Life Movements in Plants

TABLE 11. —SHOWING THE RELATION BETWEEN HOUR OF THE DAT, TEM¬ From the data given in the table, two curves have been obtained. One of these shows the relation between the hours of the day and temperature ; the other exhibits the relation between the hours of the day and the excitability as gauged by the amplitude of response (Fig. 28). It will Fig, '2$. The continuous curve shows the relation bet ween' t]ie hour of thd* day and temperature. The dotted carve, exhibits relation between the hour of the day and •excitability:.

be seen that there is, broadly speaking, a marked resemb¬ lance between the two curves, which demonstrate the It has been shown (page 59) that owing to physiological inertia, the change of excitability, generally speaking, lags behipd the inducing cause. This fact finds striking illustra¬ tion in the lag„ exhibited by the curve of excitability in reference to the temperature curve. The minimum tem¬ perature was attained at about 4 A.M., but the excitability was not reduced to a minimum til) four hours later and .again' there is a marked fall of temperature after 5 P.M., hut the excitability did not become depressed till two hours later# .

There is 'again the factor of variation of light, the effect of which is not so great as that of temperature. The periods of maximum of light and temperature are, however, not coincident. We may now discuss in greater detail the diurnal varia¬ tion of excitability in Mimosa , taking the typical case, the record of‘ which is given in Fig. 23. The temperature here is seen to remain almost constant, and at an optimum, from 1 to 5 P.M., the condition of light is also favourable. Hence the excitability is found to be constant, and at its maxi¬ mum between these hours. The'temperature begins to fall after 6 P.M., and there is, in addition, the depressing action of gathering darkness. Owing to the time-lag, the fall of excitability does not commence immediately at 6' p.M., but an hour afterwards, and continues till the Tiext morning. During this period we have the cumulative' effect of twelve hours’ darkness and the

increasing depression due to cold, the temperature minimum occurring at 4 a.m. 6a account of the combined' effects of these various factors, and phenomenon of lag, the period of minimum excitability is in general reached about 8 A*.M. In certain other cases this may occur earlier. After the attainment of this minimum, the excitability is gradually^ and con.tinuou.sly increased, under the action of light*. of rising temperature, till the maximum is reached in the' afternoon®

It was said that temperature exerted. a predominant* influence in inducing variation of excitability. We may, therefore, expect that the diurnal period would lie modified in. a certain way according to the season. In winter the night temperature falls very low ; hence the depression of excitability is correspondingly great, and results in 'the complete abolition of excitability. The after-effect of intense cold is seen in the condition of inexcitability persisting for a very long period in . the morning. In summer the prevailing high temperature modifies the diurnal periodicity in a different manner. .When the night is warm, the Ml of excitability is slight. In the day, on the other hand, the temperature may rise above the optimum, bringing about a depression. In such a case the excitability in. the earlier part of the evening may actually be .greater than in the middle of the day. These modifications are shown in a very interesting way in the, following record (Fig., 21), taken at . the end of .April. The temperature of Calcutta at this season often rises above lOQ'F. or 38 rC. fable III. also exhibits, in the .case of the summer specimen.,. : the relation between the . hours of. the. day, temperature, and"' excitability.

An inspection of the record given in Fig. 29 shows that the amplitude of response was enhanced after 4 P.M. Jfhe temperature up to that time was unusually high (38 C.), Fig. 29. Diurnal variation of excitability exhibited by summer specimen. and there was in consequence a depression of excitability. After that hour there was a mitigation of neat, the temperature returning towards the optimum. . Hence we find that the maximum excitability was attained between the hours 4 and 6 P.M. The minimum temperature at night was higher in the present case than that of the experiment carried out in February; in the former the minimum wgs 25 - 5 c C., while in the latter it was 19 - 5 C. On account of this difference the night record in summer shows a fall of’ excitability which is far more gradual than that obtained in spring. The excitability is here not totally abolished in the morning, but reaches a minimum

after 8 A.M. ; th£. sensitiveness Is then gradually enhanced in a staircase manner. TABLE Ill —SHOWING THE ^ELATION BETWEEN HOURS OF THE DAY, TEM¬ The mpto-excitabilifcy of Mimosa was ganged every hoar of the day and night,, by. the amplitude of the response to a testing stimulus. This is effected by means of auto¬ matic devices which. excite the plant periodically by an absolutely constant stimulus, and record the corresponding mechanical response..

From the record thus obtained, it was found that, tbfe excitability of the" plant is not. the same., throughout the day, but. undergoes a .variation characteristically different at different times of the day. In a typical cane in spring the excitability attained its maximum value after 1 p.m. and remained constant for several hours. There was then % continuous fall of excitability, the minimum being reached. ^ at about eight in the morning. The plant at this time was practically insensitive. The moto-excitability was then gradually enhanced in a staircase manner till it again reached a maximum next afternoon.

