Life Movements in Plants Part 21

_Experiment 173._--The mechanical response with its drawbacks is thus incapable of giving an accurate value of the latent period. The electrical method of investigation labours under no such disadvantage, since the excitation is here detected even in the absence of movement.

The perception of stimulus will thus be followed by response without undue delay. I shall in this connection give a record of electric response of the quickly reacting petiole of _Tropaeolum_, when the angle of inclination is increased from zero to 90. The responsive movement of the galvanometer spot of light was initiated in less than 5 seconds and the maximum deflection was reached in the course of 90 seconds. The angle was next reduced to zero, and the deflection practically disappeared in the further course of a minute and a half (Fig. 168).

There was a small "excitation remainder". But with vigorous specimens the recovery is complete.

[Ill.u.s.tration: FIG. 169.--Geo-electric response of the scape of _Uriclis_.]

The latent period of quickly reacting petiole of _Tropaeolum_ is thus about 5 seconds, a value which is more consonant with the idea of particles inducing excitation by their fall through an exceedingly short distance. In very sluggish organs latent period may be as long as a minute (Fig. 169), which is considerably shorter than an hour, the generally accepted value. Further even in the electric response, the latent period will be delayed beyond the period of perception. For this perception takes place in some unknown sensitive layer in the interior of the tissue, while electric contact is made with the epidermis outside. It is obvious that certain time must elapse before the excitation, initiated at the sensitive layer, should reach the epidermis. Under ideal conditions of experiment which will be described in a subsequent chapter, the latent period for geotropic excitation, I find, to be sometimes as short as a second.

PHYSIOLOGICAL CHARACTER OF GEO-ELECTRIC RESPONSE.

The intensity of the electro-motive variation is found to depend on the physiological vigour of the specimen. The _Tropaeolum_ plant, used for most of the above experiments, are at the best condition of growth in Calcutta in February; after this the plants begin to decline in March and die off by the end of April.

_Experiment 174._--In February the intensity of electric response was nearly double of that in March; it was only in March that I made quant.i.tative determination of the induced electro-motive force between the upper and lower contacts on rotation of the specimen from zero to 90. The E. M. F. was determined by the potentiometer method. I give below the following typical values obtained with two different specimens:--

Specimen Induced E. M. F.

(1) 12 millivolts.

(2) 15 "

In the most favourable season the induced electro-motive force is likely to exceed the above value very considerably.

_Effect of Age._--While a young petiole gave the above value, an old specimen from the same plant exhibited no response. The plants were in a dying condition in April and all indications of electrical reaction were found abolished. The physiological character of the response was also demonstrated by first obtaining the normal electric response in a vigorous specimen; after death, by immersion in boiling water, the plant gave no electric response to geotropic stimulus.

EFFECT OF DIFFERENTIAL EXCITABILITY OF THE ORGAN.

I have hitherto described the geo-electric effect of radial and isotropic organs. The induced E. M. F. at 90 was found practically the same whether A was above and B below, and _vice versa_. In the mechanical response of the pulvinus of _Mimosa_, the geotropic excitability was, however, found to be greater in the lower half than in the upper (p. 440). I wished to investigate the question of differential geotropic excitability anew, by means of electric response.

_Experiment 175._--Electric connections with the galvanometer were made with the upper and lower halves of the pulvinus, the organ being placed in the vertical or neutral position. The angle of inclination was then increased to 90 in the positive and negative directions alternately.

TABLE x.x.xIV.--DIFFERENCE OF GEO-ELECTRIC RESPONSE OF UPPER AND LOWER HALVES OF THE PULVINUS OF _Mimosa_.

+---------------------------------------------------------+ Specimen. Position of particular Induced E. M. F. half of pulvinus. +-----------+--------------------------+------------------+ (1) { Upper half above 23 millivolts. { Lower half above 30 " (2) { Upper half above 16 " { Lower half above 29 " +---------------------------------------------------------+

In the former case the upper half of the pulvinus occupied the up-position; in the second case the up-position was occupied by the lower half of the pulvinus. In both cases strong electric responses were obtained, the upper point of contact being always galvanometrically negative. There was, however, a difference between the two responses, the excitatory electro-motive variation was invariably greater when the lower half of the organ occupied the favourable up-position. This will be seen from the results of two typical experiments in table given above.

The electrical mode of investigation thus leads to confirm the result obtained with mechanical method that the lower half of the pulvinus of _Mimosa_ is geotropically more excitable than the upper half.

RELATION BETWEEN ANGLE OF INCLINATION AND GEOTROPIC EFFECT.

In the Method of Axial Rotation, the condition of the experiment is ideally perfect; in the neutral position the sides A and B are both parallel to the vertical lines of gravity, and are little affected by geotropic reaction. As the specimen is rotated on its long axis the vertical component of the force of gravity increases with the angle of inclination. The hypothetical statolithic particles will become displaced all along the cell, and the vertical pressure exerted by them will also increase with the angle.

The geo-electric response will then afford us a measure of the intensity of excitation induced at various angles of inclination. The mechanical response on account of its inherent defects does not afford us the true relation between the angle of inclination and intensity of geotropic reaction. But the electric method of inquiry is free from the defects of the mechanical method.

_Experiment 176._--The specimen was rotated so that the angle of rotation was 45, and the maximum electric response observed. The angle was next increased to 90 and the reading for the enhanced response taken. The ratio of the geo-electric response at 90 and 45, thus affords us a measure of the effective stimulations at the two angles.

I give below a table which gives results obtained with 24 different specimens.

