Plant Response as a Means of Physiological Investigation

tion per minute at that point. The rise of temperature in one minute, then, under the experimental conditions described, is through '4° C. The growth-elongation of the specimen, therefore, while the temperature is rising from 3 3 -8° C. to 34-2° C, gives us the rate of growth for the mean temperature of 34° C. This is found in the magnified record to be 10 mm. The absolute value of the rate of growth is thus •01 mm. per minute. In this way we can determine from the curve the rate of growth corresponding to any temperature. It will thus be seen how in the course of an experiment lasting for thirty-five minutes only, we are able to obtain data which give us the various rates of growth through a wide range of temperatures.

This operation can, moreover, be made entirely automatic. The breadth of the circular electrolytic trough may be appropriately varied at different parts of the circle, so that turning the handle through equal arcs raises the temperature of the plant chamber by equal degrees. The handle of the rheostat may then be rotated by the recording drum itself. Hence in the record, equal lengths of the abscissa will represent not only equal times, but also equal rises of temperature. And finally by taking the record photographically, the whole process becomes automatic. From the data furnished by fig. 184 we obtain the following table :

Table showing Rates of Growth at Different Temperatures in Flower of Crinum Lily. The curve shown in fig. 184 exhibits the relation between these various temperatures and their corresponding rates of growth. It is here seen that the rate gradually rises till we approach the optimum point, which in the present case is 35-5° C. After this there is a steep fall, and growth is almost abolished at a maximum temperature at or near 45 ° C. This arrest of growth does not mean arrest of internal activity. I have in the last chapter (fig. 176) explained why, in spite of the persistence of internal activity, there is in this case no resultant growth. It is to be borne in mind that as the curve of growth in fig. 183 is continuous, the rate of growth at

Fig. 184. Curve showing Relation between Temperature and Rate of Growth, as deduced from the Thermo-crescent Curve in fig. 181 points intermediate to those specified may be determined from it. From a large number of experiments which I have carried out on Crinum Lily, I find that the optimum temperature is very constant, not varying by as much as one-tenth of a degree from the mean value of 35*5° C. as the optimum temperature. In the first portion of the curve, as the temperature rises from 300 C. to 35-5° C. the rate of growth is seen to increase, from -004 to -01125 mm. per minute, or to nearly three times its first value. The fall, beyond the optimum, is steeper than this rise. At 3 7° C. the rate of growth has

fallen to "0045 mm. per minute, or .almost the same as at 300 C. Thus, while 5-5° C. of rise of temperature before the optimum enhanced the rate of growth by three times, a further rise of only 1*5° C. beyond that point was sufficient to bring it back to almost the same value as at 300 C. The individual characteristics of each specimen are seen, not by any perceptible variation of the optimum point, but rather by differences in the steepness, during rise or fall, of the curve. With some specimens, for example, the increase of rate of growth during an equal rise of temperature from 300 C. to 35'5° C. is only half of that seen in the figure. The steepness of fall, on the other hand, beyond the optimum may be much greater; that is to say, a rise of i° C. or less above the optimum will sometimes reduce the rate of growth to its value at 300 C.

(3) The Method of Balance. — I shall now describe an extremely delicate method of determining the rate of growth at different temperatures, which is especially suited for the exact determination of the optimum point. A balanced line of record is first obtained by the turning of the balancing wheel of the Crescograph (fig. 168). This regulates the difference of level of the syphon tube, until the spot of light is stationary at the temperature of the room. As the temperature is now raised and the rate of growth increased, the balancing wheel has to be rotated, say to the right, in order to keep the spot of light stationary. The reading of the circular scale at different temperatures thus gives the balanced readings for the corresponding rates of growth at those temperatures. A previous calibration of the value of the circular scale enables us to determine the absolute growthmovements at various temperatures. For the determination of the optimum point, however, this is not necessary. All that has to be done in this case is to keep the spot, which would otherwise drift to the right under a constantly increasing rate of growth, on the point of balance, by the right-handed rotation of the balancing wheel. This must be done as long as the temperature and rate of growth are

