#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_) 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]