It is remarkable that physiological reaction of opposite kinds should occur on the upper and lower sides of an organ under the identical stimulus of gravity. The difference of reaction may conceivably be connected with the fact that the vertical lines of gravity enter by the upper, and leave by the lower side of the organ. The statolithic particles rest on the inner tangential walls of the perceptive cells of the upper layer, and on the outer tangential walls of the lower layer. Similar difference of physiological reactions of a polar character are also known in responses of plants under the action of an identical electric current; here with different ionic distributions, contraction takes place at the kathode, and expansion at the anode.
The geo-electric reactions that have been described were obtained under unfavourable conditions of climate and of temperature. But under better conditions the reaction becomes very greatly enhanced, as would appear from the following account of results which I obtained on two separate occasions in the beginning of August. The season had not become quite as unfavourable as towards the end of the month, but the prevailing sultry weather had caused great depression of the geo-electric excitability. On the first occasion referred to, thunderstorm had broken out at night, and it was refreshingly cool in the morning. It was with the utmost surprise that I noted the astonishing violence of the geo-electric response which the plants gave that morning; the maximum response hitherto obtained was about 100 divisions of the galvanometer scale; but on the present occasion the displacement of the plant, from vertical to horizontal position, induced responsive deflection so great that the galvanometer spot of light flew off the scale of 3,000 divisions. I was at first incredulous of the results and wasted the valuable occasion in trying to discover some hidden source of error. Subsequent tests showed that my misgivings were groundless, and that the extraordinary large deflection was really due to geo-electric reaction. On the second favourable occasion, which lasted for three hours (during the cool hours of the morning), I was able to secure a number of important observations. Thus displacement of the flower stalk of Nymphæa through +90° was immediately followed by geo-electric response, the deflection being about 3,000 divisions of the scale. The latent period hardly exceeded a second; the return of the plant to the vertical position was quickly followed by electric recovery which was complete. The above results were obtained with the same specimen time after time without a single failure. The successive responses showed no sign of fatigue. Another remarkable effect was noticed during gradual increase of the angle of inclination. Nothing happened till a critical angle was reached, which was roughly estimated to be about 33°; when this critical angle was exceeded by a single degree, there was a sudden precipitation of geo-electric response. The experiments were repeated time after time with the identical result. It appeared as if some frictional resistance obstructed the displacement of the geotropic particles accumulated at the basal end of the cell, and it was not till the organ had been tilted beyond 33° that this resistance to sliding was overcome.
SUMMARY.
The electric distribution induced in an organ under the stimulus of gravity may be mapped out by means of an exploring Electric Probe.
The induced galvanometric negativity of the upper side of an organ (indicative of excitation) undergoes variation in different layers of the organ. The excitatory reaction attains a maximum value at a definite layer, beyond which there is a decline.
The geo-perceptive layer is experimentally localised by measuring the depth of intrusion of the probe for maximum deflection of galvanometric negativity.
The geo-perceptive layer thus determined is found to be the starch sheath which contains a number of large-sized starch grains.
The power of geo-perception undergoes seasonal variation. It is also lowered by high temperature.
The geo-electric response undergoes decline with growing sub-tonicity of the specimen; such specimens exhibit abnormal positive electric response under the stimulus of prick and feeble curvature under geotropic stimulus. The large-sized starch-grains, normally observed in the endodermis, are found to disappear in specimens which have become geo-electrically insensitive.
The electric response of the lower side of the organ to gravitational stimulus is of opposite sign to that of the upper side. The electric distribution on the lower side exhibits variations in different layers, the maximum positivity occurring at the perceptive layer. In vigorous specimens the excitatory negative electric change on the upper side is greater than the positive electric change on the lower side. Depressed condition of the tissue is attended by a relatively greater decline of the negative in comparison with the positive.