ve sometimes found similar reversal of response when the
plant is subjected to the abnormal conditions of excessively high or low
temperature.

#Radial E.M. variation.#--We have seen that a current of response flows in
the plant from the relatively more to the relatively less excited. A
theoretically important experiment is the following: A thick stem of
plant stalk was taken and a hole bored so as to make one contact with
the interior of the tissue, the other being on the surface. After a
while the current of injury was found to disappear. On exciting the stem
by taps or torsional vibration, a responsive current was observed which
flowed inwards from the more disturbed outer surface to the shielded
core inside (fig. 29). Hence it is seen that when a wave of disturbance
is propagated along the plant, there is a concomitant wave of radial
E.M. variation. The swaying of a tree by the wind would thus appear to
give rise to a radial E.M.F.

[Illustration: FIG. 29.--RADIAL E.M. VARIATION]

FOOTNOTES:

[12] For general purposes it is immaterial whether the responses are
recorded up or down. For convenience of inspection they are in general
recorded _up_. But in cases where it is necessary to discriminate the
sign of response, positive response will be recorded up, and negative
down.

CHAPTER VII

PLANT RESPONSE--ON THE RELATION BETWEEN STIMULUS AND RESPONSE

Increased response with increasing stimulus--Apparent diminution of
response with excessively strong stimulus.

As already said, in the living tissue, molecular disturbance induced by
stimulus is accompanied by an electric disturbance, which gradually
disappears with the return of the disturbed molecules to their position
of equilibrium. The greater the molecular distortion produced by the
stimulus, the greater is the electric variation produced. The electric
response is thus an outward expression of a molecular disturbance
produced by an external agency, the stimulus.

#Curve of relation between stimulus and response.#--In the curve showing
the relation between stimulus and response in nerve and muscle, it is
found that the molecular effect as exhibited either by contraction or
E.M. variation in muscle, or simply by E.M. variation in nerve, is at
first slight. In the second part, there is a rapidly increasing effect
with increased stimulus. Finally, a tendency shows itself to approach a
limit of response. Thus we find the curve at first slightly co