it is alive.... The most general and most delicate sign of life is
then the electrical response.'--Waller, in _Brain_, pp. 3 and 4. Spring
1900.

[6] Kunkel thought the electric disturbance to be due to movement of
water through the tissue. It will be shown that this explanation is
inadequate.

[7] My assistant Mr. J. Bull has rendered me very efficient help in
these experiments.

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 (_AEsculus Hippocastanum_), turnip (_Brassica
Napus_), cauliflower (_Brassica oleracea_), celery (_Apium graveolens_),
Eucharis lily (_Eucharis amazonica_).

#Flower-stalk.#--Arum lily (_Richardia africana_).

#Fruit.#--Egg-plant (_Solanum Melongena_).

#Negative variation.#--Taking the leaf-stalk of turnip we kill an area on
its surface, say B,