on will be thrown
into a greater excitatory state than an injured, by the action of the
same stimulus.

CHAPTER V

PLANT RESPONSE--ON THE EFFECTS OF SINGLE STIMULUS AND OF SUPERPOSED
STIMULI

Effect of single stimulus--Superposition of stimuli--Additive
effect--Staircase effect--Fatigue--No fatigue when sufficient
interval between stimuli--Apparent fatigue when stimulation
frequency is increased--Fatigue under continuous stimulation.

#Effect of single stimulus.#--In a muscle a single stimulus gives rise to
a single twitch which may be recorded either mechanically or
electrically. If there is no fatigue, the successive responses to
uniform stimuli are exactly similar. Muscle when strongly stimulated
often exhibits fatigue, and successive responses therefore become
feebler and feebler. In nerves, however, there is practically no fatigue
and successive records are alike. Similarly, in plants, we shall find
some exhibiting marked fatigue and others very little.

#Superposition of stimuli.#--If instead of a single stimulus a succession
of stimuli be superposed, it happens that a second shock is received
before recovery from the first has taken place. Individual effects will
then become more or less fused. When the frequency is sufficiently
increased, the intermittent effects are fused, and we find an almost
unbroken curve. When for example the muscle attains its maximum
contraction (corresponding to the frequency and strength of stimuli) it
is thrown into a state of complete _tetanus_, in which it appears to be
held rigid. If the rapidity be not sufficient for this, we have the
jagged curve of incomplete tetanus. If there is not much fatigue, the
upper part of the tetanic curve is approximately horizontal, but in
cases where fatigue sets in quickly, the fact is shown by the rapid
decline of the curve. With regard to all these points we find strict
parallels in plant response. In cases where there is no fatigue, the
successive responses are identical (fig. 16). With superposition of
stimuli we have fusion of effects, analogous to the tetanus of muscle
(fig. 17). And lastly, the influence of fatigue in plants is to produce
a modification of response-curve exactly similar to that of muscle (see
below). One effect of superposition of stimuli may be mentioned here.

[Illustration: FIG. 16.--UNIFORM RESPONSES (RADISH)]

[Illustration: FIG. 17.--FUSION OF EFFECT OF RAPIDLY SUCCEEDING STIMULI
(_a_)