TABLE x.x.xV.--RELATION BETWEEN ANGLE OF INCLINATION AND GEOTROPIC EFFECT.

+-------------------------------------------------------------+ No. of specimen. Galvanometric deflection. Ratio b/a. +---------------------------------+ (_a_) at 45 (_b_) at 90 +----------------+---------------+-----------------+----------+ 1 70 divisions 110 divisions 15 2 30 " 45 " 15 3 90 " 126 " 14 4 70 " 100 " 14 5 21 " 33 " 16 6 30 " 50 " 16 7 12 " 20 " 16 8 14 " 20 " 14 9 10 " 16 " 16 10 45 " 75 " 15 11 25 " 40 " 16 12 14 " 20 " 14 13 13 " 20 " 15 14 30 " 50 " 15 15 38 " 54 " 14 16 50 " 75 " 15 17 55 " 90 " 15 18 13 " 20 " 15 19 17 " 25 " 14 20 80 " 130 " 15 21 15 " 22 " 14 22 45 " 75 " 15 23 135 " 220 " 16 24 55 " 93 " 15 +-------------------------------------------------------------+ Mean ratio = 149 +-------------------------------------------------------------+

The mean ratio 149 may thus be regarded as the relative geotropic effects at 90 and 45; this is practically the same as Sin 90/Sin 45 = 14. Hence we arrive at the following law:

_The intensity on geotropic action varies as the sine of the directive angle._

METHOD OF VERTICAL ROTATION.

I have hitherto described results obtained with the Method of Axial Rotation; I shall now take up the second method, that of Vertical Rotation, diagrammatic representation of which is given in figure 166V.

The specimen is held vertical and two electrical contacts, A and B, made with the two lateral sides; it is then rotated round a horizontal axis perpendicular to the length of the specimen. Rotation may be carried in a right-handed direction with increasing angle with the vertical. The point A is thus subjected to enhanced geotropic stimulation and exhibits increasing electric change of galvanometric negativity; continuous decrease of angle of inclination to zero by rotation in the reverse direction causes a disappearance of the induced electric change. The rotation is next continued in the negative direction by which the point B is increasingly subjected to geotropic action. B is now found to exhibit excitatory reaction, the current of response having undergone a reversal. Rotation to the right and left will be distinguished by plus and minus signs.

ELECTRIC RESPONSE THROUGH AN ENTIRE CYCLE.

_Experiment 177._--When the specimen is vigorous, characteristic response with its changing sign may be obtained through an entire cycle from 0 to +45 to +90; then back to 45 to 0 to -45 to -90. With less vigorous specimens the responses becomes enfeebled under fatigue. I give below the results of a typical experiment carried out with a vigorous specimen, the response being distinguished as - when A is above, and + when A is below, the inversion bringing about a reversal direction of the responsive current.

+------------------------------------------------------+ Angle of inclination +45 +90 +45 0 -45 -90 -------------------------+----+----+----+---+----+---- Galvanometer deflection -19 -35 -18 0 +14 +25 +------------------------------------------------------+

RELATION BETWEEN ANGLE OF VERTICAL ROTATION AND INTENSITY OF GEOTROPIC REACTION.

The relation between the angle of inclination and the resulting geotropic action has already been determined by the Method of Axial Rotation. The ratio between the geotropic effects at 90 and 45 was thus found to be 149, which is nearly the same as Sin 90/Sin 45. I was next desirous of determining the relative excitations at the two angles by the Method of Vertical Rotation. It is necessary here to refer to certain differences of condition in the two methods. In the Axial Method, the hypothetical statoliths are distributed uniformly through the length of the cell, and rotation round the long axis causes displacement of the statoliths, the resulting pressure thus increasing with the sine of the angle of inclination. But in the case of vertical rotation through 45 to the right, the statoliths originally at the base of the cell acc.u.mulate to the right hand corner of the cell; a portion of the basal side of the cell is thus subjected to pressure. When the angle is increased to 90 the statoliths pa.s.s along the whole length including the basal and apical sides of the cell; but the excitability of the apical half may prove to be greater than that of the basal half.

Hence excitatory geotropic effect is not likely to vary strictly as in sine of angle of inclination.

Whatever the reason may be, I find as a result of experiments with 12 different specimens that the mean ratio of the effects at 90 and 45, obtained by the Method of Vertical Rotation, is 18:1 which is greater than 149:1 obtained by the Method of Axial Rotation, this latter value being practically the same as Sin 90/Sin 45.

SUMMARY.

It is shown that the state of excitation under direct stimulus is exhibited by an electrical change of galvanometric negativity; the effect of indirect stimulus induces, on the other hand, an electrical change of galvanometric positivity. The negative electric change corresponds to contraction and diminution of turgor; the positive electric change indicates, on the other hand, an expansion and increase of turgor.

The electric response to geotropic stimulus is studied by the two methods of Axial and Vertical Rotation. The upper side of a horizontally laid shoot is found to undergo an excitatory change of galvanometric negativity.

In quick reacting organs the latent period of geo-electric response is about 5 seconds, and the maximum excitation is induced in the course of 2 minutes.

The geo-electric response is due to physiological reaction. The intensity of response declines with age and is abolished at the death of the plant.

Under symmetrical conditions, the intensity of geotropic reaction is found proportional to the sine of the angle of inclination.

Electric investigation shows that the lower half of the pulvinus of _Mimosa_ is geotropically more excitable than the upper half.

XLI.--THE MECHANICAL AND ELECTRICAL RESPONSE OF ROOT TO VARIOUS STIMULI

_By_

SIR J. C. BOSE.