the plant previously under drought, with its roots greedy for absorp- tion of water, was unable to absorb even when immersed in water. In fact the record (fig. 21, h) shows that the plant continued to exhibit a drooping movement, as if the roots were buried in a very dry soil. The temperature of the water in the vessel was now allowed to rise ; the drooping movement was arrested, and the reverse erectile movement commenced at 23° C. The critical point for arrest is again found to be at or about 22° C.

Drooping of leaves during frost. — The experiments described above offer a very satisfactory explanation of the drooping of the leaves which is observed during frost, and the recovery when the plant is brought into a warmer atmosphere. The ascent of sap, as we found, becomes arrested below the critical temperature, which is lower in cold climates than in the tropics. The temperature during frost would, in most cases, prove to be below the critical point ; hence the drooping of the leaves is due to the arrest of the ascent of sap. The suctional activity is restored by the higher temperature, and by the renewal of the ascent of sap and the restoration of turgor, the leaves regain their normal condition.

The Potographic method. — I first tried to find out if suction by a cut stem of Impatiens was arrested at a suffi- ciently low temperature. The stem was mounted in the apparatus, the vessel being filled with water at 12° C. ; after this the temperature was allowed to rise gradually. It was found that no suction was recorded at the low temperature ; but as the temperature of water in the vessel rose to i6° C. there was an initiation of suction. The critical point of suction is thus about 15° C. Suction could be renewed or arrested repeatedly by variation of temperature above and below the critical point.

Phenomenon of accommodation.- — While repeating the above experiment, I became aware of the very interesting phenomenon of accommodation by which the plant adjusts itself to changing external conditions. The critical point for the arrest of suction obtained from the first experiment was 15° C. ; repeating it a second time gave 14° C. for the arrest. A third repetition gave 13° C. as the critical point ; this was found to be the lowest obtained with this specimen, as further repetition did not exhibit any variation. The average critical point for the cut stem of Impatiens is thus 14° C, which is also the average critical point for the Desmodium leaflet. This coincidence is certainly very remarkable.

The critical point for Impatiens with roots was next determined. The experiment was commenced with the temperature of water at 14° C, when suction was found to be completely arrested ; it was found to be feebly renewed at 23° C. ; the critical point is therefore below this, i.e., 22° C. A second experiment gave an identical result. This is a remarkable confirmation of the result obtained by the method of Erectile Response which has already been described.

Thus by the independent methods of the Erectile and of the Potographic Response, we arrive at the same value for the critical point for the activity of the root, which is 22° C. The critical point for the cut stem is 14° C, or 8 degrees lower. The root is therefore more sensitive than the stem to the adverse effect of lowering of temperature. It is very remarkable that in certain tropical plants the critical point, 22° C, for the suctional activity of the root should be the same as the critical point for growth. This

may be due to the fact that it is the growing portions of the root which are more actively concerned in the absorp- tion of water ; or it may be that the irritability of the root is, in general, greater than that of the stem, on which account its activity is arrested at a relatively higher temperature. A rhythmic tissue may thus be alternately rendered active and inactive, above and below a critical temperature. Above the critical point, rhythmic activity is exhibited by the pulsation of the Desmodium leaflet and in the move- ment of growth : below the critical point, in the inactive state, these manifestations become arrested. Since varia- tions of temperature above and below the critical point induce a similar alternation of states of activity and in- activity in the movement of the sap, the conclusion is inevitable that the movement is effected by a rhythmic tissue.

We found that a small dose of ether induced an enhance- ment of pulsation in the Desmodium leaflet, and an enhance- ment of the rate of growth. Chloroform gave a preliminary enhancement followed by a decline and arrest (p. 19). The effect of anaesthetics on the ascent of sap is precisely similar, as will be seen in the following experiments. After the attainment of the uniform erectile response, dilute ether was applied at the cut end of a stem of Chrysanthemum ; this is seen to induce a great enhancement of response, which continued for a considerable length of time (fig. 22, a). A similar effect was induced when the anaesthetic was applied to the root (fig. 22, h).

Application of a dilute solution of chloroform causes an extraordinary increase of the rate of ascent as the immediate effect. The record (fig. 23, b) gives us a striking example of the immediate stimulation caused by the appli- cation of chloroform, the activity being enhanced more than fifteen-fold, as seen in the 'jumps' of successive dots inscribed by the lever on the oscillating plate. Continued action of chloroform causes depression and arrest.

