the method of injury depends on this
resting difference. It is thus hypothetically possible that on the
method of negative variation there might be changes in the responses
caused by variation of the resting difference, and not necessarily due
to the stimulating or depressing effect of the reagent on the tissue.

But by the block method the two contacts are made with uninjured
surfaces, and the effect of reagents on both is similar. Thus no
advantage is given to one contact over the other. The changes now
detected in response are therefore due to no adventitious circumstance,
but to the reagent itself. If further verification be desired as to the
effect of the reagent, we can obtain it by alternate stimulation of the
A and B ends. Both ends will then show the given change. I give below a
record of responses given by two ends of leaf-stalk of turnip,
stimulated alternately in the manner described. The stalk used was
slightly conical, and owing to this difference between the A and B ends
the responses given by one end were slightly different from those given
by the other, though the stimuli were equal. A few drops of 10 per cent.
solution of NaOH was applied to both the ends. It will be seen how
quickly this reagent abolished the response of both ends (fig. 47).

[Illustration: FIG. 47.--ABOLITION OF RESPONSE AT BOTH A AND B ENDS BY
THE ACTION OF NaOH
Stimuli of 30 deg. vibration were applied at intervals of one minute to A
and B alternately. Response was completely abolished twenty-four
minutes after application of NaOH.]

#Effect of dose.#--It is sometimes found that while a reagent acts as a
poison when given in large quantities, it may act as a stimulant in
small doses. Of the two following records fig. 48 shows the slight
stimulating effect of very dilute KOH, and fig. 49 exhibits nearly
complete abolition of response by the action of the same reagent when
given in stronger doses.

[Illustration: FIG. 48.--STIMULATING ACTION OF VERY DILUTE KOH]

So we see that, judged by the final criterion of the effect produced by
anaesthetics and poisons, the plant response fulfils the test of vital
phenomenon. In previous chapters we have found that in the matter of
response by negative variation, of the presence or absence of fatigue,
of the relation between stimulus and response, of modification of
response by high and low temperatures, and even in the matter of
occasional abnormal variations such as positive r