ascending towards the optimum. On reaching and passing that point, however, it is found that the spot of light, which has hitherto tended to move to the right, now has its movement suddenly reversed to the left, thus necessitating a corresponding reversal of the balancing rotation. This turning point is extremely sharp and well defined, and enables us to make an accurate determination of the optimum temperature within less than a tenth of a degree. From a previous knowledge that the optimum point lies, say, between 350 C. and 360 C, the rise of temperature from 350 C. to 360 C. within the chamber may be adjusted to take place in five minutes, that is to say a rise of one-twentieth of a degree per fifteen seconds. The second observer, watching the delicate thermometer in the plant chamber, calls out at every twentieth of a degree of rise of temperature. The first observer, at the recording drum, notes the temperature of the turning point. It has been said before that the permanent rate of growth for any given temperature is always established in less than fifteen seconds of reaching it. The possible error, owing to this lag, could not therefore exceed one-twentieth of a degree.

Table showing Circular Readings of Balancing Wheel at Different Temperatures. I give above two sets of readings of the balancing wheel, made during two experiments for the determination of the optimum point, for Crinum Lily and the hypocotyl ot Balsam The balanced reading at 300 C. is taken as zero. The adjustment of the stop-cocks for regulation of outflow was different in the two cases. (4) The method of excitatory response.— The method which I am about to describe — and by which the relative rates of growth at different temperatures are afforded indirectly—is one of much theoretical importance, for it proves what I have already suggested, that growth is a phenomenon of excitatory response. This being so, it would follow that the reason why growth is at its optimum at about 350 C. in the case of most tropical plants, is that the excitability of the tissue is greatest at that temperature. The different excitabilities at different temperatures might further be expected, this being true, to offer an independent indication of the characteristic rate of growth of the tissue at those temperatures.

The excitability of the tissue can be tested, in the case of radial organs, by its longtitudinal contractile response to external stimulus, which, as we have seen, will be represented in growing organs by a retardation of growth, proportionate to the excitability. We must bear in mind, at this point, certain differences between responsive effects in mature and in growing organs. In the former, owing to the increase of internal energy brought about by rise of temperature, the tissue becomes over-turgid and the internal hydrostatic pressure is greatly increased. The contractile action of external stimulus is thus strongly resisted by the tissue, which in this way antagonises the normal extent of response (P- 338). Similarly, a closed india-rubber ball, fully distended with water, will not yield to any great extent when struck. But if we have, instead, a tube through which water is running, the flexible pipe when struck will yield, and cause a proportionate retardation or reversal of current behind. We have a case somewhat analogous in a growing organ. For here the tissue cannot be regarded as closed, since it is constantly elongating. It therefore represents, not a static

condition of rest, but a dynamic condition of equilibrium, and it will offer little effective resistance to excitatory contraction. We shall therefore expect that in growing organs similar stimuli will induce responsive effects varying in proportion to the changes of excitability in the tissue, under different conditions. We saw, in the case of Crinum Lily, that the optimum temperature was near 35° C, and that at this optimum the rate of growth was something like one and a half to three times as great as at 300 C. At 370 C. we saw, further, that the rate of growth was again reduced, and had become equal to, or lower than, that at 300 C. We might therefore expect that, on recording the retardation of growth in response to external stimulus at three definite temperatures, say 300 C, 350 C, and 370 C, we should find it to be greatest at 350 C. being in fact at that point about one and a half to three times as great as at 300 C. The response at 370 C, on the other hand, which is beyond the optimum, would be much less than at 350 C, being equal to, or even less than, that at

Table showing Variation of Excitatory Mechanical Response at Different Temperatures I have made numerous experiments completely bearing out these conclusions. The mode of experimental procedure is as follows : a balanced record is taken at the given temperature, and the growing organ is then subjected to a definite intensity of stimulation, which may consist of tetanic thermal or electrical shocks, lasting for twenty seconds. Records are then made of the resulting contractile retarda-

tion of growth at the different required temperatures. Three responses were taken at each temperature, and were found to be practically the same. Some of these records will be given in the next chapter (fig. 185). I have given the results of four such experiments, carried out on different specimens. The translocation of the optimum point. — We have thus seen how constant is the optimum point in the same species, under normal conditions ; but, since we found that the otherwise constant death-point was liable to be shifted under the disturbance caused by the sudden variation of external conditions (p. 172), so it would appear probable that the optimum point also would be liable to transposition under the influence of similarly disturbing causes. The optimum point of the Crinum Lily has been seen to lie, normally speaking, between 35*4° C. and 35'5° C. After a night of heavy rain and gale, however, I found that the optimum point of a specimen of this Lily had fallen to 34*6° C. Under the action of a poison like copper sulphate, again, administered in such dilution as not to kill, but only to retard growth, I have observed the optimum point to be lowered to 34'5° C. In the case of dilute solution of sugar, however, which induces — as we shall see in the next chapter — an increase of growth-activity, I have found the optimum point to be raised to 36'6° C.

