As regards the interpretation of the record of geotropic movement, it should be borne in mind that after the perception of stimulus a certain time must elapse before the induced growth-variation will result in curvature. There is again another factor which causes delay in the exhibition of true geotropic movement; for the up-movement of stems, in response to the stimulus of gravity, has to overcome the opposite down movement, caused by weight, before it becomes at all perceptible. On account of the bending due to weight there is a greater tension on the upper side, which as we have seen (p. 193), enhances the rate of growth, and thus tends to make that side convex. The exhibition of geotropic response by induced contraction of the excited upper side thus becomes greatly delayed. In these circ.u.mstances I tried to discover specimens in which the geotropic action would be quick, and in which the r.e.t.a.r.ding effect of weight could be considerably reduced.
_Geotropic response of flower stalk of Tuberose: Experiment 164._--For this I took a short length of flower stalk of tuberose in a state of active growth; the flower head itself was cut off in order to remove unnecessary weight. After a suitable period of rest for recovery from the shock of operation, the specimen was placed in a horizontal position, and its record taken. The successive dots in the curve are at intervals of 20 seconds, and the geotropic up-movement is seen to be initiated (Fig. 159) after the tenth dot, the latent period being thus 3 minutes and 20 seconds, the greater part of which was spent in overcoming the down-movement caused by the weight of the organ.
[Ill.u.s.tration: FIG. 159.--Geotropic response of flower stalk of tube rose: preliminary down-movement is due to weight.]
[Ill.u.s.tration: FIG. 160.--Geotropic response of petiole of _Tropaeolum_: latent period shorter than 20 seconds.]
_Geotropic response of petiole of_ Tropaeolum: _Experiment 165._--I expected to obtain still shorter latent period by choosing thinner specimens with less weight. I therefore took a cut specimen of the petiole of _Tropaeolum_, and held it at one end. The lamina was also cut off in order to reduce the considerable leverage exerted by it. The response did not now exhibit any preliminary down-movement, and the geotropic up-movement was commenced within a few seconds after placing the petiole in a horizontal position (Fig. 160). The successive dots in the record are at intervals of 20 seconds and the second dot already exhibited an up-movement; the latent period is therefore shorter than 20 seconds. It will thus be seen that the latent period in this case is of the same order as the hypothetical period of migration of the statoliths.
I may state here that I have been successful in devising an electric method for the determination of the latent period, in which the disturbing effect of the weight of the organ is completely eliminated.
Applying this perfect method, I found that the latent period was in some cases as short as a second. The experiment will be found fully described in a later chapter.
THE COMPLETE GEOTROPIC CURVE.
The characteristics of the geotropic curve are similar to those of other tropic curves. That is to say the susceptibility for excitation is at first feeble; it then increases at a rapid rate; in the third stage the rate becomes uniform; and finally the curvature attains a maximum value and the organ attains a state of geotropic equilibrium (cf. page 353).
The period of completion of the curve varies in different specimens from a few to many hours.
_Experiment 166._--The following record was obtained with a bud of _Crinum_, the successive dots being at intervals of 10 minutes. After overcoming the effect of weight (which took an hour), the curve rose at first slowly, then rapidly. The period of uniformity of movement is seen to be attained after three hours and continued for nearly 90 minutes.
The final equilibrium was reached after a period of 8 hours (Fig. 161).
[Ill.u.s.tration: FIG. 161.--The Complete Geotropic curve (_Crinum_).]
For studying the effect of an external agent on geotropic action, the period of uniform movement is the most suitable. Acceleration of the normal rate (with enhanced steepness of curve) indicates that the external agent acts with geotropism in a concordant manner; depression of the rate with resulting flattening of the curve shows, on the other hand, the antagonistic effect of the outside agent.
DETERMINATION OF EFFECTIVE DIRECTION OF STIMULUS.
The experiments which have been described show that it is the upper side (on which the vertical lines of gravity impinge) that undergoes excitation. The vertical lines of gravity must therefore be the direction of incident stimulus. This conclusion is supported by results of three independent lines of inquiry: (1) the algebraical summation of effect with that of a different stimulus whose direction is known, (2) the relation between the directive angle and geotropic reaction, and (3) the torsional response under geotropic stimulus.
EFFECT OF ALGEBRAICAL SUMMATION.
_Experiment 167._--A flower bud of _Crinum_ is laid horizontally, and record taken of its geotropic movement. On application of light on the upper side at L, the responsive movement is enhanced, proving that gravity and light are inducing similar effects. On the cessation of light, the original rate of geotropic movement is restored (Fig. 163).
Application of light of increasing intensity from below induces, on the other hand, a diminution, neutralisation, or reversal of geotropic movement.
Light acting vertically from above induces a concavity of the excited upper side in consequence of which the organ moves, as it were, to meet the stimulus. The geotropic response is precisely similar. In figure 162 the arrow represents the direction of stimulus which may be rays of light or vertical lines of gravity.
[Ill.u.s.tration: FIG. 162.--Stimulus of light or gravity, represented by arrow, induces up curvature as seen in dotted figure.]
[Ill.u.s.tration: FIG. 163.--The effect of super-imposition of photic stimulus. The first, third, and fifth parts of the curve, give normal record under geotropic stimulus. Rate of up-movement enhanced under light L.]
a.n.a.lOGY BETWEEN THE EFFECTS OF STIMULUS OF LIGHT AND OF GRAVITY.
In geotropic curvature we may for all practical purposes regard the direction of stimulus as coinciding with the vertical lines of gravity.