The experiments described above prove that the action of anaesthetics on the various rhythmic activities is in every way uniform. A small dose of the anaesthetic enhances the pulsation of Desmodium leaflet, the rate of growth, Ether in Enhancement of the Erectile Response (a) Effect of apphcation at the cut end of the stem. \b) Effect of apphcation at the root. {Chrysanthemum .) and the rate of the ascent of sap. A strong dose, on the other hand, stops pulsation, growth, and the ascent of sap.

It has been pointed out (p. 21) that rhythmic activity is abolished, in the leaflet of Desmodium and in growing organs, by the action of poisons. I now give the results of experiments made in order to ascertain whether or not the ascent of sap is similarly affected. I employed two different methods for the demonstration of the arrest of the ascent of sap by poison, the method of Erectile Response and the method of Exudation. It should, however, be borne in mind that certain poisons are more toxic for a given plant than for others. Moreover,

Fig. 24. Effect of Poison on the Ascent Effect of dilute solution of formaldehyde on the response of a drooping leaf of Chrysanthemum . l', the effect of stronger solution in inducing quick arrest. The normal erectile response of drooping stem of Impatiens ; b, c, and d, the effects of increasing strengths of solutions in plants often exhibit a certain amount of accommodation to poisonous agents. The Method of Erectile Response. — After the attainment of an uniform rate of erectile response in water with a droop- ing leaf of Chrysanthemum, a dilute solution of formalde- hyde was applied at the cut end of the stem. This induced at first an arrest of ascent, followed by a feeble attempt at recovery ; but the arrest soon became permanent. In a second experiment a stronger solution was applied. This caused a quick arrest (fig. 24, l, l').

Another series of experiments was carried out with cut shoots of Impatiens. In iig. 24, a, is given the erectile record of a drooping stem, when the cut end was placed in a vessel of water. The erectile movement is seen to take place with great rapidity. After the commencement of the normal erectile response, i per cent., 1-5 per cent., and' 2 per cent, solutions of formaldehyde were applied to different specimens. The records b, c, and d exhibit the effects of increasing strengths of solution in inducing increasing retardation of ascent, culminating in an arrest.

The Method of Exudation. — In the following experiments I employed specimens of seedlings of Wheat with roots. The exudation of water at the tips of the seedlings of various GraminecB is a visible indication of the activity of the ascent of sap. The experiments to be presently described were carried out with more than 100 different seedlings, and the results obtained were, without a single exception, in perfect agreement with each other. The mode of pro- cedure was as follows : the apparatus has two trenches ; one of these was filled with water and the second with i per cent, solution of poisonous agents like potassium cyanide or sodium arsenite. A dilute solution of poison was used, as the object was to paralyse the plant and thus arrest its activity : too strong a dose would have caused immediate death and wilting of the plant. Each row of seedlings was placed with their roots in water and in the solution of the poison respectively. The specimens were placed under a glass cover, and in the course of a few hours it was found that while drops were being exuded vigorously by the seedlings with their roots in water, not a single drop was found at the tips of the poisoned plants (fig. 25). These experiments were repeated many times with the same result.

The exudation of drops of water is not the only mode of expression of spontaneous activity. This is also exhibited in active growth, and nothing could be more striking than the simultaneous arrest of exudation and of growth under the action of poison in the same seedHngs. The experi- ments described below were carried out on three groups, each group consisting of six seedlings, so selected that for every specimen of a given length in one group there were two of

Note exudation of water-drops and active growth in the former (front row), and the absence of exudation and growth in the latter. the same length in the other two groups. The first group had their roots placed in water ; the second, in dilute solution of potassium cyanide ; the third, in dilute solution of arsenious acid. The following tabular statement gives the results in the three groups, the observations being continued for forty-eight hours.

Table XII. — The Effect of Irrigation with Water anu Poisonous Solutions on Growth and on Exudation Exudation of water was copious and growth active in Group I, but completely arrested in Groups II and III. It will be noted that while the normal specimens became more than doubled in length by growth, the poisoned speci- mens showed practically no growth. These latter drooped and died in the course of a few days. The difference between the normal and poisoned specimens will be seen in the photograph of the apparatus containing two rows of four seedlings each, originally all of the same length. The seedlings with roots in water are seen exhibiting vigorous growth, and with exuded water trickling down the side. The seedlings in the second row are seen to be in a state of arrested growth and with no exudation (fig. 25).