The effect of sudden darkness was found to induce a transient depression, followed by revival of excitability. The effect of persistent darkness was to induce a depression. Exposure to light from darkness caused a transient depression, followed by an enhancement of excitability. Lowering of temperature induced a depression of excit¬ ability, culminating in an abolition of response. The after¬ effect of excessive cold was a prolonged depression of excitability.

Owing to ' physiological inertia the change of ■. excit¬ ability induced by variation of external condition lags behind the inducing cause. The 'diurnal variation of . excitability is . primarily due to diurnal variation of temperature. The effect is modified in a minor degree by variation of light. The most suitable plant for researches on irritability of plants is Mimosa pudica, which can be obtained In all parts of the world* An impression unfortunately prevails that 'the excitatory reaction of the plant can be obtained only in summer and under favourable circumstances ; this has * militated against its extensive use in physiological' experiments, but the misgiving is without any foundation; for I found no difficulty in demonstrating even the most delicate experiments on Mimosa before the .meeting of the American Association for .the Advancement of. Science held during Christmas, of 1914. The prevailing outside temper¬ ature at the time’ was considerably below the freezing point. With ! .foresight and , care it should not be, at alb difficult to maintain in a; -'hot house a large number of these plants, in a sensitive condition . all the. year, round. .

In order., to, remove the drawback connected with the supply of; sufficient material, I commenced am investigation tov'find, whether ,a.'.detached leaf preparation could- be made :aS' effective .-for ' : the; etudy; of irritability, as the whole plant. Here we have at the central ' end. of the leaf the puivinus, which acts as the contractile organ; the conducting strand in the interior of the petiole, on the other hand, is the vehicle for' transmission of excitation. The problem to be solved is 'the rendering of an isolated petiole-and- pulvinus of Mimosa as efficient for researches on irritability as the nerve-and-muscle preparation of a frog. On the success of this attempt depended the practical opening out of an extended field of physiological investigation which would be unhampered by any scarcity of experimental material.

In connection with this it is well to note the surpris¬ ing difference in vegetative growth as exhibited by plants grown in soil and in pots. A pot-speciinen of Mimosa produces relatively few leaves, but one grown in the open ground is extremely luxuriant. As an instance in point, I may state that for the last five months I have taken from a plant grown in a field about 20 leaves a day for experiment, without making any impression on it. A large box containing soil would be practically as good as the open ground, and the slower rate of growth in a colder climate could be easily made up by planting half a dozen specimens. The protection of the plants from inclemencies of weather can be ensured by means of a glass cover with simple heat-regulation by electric lamps, in place of an expensive green-house.

Returning to the question of the employment of an isolated, leaf,., which I. shall designate as a ^etiole-pulvinus preparation, instead of the entire plant, the first attempts which I made proved unsuccessful. The cut leaf kept in water would sometimes exhibit very . feeble response, at other times all signs of excitability appeared to be totally abolished. It v r as impossible to attempt an investigation on the effect of changing environment on excitability when the , normal sensitiveness itself underwent so capricious a change

These difficulties' were" ultimately overcome from know-, ledge derived through systematic investigation on the relative importance of the different parts of the motor apparatus, /On the immediate and after-effect of' section on the excitability of the leaf, and on the rate of decay of this excitability on isolation from the plant. The experience thus gained enabled me to secure long-continued and uniform sensibility under normal conditions. It was thus possible to study the physiological effects of changing external conditions by observing the responsive variation in the isolated petiole-pulvinus preparation. I propose to deal with the different aspects of the investigation in the following order

The isolated. ■ petiole-pulvinus preparation is made by cutting out a portion of. the stem bearing a single lateral leaf. The four diverging sub-petioles may.also be cut-off. In order to prevent rapid drying' the specimen has to be kept in" water. Preparations made in . this way often appeared to have lost their. sensibility.-.. I was, however, able to-trace this loss to -two.. .different factors; first, to the physiological depression due 1o injury caused by section, and, second, to the sudden increase of turgor brought oh by excessive absorption of water. I shall now proceed to show that the loss of sensibility is not permanent, but is capable of restoration.