The a.n.a.logy between the effects of light and of gravity is very close[33]; in both the induced curvature is such that the organ moves so as to meet the stimulus. This will be made still more evident in the investigations on torsional geotropic response described in a subsequent chapter. The tropic curve under geotropic stimulus is similar to that under photic stimulus. The tropic reaction, both under the stimulus of light and of gravity, increases similarly with the "directive" angle.
These real a.n.a.logies are unfortunately obscured by the use of arbitrary terminology used in description of the geotropic curvature of the shoot.
In figure 163 records are given of the effects of vertical light and of vertical stimulus of gravity, on the responses of the horizontally laid bud of _Crinum_. In both, the upper side undergoes contraction and the movement of response carries the organ upwards so as to place it parallel to the incident stimulus. Though the reactions are similar in the two cases, yet the effect of light is termed _positive_ phototropism, that of gravity _negative_ geotropism. I would draw the attention of plant-physiologists to the anomalous character of the existing nomenclature. Geotropism of the shoot should, for reasons given above, be termed _positive_ instead of _negative_, and it is unfortunate that long usage has given currency to terms which are misleading, and which certainly has the effect of obscuring a.n.a.logous phenomena. Until the existing terminology is revised, it would perhaps be advisable to distinguish the geotropism of the shoot as _Zenithotropism_ and of the root as _Nadirotropism_.
[33] Exception to this will be found in page 336, where explanation is offered for the difference.
RELATION BETWEEN THE DIRECTIVE ANGLE AND GEOTROPIC REACTION.
When the main axis of the shoot is held vertical, the angle made by the surface of the organ with lines of force of gravity is zero, and there is no geotropic effect. The geotropic reaction increases with the directive angle; theoretically the geotropic effect should vary as the sine of the angle. I shall in the next chapter describe the very accurate electrical method, which I have been able to devise for determination of relative intensities of geotropic action at various angles. Under perfect conditions of symmetry, the intensity of effect is found to vary as the sine of the directive angle. This quant.i.tative relation fully demonstrates that geotropic stimulus acts in a definite direction which coincides with the vertical lines of gravity.
The conditions of perfect symmetry for study of geotropic action at various angles will be fully described in the next chapter. In the ordinary method of experimentation with mechanical response the organ is rotated in a vertical plane. The geotropic movement is found increased as the directive angle is increased from zero to 90.
DIFFERENTIAL GEOTROPIC EXCITABILITY.
It has been shown that geotropic stimulus acts more effectively on the upper side of the organ. The intensity of geotropic reaction is, moreover, modified by the excitability of the responding tissue. It is easy to demonstrate this by application of depressing agents on the more effective side of the organ. The rate of geotropic up-movement will be found reduced, or even abolished by the local application of cold, anaesthetics like chloroform, and of poisonous pota.s.sium cyanide solution.
The different sides of a dorsiventral organ are unequally excitable to different forms of stimuli. I have already shown (p. 85) that the lower side of the pulvinus of _Mimosa_, is about 80 times more excitable to electric stimulus than the upper side. Since the effect of geotropic stimulus is similar to that of other forms of stimuli, the lower side of the pulvinus should prove to be geotropically more excitable than the upper side. This I have been able to demonstrate by different methods of investigation which will be described in the following chapters.
Under ordinary circ.u.mstances, the upper half of the pulvinus is, on account of its favourable position, more effectively stimulated by geotropic stimulus; in consequence of this the leaf a.s.sume a more or less horizontal position of "dia-geotropic" equilibrium. But when the plant is inverted the more excitable lower half of the organ now occupies the favourable position for geotropic excitation. The leaf now erects itself till it becomes almost parallel to the stem. The response of the same pulvinus which was formerly "dia-geotropic" now becomes "negatively geotropic"; but an identical organ cannot be supposed to possess two different specific sensibilities. The normal horizontal position a.s.sumed by the leaf is, therefore, due to differential geotropic excitabilities of the two sides of a dorsiventral organ.
I have explained (p. 401) that when the pulvinus of _Mimosa_ is subjected to lateral stimulation of any kind, it undergoes a torsion, in virtue of which the less excitable half of the organ is made to face the stimulus. Experiments will be described in a subsequent chapter which show that geotropic stimulus also induces similar torsional response.
The results obtained from this method of enquiry give independent proof: (1) that the lower half of the pulvinus is geotropically the more excitable, and (2) that the direction of incident geotropic stimulus is the vertical line of gravity which impinges on the upper surface of the organ.
SUMMARY.
The stimulus of gravity is shown to induce an excitatory reaction which is similar to that induced by other forms of stimulation. The direct effect of geotropic stimulus is an incipient contraction and r.e.t.a.r.dation of rate of growth.
The upper side of a horizontally laid shoot is more effectively stimulated than the lower side, the excited upper side becoming concave.
Electrical investigation also shows that it is the upper side that undergoes direct stimulation.
Tropic reactions are said to be positive, when the directly stimulated side undergoes contraction with the result that the organ moves to meet the stimulus. According to this test, the geotropic response of the stem is _positive_.
The geotropic response is delayed by the bending down of the horizontally laid shoot. Reduction of weight is found to shorten the latent period; in the case of the petiole of _Tropaeolum_ this is shorter than 20 seconds. The latent period of geotropic response is found to be of the same order as the "migration period" of the hypothetical statoliths.
The complete geotropic curve shows characteristics which are similar to tropic curves in general.
In a dorsiventral organ the geotropic excitabilities of the upper and lower sides are different. In the pulvinus of _Mimosa_ the geotropic excitability of the lower half is greater than that of the upper half.
The differential excitabilities of a dorsiventral organ modifies its position of geotropic equilibrium.
XL.--GEO-ELECTRIC RESPONSE OF SHOOT
_By_
SIR J. C. BOSE,
_a.s.sisted by_