Part 23 (1/2)
The electric method of investigation described in the last chapter, holds out the possibility of discovering the character of the responsive reaction induced in the root by its displacement from vertical to horizontal position; we shall, moreover, be able to make an electrical exploration of the root-tip and the zone of growth, and thus determine the qualitative changes of response, induced in two regions of the root under the action of gravitational stimulus. For the detection of geotropic action in the shoot, electric contacts were made at two points diametrically opposite to each other. Displacement of the shoot from vertical to horizontal position induced excitatory change of galvanometric negativity at the upper side of the organ, demonstrating the effect of direct stimulation of that side; this excitatory reaction of the upper side finds independent mechanical expression in the induced contraction and concavity of that side of the organ.
I employ a similar electric method for detection of geotropic excitation of the root, responses to geotropic stimulus being taken at the root-tip and also at the zone of growth in which geotropic curvature is effected.
I shall now proceed to give a detailed description of the characteristic electric responses of the tip and of the growing region.
The two diametrically opposite contacts at the tip will be distinguished as _a_ and _b_, the corresponding points higher up in the growing region being A and B. When the root is vertical the electric conditions of the two diametrically opposite points are practically the same. But when the root is rotated in a vertical plane through +90 a geo-electric response will be found to take place; the direction of the responsive current disappears when the root is brought back to the vertical. Rotation through -90 gives rise again to a responsive current, but its direction is found reversed.
GEO-ELECTRIC RESPONSE OF THE ROOT-TIP.
_Experiment 182._--I took the root of the bean plant and made two electric contacts with the diametrically opposite points, _a_ and _b_, of the root-tip at a distance of about 15 mm. from the extreme end.
Owing to the very small size of the tip this is by no means an easy operation. Two platinum points tipped with kaolin paste are very carefully adjusted so as to make good electric contacts at the two opposite sides, without exerting undue pressure. For geotropic stimulation the root has to be laid horizontal, and as the root of the bean plant is somewhat long and limp, displacement from the vertical position is apt to cause a break of the electric contact. This is avoided by supporting the root from the top and also from the sides; for the latter purpose, I use paddings of cotton wool.
[Ill.u.s.tration: FIG. 172.--Diagrammatic representation of geo-electric response of root-tip. The middle figure shows root in vertical position.
Rotation through +90 places _a_ above, which becomes galvanometrically negative. Rotation through -90, places _b_ above and makes it negative.]
After due observance of these precautions the electric response obtained is found to be very definite; when the root is made horizontal, by rotation of the root through +90, the point _a_ is above, and the responsive current is found to flow from _b_ to _a_, _the upper side of the tip_ becoming galvanometrically negative; when the root is brought back to the vertical, the responsive current disappears; rotation through -90 makes the point _b_ occupy the upper position, and the responsive current is from _a_ to _b_; the upper side thus exhibits in every case, an excitatory electric change of galvanometric negativity (Fig. 172). The root-tip thus exhibits the characteristic response to direct stimulation. Experiments carried out with 12 different specimens gave concordant results. The following table gives the absolute values of electro-motive force induced at the tip under geotropic stimulus.
TABLE x.x.xVII.--GEO-ELECTRIC RESPONSE OF THE ROOT TIP (_Vicia Faba_).
+--------------------------+
Specimen.
Induced E. M. F.
+---------+----------------+
1
00005 volt.
2
00011 ”
3
00010 ”
4
00015 ”
+--------------------------+
ELECTRIC RESPONSE IN THE GROWING REGION.
_Experiment 183._--I next undertook an investigation on the electric variation induced in the growing region under the stimulus of gravity.
The experimental difficulties are here greatly reduced, since the available area of contact for galvanometric connection is not so restricted as in the case of the root-tip. The specimen is securely mounted so that the root is vertical. It is next rotated in the vertical plane through +90, so that the point A in the growing region occupied the upper position. The electric response in the growing region took place in a short time and was very distinct. The induced electric change at A was now galvanometric _positivity_ indicative of increase of _turgor and expansion_.
The series of experiments were carried out in the following order. The specimen was first rotated through +90 so that A was above. The responsive electric variation rendered it galvanometrically positive.
The root was rotated back to neutral position when the current disappeared. The root was next rotated through -90 and the responsive current became reversed, the upper B becoming electro-positive (Fig.
173). The alternative rotations through +90 and -90 were carried out six times in succession with consistent results. The interval allowed between one stimulation and the next was determined by the period of complete recovery. Growing fatigue was found to increase this period; at first it was seven minutes, at the second repet.i.tion it was ten minutes, and at the third time it was prolonged to fifteen minutes.
[Ill.u.s.tration: FIG. 173.--Diagrammatic representation of geo-electric response of growing region of root. (_a_) Rotation through -90 makes B, galvanometrically positive. (_b_) Vertical and neutral position. (_c_) Rotation through +90 places A above and renders it galvanometrically positive. (_d_) Additive effect on current of response, root-tip a negative, and growing region A positive.]
I give below the series of electric responses induced by alternate rotations through +90 and -90. The upper position was occupied by A in the odd series, and by B in the even series. In every case the upper side became galvanometrically positive.
TABLE x.x.xVIII.--GEO-ELECTRIC RESPONSE OF ROOT IN THE REGION OF GROWTH.
+---------------------------------------------------------------+
Odd
Galvanometer deflection
Even
Galvanometer deflection
series.
A, positive.
series.
B, positive.
+-------+-----------------------+-------+-----------------------+
1
20 divisions.
2
18 divisions.
3
16 ”
4
18 ”