lance each other,
and the galvanometer will show no current. On taking a cylindrical root
of radish I have sometimes succeeded in finding a neutral point, which,
being disturbed, did not give rise to any resultant current. But
disturbing a point to the right or to the left gave rise to opposite
currents.

It is, however, difficult to obtain an absolutely cylindrical specimen,
as it always tapers in one direction. The conductivity towards the tip
of the root is not exactly the same as that in the ascending direction.
It is therefore difficult to fix an absolutely neutral point, but a
point may be found which approaches this very nearly, and on stimulating
the stalk near this, a very interesting diphasic variation has been
observed. In a specimen of cauliflower-stalk, (1) stimulus was applied
very much nearer A than B (the feeble disturbance reaching B was
negligible). The resulting response was upward and the recovery took
place in about sixty seconds.

[Illustration: FIG. 26.--DIPHASIC VARIATION]

(2) Stimulus was next applied near B. The resulting response was now
downward (fig. 26, _b_).

(3) The stimulus was now applied near the approximately neutral point N.
In this case, owing to a slight difference in the rates of propagation
in the two directions, a very interesting diphasic variation was
produced (fig. 26, _c_). From the record it will be seen that the
disturbance arrived earlier at A than at B. This produced an upward
response. But during the subsidence of the disturbance at A, the wave
reached B. The effect of this was to produce a current in the opposite
direction. This apparently hastened the recovery of A (from 60 seconds
to 12 seconds). The excitation of A now disappeared, and the second
phase of response, that due to excitation of B, was fully displayed.

#Positive after-effect.#--If we regard the response due to excitation of A
as negative, the later effect on B would appear as a subsequent positive
variation.

In the response of nerve, for example, where contacts are made at two
surfaces, injured and uninjured, there is sometimes observed, first a
negative variation, and then a positive after-effect. This may sometimes
at least be due to the proximal uninjured contact first giving the usual
negative variation, and the more distant contact of injury giving rise,
later, to the opposite, that is to say, apparently positive, response.
There is always a chance of an after-effect due to this cause, unl