CHAPTER III
ELECTRIC RESPONSE IN PLANTS—METHOD OF NEGATIVE VARIATION
- Negative variation
- —Response recorder
- —Photographic recorder
- —Compensator
- —Means of graduating intensity of stimulus
- —Spring-tapper and torsional vibrator
- —Intensity of stimulus dependent on amplitude of vibration
- —Effectiveness of stimulus dependent on rapidity also.
I shall first proceed to show that an electric response is evoked in plants under stimulation.[8]
In experiments for the exhibition of electric response it is preferable to use a non-electrical form of stimulus, for there is then a certainty that the observed response is entirely due to reaction from stimulus, and not, as might be the case with electric stimulus, to mere escape of stimulating current through the tissue. For this reason, the mechanical form of stimulation is the most suitable.
I find that all parts of the living plant give electric response to a greater or less extent. Some, however, give stronger response than others. In favourable cases, we may have an E.M. variation as high as ·1 volt. It must however be remembered that the response, being a function of physiological activity of the plant, is liable to undergo changes at different seasons of the year. Each plant has its particular season of maximum responsiveness. The leaf-stalk of horse-chestnut, for example, exhibits fairly strong response in spring and summer, but on the approach of autumn it undergoes diminution. I give here a list of specimens which will be found to exhibit fairly good response:
Root.—Carrot (Daucus Carota), radish (Raphanus sativus).
Stem.—Geranium (Pelargonium), vine (Vitis vinifera).
Leaf-stalk.—Horse-chestnut (Æsculus Hippocastanum), turnip (Brassica Napus), cauliflower (Brassica oleracea), celery (Apium graveolens), Eucharis lily (Eucharis amazonica).
Flower-stalk.—Arum lily (Richardia africana).