Thus under normal conditions the optimum temperature for each species is extremely definite. But circumstances which increase or decrease the rate of growth abnormally, operate also to transpose the optimum point, in the same manner as the death -point was found to be translocated by external influences. The difficulties usually encountered in the accurate determination of the effect of temperature on growth have been successfully overcome in the case of four distinct methods.

is found that the variation from one to another acts as a stimulus, and induces a transient retardation of growth. But this cause of disturbance is eliminated when the rise of temperature is made gradual and continuous. In this way, by taking a continuous record of growth under uniform rise of temperature, a thermo-crescent curve is obtained, that gives data from which the absolute values of growth at all temperatures may be obtained. From this curve we are also able to obtain an accurate determination of the optimum and maximum points.

The Method of Balance also affords us, by means of a sharply defined turning point, an exact indication of the optimum point. The optimum point is very definite, and under normal conditions is always constant for a given species; but just as the death-point was found liable to be shifted under abnormal external conditions, so the optimum point also is apt to be transposed under similarly disturbing causes. That growth is a phenomenon of excitatory response is demonstrated by the fact that the growth-rate is increased or decreased at different temperatures, in proportion to the excitability of the tissue at the same points, as indicated by its contractile response.

Positive and negative after-effects — Extreme delicacy of the Method of Balance — Detection of absorbed stimulus by negative after-effect — Constancy of sum of direct and indirect after-effects — Latent component almost vanishing above the optimum — Variation of receptivity— Direct and indirect response of plant in sub-tonic condition — Table showing direct and indirect effects at different temperatures— Is the change induced by stimulus always of an explosive chemical character ? — Relation between stimulus and response in different tonic conditions— After-effect— Factors which determine periodic after-effects : (i) Stimulus of light — (2) Temperature — (3) Chemical stimulus— (4) Turgidity— Continuous photographic record of the pulsations of Desmodium — Record of periodic variation of rate of growth — Continuous photographic record of periodic variations of transpiration — Continuous photographic record of the variation of the rate of growth — Annual rings and seasonal periodicity.

By regarding the plant as a machine, as we did in the course of the earlier chapters, we were enabled to understand the possibility of its absorbing, and holding latent, more or less of the incident stimulus (p. 124). The experimental demonstration of this would, however, be difficult, in the case of the ordinary response of motile organs ; for thougn we have seen that external stimulus and the absorbed internal energy are opposite in their responsive effects, yet in the ordinary records of mechanical response it is not easy to discriminate that part of the effect which is due to the latter element ; for while it is true that the presence of internal energy would tend to hasten the recovery, it is still impossible to distinguish with certainty a recovery so hastened from one which is natural. The fact that excess of stimulus is transformed into latent energy is demonstrated, however, by the occurrence of multiple response.

Positive and negative after-effects. — We have seen that when a moderately strong stimulus acts on a responding organ, a short time elapses before the initiation of the response, and this is known as the latent period ; but when response has been initiated, it persists for some time, even on the cessation of the stimulus, and this is known as the aftereffect. I shall, however, for important reasons, which will appear later, further distinguish it as the positive after-effect. By the term positive after-effect, then, is meant the continuation of a response evoked by external stimulus, on the cessation of that stimulus. We may, for example, imagine a heavy elastic spring immersed in a viscous fluid. If this be subjected to a sudden compressional blow, then, after a short latent period, it will begin to undergo compression, and this compressional movement will continue for some time, even on the cessation of the blow that caused it, thus exhibiting a positive after-effect. But a spring compressed in this manner contains some amount of latent or potential energy, on account of which it next begins to expand, exhibiting a movement opposite to the first. This second movement, due to the latent energy, we may distinguish as the negative aftereffect. This negative after-effect, it should further be stated, may sometimes be separated from the direct effect by a considerable interval of time. This may be seen in a viscous wire subjected to a torsional impulse. After the twisting has ceased, some time elapses before the wire is seen to begin the contrary, or negative, movement of untwisting, which is accomplished very slowly, and may even take hours to complete.