In connection with the question of effect of injury, it is to be borne in mind that after each excitation the plant becomes temporarily irresponsive and that the excit¬ ability is .fully restored after the completion ' of protoplas¬ mic recovery. A cut or a section acts as a . very intense stimulus, from the effect of which the recovery is very slow. If the stem be cut very near the leaf, the excita¬ tion of the pulvmus is very intense, and the consequent loss of excitability becomes more or less persistent. But if the stem be cut at a greater distance, ■. the transmitted excitation is less intense, and the cut specimen recovers its excitability within a moderate time. I have also suc¬ ceeded in reducing the .excitatory depression by previously benumbing the tissue by physiological means. The isolated specimen can be made still more compact by cutting off the sub-petioles bearing the leaflets; the preparation now consists .of a short length of stem of about 2 cm. and

an .equally short .length of., primary petiole, the motile . For the restoration of sensitiveness, and to meet work¬ ing conditions, the lower end of the cut stem is mounted on a T-tube,^ with funnel-attachment and exit-tube, as shown in Fig. hd. The other two cut ends—of the stem and of the petiole—may be covered -with moist eioth or may be closed with collodion flexile to prevent rapid evaporation and drying up of the specimen. A slight Wig. SO—The Resonant Recorder, with .petiole-pulvimia preparation. ' .(From

insensitive at the beginning, but • if left undisturbed slowly recovers its excitability. The history of Che depr< sion of excitability after shock of preparation and : gradual restoration is graphically illustrated by a series of The petiole-pulvinus preparation thus made offers all facilities for experiment. . Owing to its small size it can be' easily manipulated ; it can be enclosed in a small chamber and subjected to varying conditions of temperature and to the action of different vapours and gases. Drugs are easily absorbed at the cut .end, and poison and its antidote. can be successively applied through the funnel without any disturbance of the continuity of record. In fact, many experiments which would be impossible with the entire plant are quite practicable with the isolated leaf.

The arrangement for; talcing records of response Is seen in Fig. 30, which is reproduced from a photograph of the actual apparatus. For recording the response and recovery of the leaf under stimulation, I use my Resonant Recorder fully described in the 'Philosophical Transactions’ (1913). Tpie petiole is attached to one arm of the horizontal lever. The writer, made of fine steel wire with a bent tip, is *at right angles to the , lever, and is maintained by electro¬ magnetic means in a state of to-and-fro vibration, say, ten times in a second. The record, consisting of a series of dots,. Is .free from errors arising from friction of continuous contact of the writer with the recording surface. .The suc¬ cessive dots in the record at definite intervals of .a tenth, of ..a second also give the time-relations of the response curve.

On account of its small size, the petiole-pulvinus prepara¬ tion offers great facilities for mounting in different ways suitable for special investigations. Ordinarily, the cut stem with its lower end enclosed in moist cloth is supported below. A very suitable form of stimulus is that of induc¬ tion shock from a secondary coil, the intensity of which is capable of variation in the usual manner by adjusting the distance between the primary and the secondary coils. The

motile pulvinus, P, may be excited directly. For investiga¬ tions on velocity of transmission of excitation, stimulus is applied on the petiole at some distance from the pnlvinus, by means of suitable electrodes. Excitation is now trans¬ mitted along the intervening length of petiole, the/conduct¬ ing power of which will be found appropriately modified under the action of chemical and other agents. In this normal method of mounting, the mors excitable lower half of the pulvinus is below ; excitatory reaction produces the fall of the petiole, gravity helping the movement. The pre¬ paration may, however, be mounted in the inverted position, with the more excitable lower half of the pulvinus facing upwards. The excitatory movement will now be the erection of the petiole, against gravity.

Under natural conditions the stem is fixed, and it is the . petiole which moves under excitation. But a very interest¬ ing case presents itself when the petiole is fixed and the stem free. Here is presented the unusual spectacle of the plant or the stem “wagging” in response to excitation. ' The isolated specimen can be kept alive ter several days immersed in water. The excitability of the pulvinus, how¬ ever, undergoes great depression, or even abolition, by the sudden change of turgor brought on by excessive absorption of water. The plant gradually 0 accommodates itself to the ehangjd condition, and the excitability is restored in a stair- case manner from zero to a .maximum.

' In > studying ' the .action of. a chemical. solution on excit¬ ability, the solution may be applied .through the cut end'or directly on the pulvinus. The sadden variation of-turgor, due to the liquid, always induces a depression, irrespective of the stimulating or the depressing action of the'* drug-. The difficulty may be eliminated by previous long-continued application of water on the pulvinuk and waiting till the attainment of pmiform excitability which generally takes place in the course of about'three hours,' Subsequent appli¬ cation of a chemical solution gives rise to characteristic variation in the response.