As regards the effect of poison on erectile response of drooping stems, nothing could be more striking than the photographs reproduced below of the effect of formalde- hyde solution in the arrest of ascent of sap in drooping shoots of Chrysanthemum (fig. 26). In a previous illustration (fig. 9) it is shown how the stem with its cut end in water becomes fully erected, with its leaves outspread in a turgid condition, in so short a time as fifteen minutes. In the present case, however, the cut stem in formaldehyde solu- tion persisted in the drooping condition ; so presumably the ascent of sap was completely abolished. The specimen never recovered, but exhibited even greater drooping after

eight hours ; subsequently it died from the effect of the poison and became decomposed. These results give conclusive evidence that poisons affect the ascent of sap just as they do the movements of the Desmodium leaflet and the process of growth ; it may therefore be inferred that, like them, the ascent of sap is dependent upon the activity of living cells. The experiments described above on the effect of poison in the arrest of ascent of sap have an important bearing upon Strasburger's results, already referred to (p. 22). The

Fig. 26. Photographs of Drooping cut shoot of Chrysanthetnuiii placed in solution of Formaldehyde, which caused increased drooping, instead of full erection by ascent of water as in fig- 9 erroneous inferences drawn from them have had the most disastrous effect on the advance of investigation of this subject, as will be seen from the following extract : ' Owing to the researches of Strasburger, all vital theories have re- ceived a severe blow, if indeed they have not been directly disproved. Further, no positive evidence has been advanced in support of these theories, and one accepted them because purely physical explanation appeared to be inadequate.' ^ Now, no evidence could be more direct and convincing in support of the physiological theory than the continued arrest of ascent in a drooping stem with its cut end in a poisonous solution, and the renewal of ascent in a similar

specimen with its cut end in water. The results of a com- plementary experiment described below will be found to be even more convincing. In this, we take two vigorous specimens, a and b, with their cut ends in water. They are in every respect similar to each other, the rate of suction in a being i-i c.c, that in b, i-o c.c. per hour, a and b were then placed in two similar test-tubes, one filled with water and the other with a 10 per cent, solution of formalde- hyde ; a layer of oil was spread over the surface of the two liquids to prevent evaporation. The two test-tubes being previously calibrated (making allowance for the volume of the immersed stem), the rates of subsidence of the liquids will show the rate of suction and the ascent of sap in the two cases. The experiments were carried out inside the laboratory before a window. It was a rainy day, and the variation of temperature during the five hours of the experi- ment was slight. The specimen b, which was as erect and outspread as a, being placed in the poisonous solution, exhibited a collapse of the first pair of leaves in the course of five minutes, similar effects being produced in others in sequence from below upwards. The rise of poison could be followed by the discoloration ; the stem also collapsed, and the plant became a huddled mass of dying tissue (fig. 26A).

The difference in the rate of suction observed in the two cases ofiers the most striking and conclusive proof of the activity of living cells in the ascent of sap. In specimen A the rates of suction and ascent were practically uniform throughout the five hours of the experiment. The suction continued unabated for several days in succession. In con- trast with this is the rapid fall of the rate in B due to gradual rise of poison, which put the successive zones of the living stem out of operation. The normal rate of i -o c.c. fell to 0 -6 c.c. one hour after the action of the poison ; after two hours it was reduced to o -35 c.c, after three hours to o -2 c.c, and after four hours to o-i c.c. Suction was completely abolished after five hours. The difference in the two cases

is exhibited in a striking manner by the two curves given in fig. 26b. It may be thought that the effect of poison in woody trees might be different from that in the herbaceous stem of Chrysanthemum. The next experiment was, therefore, undertaken with two similar shoots of a Mango-tree, each bearing a rosette of eleven leaves. The average rate of suction of the specimen maintained with its cut end in water Fig. 26a. Photographs of two Shoots of Chrysanthemum, originally erect^: the one to the left with cut end in Water, and the other in Formaldehyde Solution

was o "9 c.c. per hour ; the normal rate of the other specimen was I -I c.c. After treatment with the poisonous solution, the rates at successive hours were o -7 c.c, 0-4 c.c, o -21 c.c, and 0 • 12 c.c The suction was practically abolished after five hours. But to return to the consideration of Strasburger's experiments on the ascent of poisonous solutions in the trunks of trees. They do not afford conclusive evidence that the ascent of sap is independent of living cells : for it is only reasonable to attribute the ascent, in his experi-