Now, it occurred to me that, in the response of growth, it was possible to find a means of detecting whether the external stimulus in the case of living tissue might, or might not, become partially latent, to be similarly manifested later, in the form of the negative after-effect ; for if we take a balanced horizontal record of growth, then the direct effect of external stimulus will be seen in that retardation which is shown in the shifting of the line — here, for convenience of

inspection, represented upwards (fig. 185). On the cessation of external stimulus, if the recovery of the excited region be merely passive, it is evident that this ascending line will gradually return to the horizontal, as in the third record of fig. 185 ; that is to say, the retarded will be exchanged by degrees for the normal rate of growth. But if some portion of the external stimulus be held latent in the tissue, this will go to increase the internal energy of the plant. Now, we have already seen that the effect of augmented internal energy is exhibited in an increase of the rate of growth above the normal, shown in a balanced response-curve by an opposite movement to that of retardation, constituting the negative after-effect. Such a negative after-effect, consisting of an enhanced rate of growth, will persist until the energy thus held latent is exhausted, when the curve will again return to the horizontal. Thus, the up curve will represent the direct effect of external stimulus, and the down curve the acceleration of growth due to absorbed stimulus, or the negative after-effect.

Extreme delicacy of the Method of Balance. — Such transient variations in the rate of growth, occurring as the expression of the absorbed fraction of incident stimulus, would have been incapable of detection by the ordinary auxonometric method of growth-record ; for here, owing to the relatively slight magnification which is possible, it takes nearly half an hour to obtain data from which the normal rate of growth may be inferred. Another half-hour's observation would be necessary before we could infer the occurrence of variation under changed conditions, and it is clear that, during a period relatively so long, the plant may undergo spontaneous changes. The after-effects, however, which we now wish to detect, are found to take place immediately, and to last for a few minutes only, in the case of moderate stimulation. Even with our crescographic arrangement, though the usual magnification is a thousand times, the variation constituting the after-effect is seen only in a slight change of the slope of the curve ; but when the Method of

Balance is employed, the ordinary magnification is enough to show, in a very marked manner, all the phases of these transient variations. The records here reproduced have been in fact reduced to one-third of the originals. The sensitiveness of the arrangement can be very much exalted by observing the balanced line of light with its deviations, through a telescope placed at a distance. In this way, I have been able to detect a variation from the normal rate of growth, of so little as a two millionth part of 1 mm. per second, within so short a period of observation as ten seconds.

Detection of absorbed stimulus by negative aftereffect.— We must now revert to the question of the detection of latent stimulus by an increase in the rate of growth, and we shall first take that simple case in which there is no loss of energy from irreversible effects due to molecular friction. The energy of external stimulus will here find complete expression in doing external and internal work. If the external stimulus remain constant, the sum of these two— that is to say, the direct or immediate, and the indirect effects — will also remain constant ; but if, under the same circumstances, one of these factors, say the direct effect, should for any reason be enhanced, we might then expect that its complement, the indirect effect, would undergo a corresponding diminution ; while, if the direct effect should be small, the indirect effect would show augmentation.

These theoretical considerations are found very strikingly verified in the experiment which I shall now describe. I first took a balanced record of growth in a specimen of Crinum Lily, at 300 C. This was then subjected to thermal shocks for five seconds. The direct response, as will be seen from the first record in fig. 185, which is reduced to one-third of the original, was a retardation of growth represented by 33 divisions. On the cessation of stimulus, however, the rate of growth did not at once return to the normal, but exhibited the effect of absorbed energy by an acceleration shown in the down curve, and represented by 13 divisions, after which it became normal. The same stimulus was now repeated, and