Variation of excitability after section : Experiment 28 .— In order to test the history of the change of excitability resulting from the immediate and after-effect of section, I took an intact plant and fixed the upper half of the stem in a clamp. The response of a given leaf was now . taken to the'Stimulus of an induction shock of 0*1 unit intensity, the unit chosen being that which/ causes a bare perception of shock in a human being. The specimen was vigorous and' the response, obtained was found, to be a- maximum. The stem:bearing the leaf was;cut at the moment; marked in. the record, with .a-cross, and. water was applied at the cut end. The effect .of■ section was to cause the maximum fall of the leaf, with subsequent recovery. After this, succes¬ sive responses to uniform stimuli at intervals of 15 minutes show, in (1) of Fig. 31, that a depression of excitability; has been induced owing to the shock caused by section. In course of an hour, however, the excitability had been restor¬ ed almost to its original value before the section. .'This was the case with a vigorous specimen, but with less vigorous ones. a longer period of about three hours Is required for restoration. In certain other cases the response after section exhibits alternate fatigue; that is to say, one response is large and the next feeble, and this alternation

Fig. 31—Variation o! excitability after section. (1) Immediate effect; (2) , variation of excitability in a second specimen daring SO hours: *«) response 4 hours after section; (6) response after 2' hours; (e) after 49 hours. TJp-ime of record represents responsive fall of the leaf, down-line indicates recovery from ■■■ excitation. bours, which is maintained, with very slight deeiine under constant external conditions, for about 24 hours. On the third day the fall of excitability is very rapid, and the sensibility declines to zero ia about 50 hours after isolation

£Fig. 31 (2)]. We may describe the whole cycle of change' ^•follows s by the shock of operation the isolated prepara¬ tion "ta tendered insensitive for nearly an hour, the excitabi¬ lity is,. then gradually restored almost to its normal value before operation. Under constant external conditions, this excitability remains ' fairly constant for about 24 'hours after which depression sets in. The rate of fall ‘of excitabi¬ lity becomes very rapid 40 hours after the operation, being finally abolished after the fiftieth hour. It is probable that in a colder climate the fall of excitability would be much slower. The most important outcome of this inquiry is the ''demonstration of the possibility of 'obtaining ■ per¬ sistent and uniform sensibility in isolated preparations. On account of this, not only is the difficulty of supply of material entirely removed but a very high degree of accuracy secured for the investigation itself.

’ "Experiment 24 .—The determination - of the role played by different parts of the pulvinus in response and recovery is Gf much theoretical importance. Our knowledge on this subject is unfortunately very scanty. The generally accept¬ ed view is that on excitation “ the actual downward curva¬ ture of the pulvinus is partly due to a contraction of the walls of the motor cells consequent upon the decrease of turgor, hut is accentuated by expansion of the insensitive adaxial half of the pulvinus—which was strongly compressed in the unstimulated condition of the organ—and also by the weight of the leaf.”* According 'to Pfeifer, after excitation of the organ, “ the original condition of turgor is gradually reproduced in the lower half of the pulvinus, which ex¬ pands, raising the leaf and producing compression of the

upper half of the pulvinus, which aids in the rapid curva¬ ture of the stimulated pulvinus.”* under stimulus is materially aided (1) by the expansion of the upper half of the pulvinus, which is normally in a state of compression, and (2) by the weight of the leaf. So much for theory. The experimental evidence available regarding the relative importance of the upper and lower halves of the pulvinus is not very conclusive. Lindsay attempted to decide the question by his amputation experi¬ ments. He showed that when the upper half was removed the leaf carried out the response, but rigor set in when the low^r half was amputated, Pfeffer’s experiments on the subject, however, contradicted the above results. He -found that u after the upper half of the pulvinus was carefully removed, no movement was produced by stimu¬ lation, whereas when the lower half is absent a weakened power of movement is. retained.” Pfeifer, however, adds,.

since the operation undoubtedly affects, the irritability, it is impossible to determine from such experiments, the exact part played by the active contraction of the lower half of the pulvinus.”* The cause of uncertainty in this investigation is twofold. First, it arises from the unknown change in irritability consequent on amputation; and, secondly, from absence of any. quantitative standard by which the effect of selective amputation of the pulvinus may be measured. As regards the first, I have been able to reduce the depressing action caused by injury to a minimum by benumbing the tissue before operation, through local application of cold, and also .allowing the shock-effect to disappear after a rest of several hours. As regards the physiological gauge of -efficiency of the motor mechanism, such a measure is afforded by the

relation between a definite testing stimulus and the result¬ ing response with its time-relations, which is secured by my Resonant Recorder with the standardised electrical stimulator. In carrying out this investigation I first took the record of normal response of an intact leaf on a fast moving plate. A second record, with the same stimulus, was taken after the removal of the upper half of the pulvinus, having taken the necessary precautions that have been described. Comparison of the two records (Fig. 32) shows that the only