in muscle; (_b_) in carrot.]

#Additive effect.#--It is found in animal responses that there is a
minimum intensity of stimulus, below which no response can be evoked.
But even a sub-minimal stimulus will, though singly ineffective, become
effective by the summation of several. In plants, too, we obtain a
similar effect, i.e. the summation of single ineffective stimuli
produces effective response (fig. 18).

[Illustration: FIG. 18.--ADDITIVE EFFECT
(_a_) A single stimulus of 3 deg. vibration produced little or no effect,
but the same stimulus when rapidly superposed thirty times,
produced the large effect (_b_). (Leaf-stalk of turnip.)]

#Staircase effect.#--Animal tissues sometimes exhibit what is known as the
'staircase effect,' that is to say, the heights of successive responses
are gradually increased, though the stimuli are maintained constant.
This is exhibited typically by cardiac muscle, though it is not unknown
even in nerve. The cause is obscure, but it seems to depend on the
condition of the tissue. It appears as if the molecular sluggishness of
tissue were in these cases only gradually removed under stimulation, and
the increased effects were due to increased molecular mobility. Whatever
be the explanation, I have sometimes observed the same staircase effect
in plants (fig. 19).

[Illustration: FIG. 19.--'STAIRCASE EFFECT' IN PLANT]

#Fatigue.#--It is assumed that in living substances like muscle, fatigue
is caused by the break down or dissimilation of tissue by stimulus. And
till this waste is repaired by the process of building-up or
assimilation, the functional activity of the tissue will remain below
par. There may also be an accumulation of the products of
dissimilation--'the fatigue stuffs'--and these latter may act as poisons
or chemical depressants.

In an animal it is supposed that the nutritive blood supply performs the
two-fold task of bringing material for assimilation and removing the
fatigue products, thus causing the disappearance of fatigue. This
explanation, however, is shown to be insufficient by the fact that an
excised bloodless muscle recovers from fatigue after a short period of
rest. It is obvious that here the fatigue has been removed by means
other than that of renewed assimilation and removal of fatigue products
by the circulating blood. It may therefore be instructive to study
certain phases of fatigue exhibited under simpler conditions in
vegetable tissue, where