Part 3 (1/2)
TURGOR-VARIATION UNDER TRANSVERSE TRANSMISSION OF STIMULUS-EFFECT.
_Unilateral photic stimulation: Experiment 104._--A _Mimosa_ plant was taken, and its stem was held vertical by means of a clamp. We apply a stimulus at a point on one side of the stem, and observe the effect of this on the state of turgor at the diametrically opposite side. In my first experiment on the subject of detection of induced change of turgor I employed the stimulus of light. A narrow beam from a small arc lamp was made to fall on the stem, at a point diametrically opposite to the motile leaf, which was to serve as a indicator for induced variation of turgor at the distal side. The leaf was attached to the recording lever, the successive dots in the record being at intervals of ten seconds. Stimulation by light caused a positive or erectile movement within 20 seconds of application. The positive response afforded a conclusive proof of the induction of an increase of turgor at the distal point. When the stimulus is moderate or of short duration, the response remains positive. But with strong or prolonged stimulation, the slower excitatory negative impulse is conducted to the distal point and brings about the sudden fall of the leaf (Fig. 100). In the present case the excitatory impulse reached the motile organ 200 seconds after the initiation of the positive response. The stem was thin, only 2 mm. in diameter. The velocity of excitatory impulse in a transverse direction is thus 001 mm. per second; transverse transmission is, for obvious reasons, a much slower process than longitudinal transmission of excitation; in the _Mimosa_ stem this is about 4 mm. per second.
[Ill.u.s.tration: FIG. 101.--Response of leaf of _Mimosa_ under transverse transmission of electric stimulus. (Compare this with fig. 100.)]
_Unilateral electric stimulation: Experiment 105._--In order to show that the effects described above are not due to any particular mode of stimulation but to stimuli in general, I carried out an additional experiment, the stimulus employed being electrical. Two fine pin-electrodes were p.r.i.c.ked into the stem, opposite to the responding leaf of _Mimosa_; these electrodes were placed vertically one above the other, 5 mm. apart. After a suitable period, allowed for recovery from mechanical irritation, feeble tetanising electric shock was pa.s.sed through the electrodes. The responsive effects at the distal side of the stem is precisely similar to those induced under unilateral photic stimulation; that is to say, the first effect was an erectile movement of the leaf, indicative of an induced enhancement of turgor; the excitatory negative impulse then reached the distal point and caused a sudden fall of the leaf (Fig. 101).
The experiments that have just been described are of much significance.
An organ like the stem of _Mimosa_, since it exhibits no contraction, may appear insensitive to stimulation; but its perception of stimulus is shown by its power of transmitting two characteristic impulses, one of which is the positive, giving rise to an enhancement of turgor, and the other, the true excitatory negative, inducing the opposite reaction or diminution of turgor. Unilateral stimulation gives rise to both these effects in all organs: pulvinated, growing, and non-growing. It was the fortunate circ.u.mstance of the insertion of the motile leaf on one side of the _Mimosa_ stem that enabled us to demonstrate the important facts given above.
The underlying reactions, which give rise to tropic curvature, could have been foretold from the Laws of effects of Direct and Indirect stimulation, established in previous chapters (pp. 136, 216). The resulting curvature is thus brought about by the joint effects of direct stimulation of the proximal, and indirect stimulation of the distal side. We may now recapitulate some of the important facts relating to tropic curvatures:
Indirect stimulation gives rise to dual impulses, positive and negative; of these the positive impulse is practically independent of the conducting power of the tissue; but the transmission of the excitatory negative impulse is dependent on the conducting power. No tissue is a perfect conductor, nor is any a perfect non-conductor of excitation, the difference is a question of degree. In a petiole or a stem the conducting power along the direction of length is considerable, but very feeble in a transverse direction. In a semi-conducting tissue, a feeble stimulus will transmit only the positive impulse; strong or long continued stimulation will transmit both positive and negative impulses, the positive preceding the negative. The transmitted positive gives rise to increase of turgor, expansion, and acceleration of rate of growth; the negative induces the opposite reaction of diminution of turgor, of contraction, and of r.e.t.a.r.dation of rate of growth. Transverse transmission is only a particular instance of transmission in general; the only difference is that the conducting power for _excitation_ is very much less in the transverse than in the longitudinal direction.
Owing to feeble transverse conductivity, the transmitted impulse to the distal side often remains positive; it is only under strong or continued stimulation that the excitatory negative reaches the distal side and neutralises or reverses the previous positive reaction. If the distal is the more excitable side, the reversed response will appear as p.r.o.nounced negative. I give a table which will clearly exhibit the effects of stimulus on the proximal and distal sides of the responding organ.
TABLE XXIV.--SHOWING RESPONSIVE EFFECTS COMMON TO PULVINI AND GROWING ORGANS UNDER UNILATERAL STIMULATION.
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Effect of direct stimulation on
Effect of indirect stimulation
proximal side.
on distal side.
+---------------------------------+--------------------------------+
Diminution of turgor
Increase of turgor.
Galvanometric negativity
Galvanometric positivity.
Contraction and concavity
Expansion and convexity.
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When stimulus is strong or long continued, the
true excitatory effect isconducted to the
distal side, neutralising or reversing the first response.
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The diagram which I have already given (Fig. 98) clearly explains the different tropic effects induced by changing the point of application of stimulus. We may thus have stimulus applied at the responding region itself (Direct Stimulation) or at some distance from it (Indirect Stimulation). The final effect will be modified by the conducting power of the tissue.
DIRECT UNILATERAL STIMULATION.
_Type I._--The tissue has little or no power of transverse conduction: stimulus remains localised, the proximal side undergoes contraction, and the distal side expansion. The result is a positive curvature.
_Type II._--The tissue is transversely conducting. Under strong and long continued stimulation the excitatory impulse reaches the distal side, neutralising or reversing the first effect.
INDIRECT UNILATERAL STIMULATION.
_Type I._--The intervening tissue is an indifferent conductor: transmitted positive impulse induces expansion and convexity on the same side, thus giving rise to negative curvature (_i.e._, away from stimulus).
_Type II._--Intervening tissue is a fairly good conductor: the effect of positive impulse is over-powered by the predominant excitatory negative impulse, the final result is a concavity and positive curvature, with movement towards the stimulus.
The following is a tabular statement of the different effects induced by Direct and Indirect stimulation.
TABLE XXV.--SHOWING DIFFERENCE OF EFFECTS INDUCED BY DIRECT AND INDIRECT STIMULATION.
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Stimulation.
Nature of the tissue.
Final effect.
+---------------+----------------------------+---------------------+
Direct (Feeble)