the direct effect was a retardation of 31, which was followed by an augmentation of 14 divisions. Thus the sum of these two effects is practically constant, being in one case 46, and in the other 45, divisions. Constancy of sum of direct effect and indirect aftereffect.— This constancy, however, becomes still more remarkable when the same plant is raised to a temperature of 350 C. and subjected once more to the same stimulation. The direct effect is now shown by a retardation which may be represented as 39, and the indirect by 9, divisions. In the second response of this second series, we have a direct effect of 37, and an indirect effect of 8, divisions. Thus the sum of the first direct and indirect effects is 48, and the sum of the second 45, divisions, the mean of the two at 35° C. being 46*5 divisions, while the mean at 300 C. was 45-5 divisions. We have found, then, not only that the sum of direct and indirect effects at a given temperature is practically constant when stimulus is the same, but also that this sum itself remains approximately constant at different temperatures within the optimum ; and, further, we see that as the excitability is increased in approaching the optimum, the direct effect also increases at the expense of the indirect. In other words, when the tissue is at its optimum tonic condition, its capacity for the absorption of stimulus being already fully satisfied, the external stimulus tends to be immediately expended, in direct response, allowing relatively little to become latent.

Fig. 185. Series of Responses of Growing Organ of Crinum Lily, taken under Balanced Conditions at Three Different Temperatures On comparing these records it will be seen that the direct effect increases up to the optimum, and that the indirect effect of accelerated growth decreases. Beyond the optimum, at 370 C, there is no latent component, as shown by recovery from direct effect to normal rate of growth. Latent component almost vanishing above the optimum.— As an extreme instance of this, we may take the response at a temperature beyond the optimum, say at 370 C. Here, by its environmental conditions, the plant is already supplied with an excess of energy. And besides this, there is the fact which we have already noticed, that its general excitability is diminished, so as to be equal to, or less than, that at 300 C. At these two temperatures, then — of 300 C. and 370 C. — we have two conditions of excitability more or less the same, but with a different history.

Below the optimum there is an unsatisfied capacity for absorption of stimulus, whereas above it this capacity has been fully met. It would therefore appear that the power of the tissue to hold stimulus latent is diminished progressively up to the optimum, till beyond that point it practically disappears. I obtained a remarkable confirmation of this inference in the course of my experiments. In the experiment just described, for example, the direct response was a retardation of twenty-four divisions, and there was no indirect effect, showing that little or no stimulus had become latent (fig. 185).

Variation of receptivity. — In the experiments described, the sum of the direct and the indirect effects, up to the optimum, had been found to be approximately constant, that is to say, a total response of about forty-six divisions. At 370 C, however, we see the total response reduced to twenty-four divisions without any latent component. And since the total response measures for us the amount of stimulus taken up by the tissue, it would appear that at 370 C. not only is the power to hold stimulus latent lost, but also that the general receptivity of the tissue is very much reduced. It is thus seen that the condition of a tissue modifies its receptive power ; hence it is possible for different parts of the same organ — say, for instance, the tip and the growing region — being in different conditions, to possess different receptivities.

Direct and indirect response of plant in sub-tonic condition. — Turning from this case of excess of energy to that in which it is below par, the plant being in a sub-tonic condition of arrested growth, we find that external stimulus gives rise to little expression in direct response. External stimulus is found under such circumstances mainly to increase the store of internal energy, in consequence of which we obtain the indirect response of renewed growth. In such a condition, the plant has a great capacity for the absorption of external energy. After growth has commenced, the energy of incident stimulus finds bifurcated expression, by inducing direct or immediate response, and by the indirect or negative after-effect, of enhanced rate of growth (p. 434). The sum total of these two — external stimulus being the same — remains approximately constant, till the optimum tonic condition is reached. The direct and indirect effects are thus, up to this point, complementary to each other. After passing this point, however, when the plant is possessed of excess of energy, its power of absorbing stimulus appears to undergo diminution. The direct effect of stimulus is then found reduced, and there is no negative after-effect, due to the absorbed component of the external stimulus.

From these considerations we are enabled to understand the curious growth-response that was observed in varying the temperature of the plant from 340 C. to 350 C. (fig. 182, p. 445). The change of temperature was in that case accomplished, as will be remembered, by changing the intensity of the electrical heating current. The change from 3 40 C. to 350 C. thus produced was not, however, brought about at once, but took place in the course of a period of three minutes. We had consequently the stimulating effect of variation of temperature bringing about contraction, followed by the accelerated rate of growth which constituted the negative aftereffect of that stimulus, plus the accelerated rate of growth due to rising temperature. That the first of these two factors played a considerable part in this acceleration, is seen from the fact that the permanent increase in the rate of growth characteristic of the higher temperature of 350 C. is smaller than the